New Fatigue Provisions For The Design Of Crane Runway Girders

New Fatigue Provisions for the Design of Crane Runway Girders

James M. Fisher

Julius P. Van de Pas

Author

Author

ames M. Fisher is vice president of Computerized Structural Design (CSD), a Milwaukee, Wisconsin consulting engineering firm. Dr. Fisher received a bachelor of science degree in civil engineering from the University of Wisconsin in 1962. After serving two years as a Lieutenant in the United States Army Corps of Engineers, Dr. Fisher continued his formal education. He received his master of science and Ph.D. degree in structural engineering from the University of Illinois in 1965 and 1968 respectively. Prior to joining CSD, Dr. Fisher was an assistant professor of structural engineering at the University of Wisconsin at Milwaukee. He is a registered structural engineer in several states. Dr. Fisher has specialized in structural steel research and development. He has spent a large part of his career investigating building systems and the study of economical structural framing systems. He was a former chairman of the American Society of Civil Engineers Committee on the Design of Steel Building Structures. Dr. Fisher is a member of the American Iron and Steel Institute (AISI) Committee on Specifications, and a member of the AISC Specification Committee for the Design Fabrication and Erection of Structural Steel Buildings. Dr. Fisher is the co-author of seven books, as-well-as the author of many technical publications in the field of structural engineering. He is a member of the American Society of Civil Engineers and honorary fraternities Tau Beta Pi, Sigma Xi, Chi Epsilon and Phi Kappa Phi. Dr. Fisher received the 1984 T.R. Higgins Lecturship Award presented by the American Institute of Steel Construction.

J

ulius P. Van de Pas is a principal at Computerized Structural Design and manages the firm's Colorado office. Mr. Van de Pas has been employed at CSD since 1988. During his tenure at CSD, he has been responsible for the structural design of numerous industrial, commercial and institutional buildings. Mr. Van de Pas received a bachelor of science degree in civil engineering from the University of Wisconsin-Platteville in 1984 and a master of science degree in civil engineering from the University of Wisconsin-Milwaukee in 1991. He is Licensed as a Professional engineer in Wisconsin, Michigan, California and Colorado. In addition, Mr. Van de Pas has served as an adjunct assistant professor at the University of Wisconsin-Milwaukee. He has also co-authored a publication on steel joist construction.

J

Summary his paper will discuss the design of crane buildings relative to fatigue requirements. Emphasis is placed on the design and detailing requirements to avoid failures due to fatigue. Typical girder configurations, details, and problem areas will be discussed, including lessons that have been learned from previous fatigue related failures. Examples are provided to illustrate how the designer can apply the new 1999 AISC Fatigue Requirements to design for the anticipated service requirements.

T

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NEW FATIGUE PROVISIONS FOR THE DESIGN OF CRANE RUNWAY GIRDERS James M. Fisher, Ph.D., P.E. Julius P. Van de Pas, P.E. INTRODUCTION

Proper functioning of the bridge cranes is dependent upon proper crane runway girder design and

detailing. The runway design must account for the fatigue effects caused by the repeated passing of the crane. Runway girders should be thought of as a part of a system comprised of the crane rails, rail attachments, electrification support, crane stop, crane column attachment, tie back and the girder itself. All of these items should be incorporated into the design and detailing of the crane runway girder system. It has been estimated that 90 percent of crane runway girder problems are associated with fatigue

cracking. To address these conditions, this paper will discuss the new AISC fatigue provisions, crane loads, typical connections and typical details. A design example is also provided. Engineers have designed crane runway girders that have performed with minimal problems while being

subjected to millions of cycles of loading. The girders that are performing successfully have been properly designed and detailed to:

• Limit the applied stress range to acceptable levels. • Avoid unexpected restraints at the attachments and supports • Avoid stress concentrations at critical locations

• Avoid eccentricities due to rail misalignment or crane travel • Minimize residual stresses

Even when all state of the art design provisions are followed building owners can expect to perform

periodic maintenance on runway systems. Runway systems that have performed well have been properly maintained by keeping the rails and girders aligned and level. Some fatigue damage will occur even in "perfectly designed" structures since fabrication and erection cannot be perfect. Fatigue provisions by their very nature have a 95 percent reliability factor for a given stress range, and expected life condition.

Thus, for a given "correct" design a 5 percent failure rate can occur.

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FATIGUE DAMAGE

Fatigue damage can be characterized as progressive crack growth due to fluctuating stress on the member. Fatigue cracks initiate at small defects or imperfections in the base material or weld metal. The imperfections act as stress risers that magnify the applied elastic stresses into small regions of the plastic

stress. As load cycles are applied, the plastic strain in the small plastic region advances until the material separates and the crack advances. At that point, the plastic stress region moves to the new tip of the crack and the process repeats itself. Eventually, the crack size becomes large enough that the combined effect of the crack size and the applied stress exceed the toughness of the material and a final fracture occurs.

The phenomena of fatigue damage or crack growth is considered to occur in three stages. These are initiation, propagation and final fracture. The crack initiation is affected by the initial flaw size, the amount of residual stress, the presence of corrosion and the applied stress range. Most of the fatigue life

of an unwelded or unnotched member is taken up in the initiation of the crack. Fabricated members typically will have small defects from the welding process that can be considered as initiated cracks. In this case, the entire useful life of the section is taken up in crack propagation. The useful life of the elements is usually met when the crack reaches an objectionable size. Crack propagation occurs when the applied loads fluctuate in tension or in reversal from tension to

compression. Fluctuating compressive stress will not cause cracks to propagate. However, fluctuating compressive stresses in a region of residual tensile stress will cause cracks to propagate. In this case, the cracks will stop growing after the residual stress is released or the crack extends out of the tensile region. THE 1999 AISC FATIGUE PROVISIONS

The 1999 AISC LRFD Specification contains revised fatigue provisions. Both the 1993 (current) and 1999 AISC Specifications are based on S-N curves that define allowable stress range values for given

detail, categories, and loading conditions. These relationships were established based on an extensive database developed in the United States and abroad. The database for the provisions was based on cyclic testing of actual joints, thus stress concentrations were accounted for in each of the detail stress

categories. Calculated stresses were determined by ordinary analysis at service loads and were not

amplified by stress concentration factors since the factors already existed in the tested real conditions. The 1993 provisions define Loading Conditions based on the number of cycles expected in the life of the

structure. The loading conditions are defined as 20,000 to 100,000 cycles, 100,000 to 500,000 cycles, 500,000 to 2,000,000 cycles or more than 2,000,000 cycles (Table A-K3.1). Stress Category

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Classifications are defined based on the configuration of the given conditions and the associated stress concentrations (Table A-K3.2). The Design Stress Range is determined based on the Loading Condition and the Stress Category Classification.

The 1996 provisions provide a more accurate method of determining the Design Stress Range. The 1996 provisions use a single table that is divided into sections which described various conditions. The sections are: 1. Plain material away from any welding. 2. Connected material in mechanically fastened joints.

3. Welded joints joining components of built-up members. 4. Longitudinal fillet welded end conditions. 5. Welded joints transverse to direction of stress.

6. Base metal at welded transverse member connections. 7. Base metal at short attachments. 8. Miscellaneous.

The 1999 AISC provisions use equations to calculate the design stress range for a chosen design life, N, for various conditions and stress categories. For the first time, the point of potential crack initiation is identified by description, and shown in the table figures. The tables also provide the detail constant, applicable to the stress category that is required for calculating the design stress range

For example,

for the majority of stress categories

where:

Constant from Table A-K3.1 Number of stress range fluctuations in design life, Number of stress range fluctuations per day x 365 x years of design life

Threshold fatigue stress range, maximum stress range for indefinite design life The tables contain the threshold design stress

for each stress category. A copy of the new fatigue

provisions are provided in the Appendix of this paper.

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The 1993 and the 1999 AISC Specifications limit the allowable stress range for a given service life based on an anticipated severity of the stress riser for a given fabricated condition. In addition to limiting the

applied stress range, the AISC Specification for certain cases requires conformance with Chapter 9 of the AWS D1.1 Structural Welding Code. Chapter 9 of the ANSI/AWS JD1.1 Structural Welding Code titled Dynamically Loaded Structures provides criteria for limiting the severity of stress risers found in weld

metal and the adjacent base metal. CRANE RUNWAY LOADS

Each runway is designed to support a specific crane or group of cranes. The weight of the crane bridge

and trolley and the wheel spacing for the specific crane should be obtained from the crane manufacturer. The crane weight can vary significantly depending on the manufacturer and the classification of the crane. Based on the manufacturer's data, forces are determined to account for impact, lateral loads, and longitudinal loads. The AISC Specification, and most model building codes address crane loads and set

minimum standards for these loads. The AISE Technical Report No. 13 Guide for the Design and Construction of Mill Buildings also sets minimum requirements for impact, lateral and longitudinal crane loads. The AISE requirements are used when the engineer and owner determine that the level of quality set by the AISE Guide is appropriate for a give project. Vertical crane loads are termed as wheel loads. The magnitude of the wheel load is at its maximum when

the crane is lifting its rated capacity load, and the trolley is located at the end of the bridge directly adjacent to the girder. The vertical wheel loads are typically factored by the use of an impact factor. The impact factor accounts for the effect of acceleration in hoisting the loads and impact caused by the wheels jumping over

irregularities in the rail. Bolted rail splices will tend to cause greater impact when welded rail splices. In

the US, most codes require a twenty-five percent increase in loads for cab and radio operated cranes, and

a ten percent increase for pendant operated cranes. Lateral crane loads are oriented perpendicular to the crane runway and are applied at the top of the rails. Lateral loads are caused by:

• Acceleration and deceleration of the trolley and loads • Non vertical lifting • Unbalanced drive mechanisms

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• Oblique or skewed travel of the bridge The AISC Specification and most model building codes set the magnitude of lateral loads at 20% of the sum of the weights of the trolley and lifted load. The AISE Technical Report varies the magnitude of the lateral load based on the function of the crane.

Longitudinal crane forces are due to either acceleration and deceleration of the bridge crane or the crane impacting the bumper. The tractive forces are limited by the coefficient of friction of the steel wheel on the rails. The force imparted by impact with hydraulic or spring type bumpers is a function of the length

of stroke of the bumper and the velocity of the crane upon impact with the crane stop. The longitudinal forces should be obtained from the crane manufacturer. If this information is not available, the AISE Technical Report provides equations that can be used for determining the bumper force.

Consideration of fatigue requires that the designer determine the anticipated number of load cycles. It is a common practice for the crane runway girder to be designed for service life that is consistent with the crane classification. The correlation between the CMAA crane designations and the anticipated number

of load cycles for the life of the structure is not easy since a given crane does not lift its maximum load, or travel at the same speed, every day or every hour. Shown in Table 1 are estimates of the number of cycles for CMAA crane classifications A through F over a 40 year period. It must be emphasized that

these are only guidelines and actual duty cycles can only be established from the buildings owner and the crane manufacturer. CMAA Crane Classification A

Design Life 20,000 50,000 100,000 500,000 1,500,000 >2,000,000

B C D E F

The AISE Guide provides specific load combinations to be used for fatigue calculations. CRANE RUNWAY FATIGUE DESIGN

Tension Flange Stress

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When runway girders are fabricated from plate material, fatigue requirements are more severe than for rolled shape girders. AISC (1996) Appendix K3 Section 3.1 applies to the design of the plate material and Section 1.1 applies to plain material. Stress Category B is required for plate girders as compared to stress Category A for rolled shapes.

Web to Flange Welds The shear in fillet welds which connect the web to the tension and compression flanges is controlled by

Section 8.2, stress Category F. Cracks have been observed in plate girders at the junction of the web to the compression flange of runway girders when fillet welds are used to connect the web to the compression flange. The AISE Guide requires that this joint be a full penetration weld with fillet reinforcement.

Tiebacks Tiebacks are provided at the end of the crane runway girders to transfer lateral forces from the girder top flange into the crane column and to laterally restrain the top flange of the crane girder against buckling.

The tiebacks must have adequate strength to transfer the lateral crane loads. However, the tiebacks must also be flexible enough to allow for longitudinal movement of the top of the girder caused by girder end rotation. The amount of longitudinal movement due to the end rotation of the girder can be significant.

The end rotation of a 40 foot girder that has undergone a deflection of span over 600 is about .005 radians. For a 36 inch deep girder this results in .2" of horizontal movement at the top flange. The tieback must also allow for vertical movement due to axial shortening of the crane column. This vertical movement can be in the range of ¼ inch. In general, the tieback should be attached directly to the top flange of the girder. Attachment to the web of the girder with a diaphragm plate should be avoided. The

lateral load path for this detail causes bending stresses in the girder web perpendicular to the girder cross

section. The diaphragm plate also tends resist movement due to the axial shortening of the crane column. Various AISC fatigue provisions are applicable to the loads depending on the exact tieback configurations.

Bearing Stiffeners Bearing stiffeners should be provided at the ends of the girders as required by the AISC Specification Paragraphs K1.3 and K1.4. Fatigue cracks have occurred at the connection between the bearing stiffener

and the girder top flange. The cracks occurred in details where the bearing stiffener was fillet welded to the underside of the top flange. Passage of each crane wheel produces shear stress in the fillet welds. The AISC fatigue provisions contain fatigue criteria for fillet welds in shear; however, the determination of

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the actual stress state in the welds is extremely complex, thus the AISE Guide requires that full penetration welds be used to connect the top of the bearing stiffeners to the top flange of the girder. The bottom of the bearing stiffeners may be fitted (preferred) or fillet welded to the bottom flange. All stiffener to girder webs should be continuous. Horizontal cracks have been observed in the webs of crane girders with partial height bearing stiffeners. The cracks start between the bearing stiffeners and the top flange and run longitudinally along the web of the girder. There are many possible causes for the propagation of these cracks. One possible explanation is that eccentricity in the placement of the rail on the girder causes distortion of the girder cross section and rotation of the girder cross section. Intermediate Stiffeners If intermediate stiffeners are used, the AISE Guide also requires that the intermediate stiffeners be welded to the top flange with full penetration welds for the same reasons as with bearing stiffeners. Stiffeners should be stopped short of the tension flange in accordance with the AISC Specification provisions contained in Chapter G. The AISE Guide also requires continuous stiffener to web welds for intermediate stiffeners. Fatigue must be checked where the stiffener terminates adjacent to the tension flange. This condition is addressed in Section 5.7, Table A-K3.1, of the new AISC Specifications. Channel Caps and Cap Plates Channel caps or cap plates are frequently used to provide adequate top flange capacity to transfer lateral loads to the crane columns and to provide adequate lateral torsional stability of the runway girder cross section. The common heuristic is that a wide flange reinforced with a cap channel will be economical if it is 20 pounds a foot lighter than a unreinforced wide flange member. It should be noted that the cap channel or plate does not fit perfectly with 100% bearing on the top of the wide flange. The tolerances given in ASTM A6 allow the wide flange member to have some flange tilt along its length, or the plate may be cupped or slightly warped, or the channel may have some twist along its length. These conditions will leave small gaps between the top flange of the girder and the top plate or channel. The passage of the crane wheel over these gaps will tend to distress the channel or plate to top flange welds. Calculation of the stress condition for these welds is nearly impossible. Because of this phenomena, cap plates or channels should not be used with Class E or F cranes. For less severe duty cycle cranes, shear flow stress in the welds can be calculated and limited according to the AISC fatigue provisions in Section 8.2 of the 1999 Specifications. The channel or plate welds to the top flange can be continuous or intermittent.

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However, the AISC design stress range for the base metal is reduced from Category B (Section 3.1) for continuous welds to Category E (Section 3.4) for intermittent welds. Crane Column Cap Plates

The crane column cap plate should be detailed so as to not restrain the end rotation of the girder. If the cap plate girder bolts are placed between the column flanges, the girder end rotation is resisted by a force couple between the column flange and the bolts. This detail has been known to cause bolt failures.

Preferably, the girder should be bolted to the cap plate outside of the column flanges. The column cap plate should be extended outside of the column flange with the bolts to the girder placed outside of the

column flanges. The column cap plate should not be made overly thick as this detail requires the cap

plate to distort to allow for the end rotation of the girder. The girder to cap plate bolts should be adequate to transfer the tractive or bumper forces to the longitudinal crane bracing. The engineer should consider using slotted holes perpendicular to the runway or oversize holes to allow tolerance for aligning the girders atop the crane columns. Laced Crane Girders A horizontal truss can be used to resist the crane lateral forces. The truss is designed to span between the

crane columns. Typically, the top flange of the girder acts as one chord of the truss while a back up beam acts as the other chord. The diagonal members are typically angles. Preferably, the angles should be bolted rather than welded. The crane girder will deflect downward when the crane passes, the back up beam will not. The design of the diagonal members should account for the fixed end moments that will

be generated by this relative movement. Walkways can be designed and detailed as a beam to transfer lateral loads to the crane columns. The lacing design may need to be incorporated into the walk design. Similar to the crane lacing, the walkway connection to the crane girder needs to account for the vertical deflection of the crane girder. If the

walkway is not intended to act as a beam, then the designer must isolate the walkway from the crane girder. The AISE Guide requires that crane runway girders with spans of 36 feet and over for AISE Building Classifications A, B and C or runway girder spans 40 feet and over in AISE Class D buildings shall have bottom flange bracing. This lacing is to be designed for 2½ percent of the maximum bottom flange force,

and is not to be welded to the bottom flange. Cross braces or diaphragms should not be added to this

bracing so as to allow for the deflection of the crane beam relative to the backup beam.

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Various AISC fatigue provisions are applicable to lacing systems depending on the detail used to connect the lacing to the runway girders and the back up girder. Rail Attachments

The rail to girder attachments must perform the following functions: • transfer the lateral loads from the top of the rail to the top of the girder.

• allow the rail to float longitudinally relative to the top flange of the girder.

• hold the rail in place laterally. • allow for lateral adjustment or alignment of the rail.

The relative longitudinal movement of the crane rail to the top flange of crane girder is caused by longitudinal expansion and contraction of the rail in response to changes in temperature and shortening of

the girder compression flange due to the applied vertical load of the crane.

There are four commonly accepted methods of attaching light rails supporting relatively small and light duty cranes. Hook bolts should be limited to CMAA Class A, B and C cranes with a maximum capacity of approximately 20 tons. Hook bolts work well for smaller crane girders that do not have adequate space on the top flange for rail clips or clamps. Longitudinal motion the crane rail relative to the runway girder

may cause the hook bolts to loosen or elongate. Therefore, crane runways with hook bolts should be regularly inspected and maintained. AISC recommends that hook bolts be installed in pairs at a

maximum spacing of 24 inches on center. The use of hook bolts eliminates the need to drill the top flange of the girder. However, these savings are offset by the need to drill the rails. Rail clips are one piece castings or forgings that are usually bolted to the top of the girder flange. Many

clips are held in place with a single bolt. The single bolt type of clip is susceptible to twisting due to longitudinal movement of the rail. This twisting of the clip causes a camming action that will tend to push the rail out of alignment.

There are two types of rail clamps, tight and floating. Rail clamps are two part forgings or pressed steel assemblies that are bolted to the top flange of the girder. The AISE Technical Report No. 13 requires that rail clips allow for longitudinal float of the rail and that the clips restrict the lateral movement to ¼ inch inward or outward. When crane rails are installed with resilient pads between the rail and the girder, the

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amount of lateral movement should be restricted to 1/32 inch to reduce the tendency of the pad to work

out from under the rail. Patented rail clips are typically two part castings or forgings that are bolted or welded to the top flange of the crane girder. The patented rail clips have been engineered to address the complex requirements of

successfully attaching the crane rail to the crane girder. Compared to traditional clips, the patented clips provide greater ease in installation and adjustment and provide the needed performance with regard to allowing longitudinal movement and restraining lateral movement. The appropriate size and spacing of

the patented clips can be determined from the manufacturer's literature. When rail clips are attached to the runway girder by welding the runway girder top flange stress must be checked using the requirement of Section 7.1 of the AISC fatigue provisions.

Miscellaneous Attachments

Miscellaneous attachments to crane runway girders should be avoided. The AISE Guide specifically prohibits welding attachments to the tension flange of runway girders. Brackets to support the runway electrification are often necessary. If the brackets are bolted to the web of the girder, fatigue consequences are relatively minor, i.e. stress category B, Section 1.3 of the AISC Fatigue Specifications.

However, if the attachment is made with fillet welds Section 7.2 of the Fatigue Specification applies. This provision places the detail into stress category D or E depending on the detail. EXAMPLE

Design a welded plate girder to support the following pair of cranes. The runway beams are to be designed for 2,000,000 cycles and the owner has required conformance with the AISE Guide for the

Design and Construction of Mill Buildings. Use the 1999 AISC fatigue provisions and the prescriptive requirements of AISE. Crane Capacity: (2) 30 ton magnet cranes Wheel Spacing: 22 feet - two wheels per end truck Crane Spacing: 11 feet between wheels Bridge Length: 100 feet Bridge Weight: 270 kips Trolley Weight: 30 kips

Maximum Wheel Load: 108 kips Rail Size: 135#/rail with welded clamps

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Runway Girder Span: 40 feet Determine the maximum moment due to the two cranes:

Position the crane with the center of the girder midway between one wheel and the centroid of the load. Allow 500 plf for the girder and attachments.

(two cranes - no impact)

Determine the maximum lateral load per wheel. Per AISE 3.4.2: V equals 100% of the lifted load

or 20% of the lifted load plus trolley

or 10% of the lifted load plus the crane weight

Determine the maximum lateral movement for two cranes: Per AISE 3.10.2 use 50% of the single maximum lateral load for multiple cranes. Position the wheels at

the same location as for the maximum vertical load.

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Determine the maximum vertical moment for one crane.

Include 25% impact per AISE 3.4.

Determine the maximum lateral moment for one crane:

Determine the required moment of inertia to limit the maximum vertical deflection of L/1000. The critical location occurs when the wheel loads are centered on the girder.

Trial Section Try a plate girder with a 28 in. x 1.5 in. top flange, 22 in. x 1 in. bottom flange and a 42 in. 1 ½ in. web. The girder has the following cross section properties.

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Check bending stresses for two crane with 50% of the maximum lateral load acting per crane.

Note the lateral loads are increased to account for the rail height of 5.75 inches. Per AISC F1-G Per AISC F2-1

Check combined stresses per AISC H103:

Check bending stresses for one crane:

Check shear on the girder web:

Check sidesway web buckling per AISC K1-7:

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Fatigue Design

The allowable stresses for fatigue design are based on the 1999 AISC Specification Appendix K. In accordance with AISE Section 3.10 fatigue loading is based on either the vertical load from one crane including impact and 50% of the maximum lateral load, or the vertical load from both cranes and 50% of

the maximum lateral load. The following fatigue conditions will be evaluated: 1. 2. 3. 4.

The tension flange flexural stress. The web to tension flange shear flow stress. The top flange at the rail clips for lateral load flexural stress. The weld at the base of the intermediate stiffeners.

1. Tension Flange Check the tension flange. Only the live load moment is used to determine the bending stress.

From the 1999 AISC Specifications Table A-K3.1, Stress Category B, Section 3.1,

2. Web to Flange Welds Determine the fillet weld size for the bottom flange attachment to the web. This fillet weld is designed to provide adequate shear flow capacity. The shear is based on the maximum live load shear on the girder.

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From the 1999 AISC Specifications Table A-K3.1, Section 8.2, Stress Category F,

Use 3/8 fillet welds NS/FS. At the top flange use a full penetration weld with contoured fillets per AISE Technical Report #13.

3. Intermediate Stiffener Welds

Assume that intermediate stiffeners are provided at equal spaces along the length of the girder. The flexural stress level at the bottom weld termination of the stiffeners needs to be checked. It should be emphasized that the flexural stress at this location is not a stress in the stiffener weld. Rather, it is the flexural stress that occurs at the location of this stress riser. Per AISC Table A-K3.1, Section 5.7, the Stress Category C is appropriate, and

Per AISC G4 terminate the intermediate stiffener between 4 and 6 times the web thickness from the near toe of the flange to web weld.

Determine the distance from the end of the stiffener to the neutral axis.

Determine the stress range at the end of the stiffener.

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4. Top Flange Rail Clips

The fatigue concern at the top flange of the girder is created by the stress due to the lateral loads. The vertical wheel loads always cause compressive stress in the top flange. Since fatigue cracks do not propagate in regions of compressive stress, a check will be made of the various combinations of minimum vertical load with maximum lateral load to determine if any of the loading conditions results in a net tension.

For the condition at the top flange, the critical location occurs at the weld of the clip to the top flange. Depending on the configuration of the attachment, the appropriate Stress Category from Table A-K3.1, Section 7.1, is either C, D, E or E'. The distance from the center of the top flange to the back of the clip is 5.25 inches.

The minimum wheel load is 72 kips. Check two cranes:

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Therefore no net tension occurs for the two crane condition.

Check one crane: Include impact and 50% lateral load for the minimum wheel load of 72 kips

No net tension occurs for the single crane loading condition. No further fatigue investigation is required for the top flange.

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APPENDIX

1999 AISC FATIGUE PROVISIONS

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DRAFT

(The following replaces the entire old Appendix K3) K3

DESIGN FOR CYCLIC LOADING (FATIGUE) This appendix applies to members and connections subject to high cycle loading within the elastic range of stresses of frequency and magnitude sufficient to initiate cracking and progressive failure (fatigue).

1.

General The provisions of this section apply to stresses calculated on the basis of Unfactored loads. The maximum permitted stress due to Unfactored loads is 0.66

Stress range is defined as the magnitude of the change in stress due to the application or removal of the Unfactored live load. In the case of a stress reversal, the stress range shall be computed as the numerical sum of maximum repeated tensile and compressive stresses or the numerical sum of maximum shearing stresses of opposite direction at the point of probable crack initiation. In the case of complete joint penetration butt welds, the maximum design stress range calculated by Equation A-K3.1 applies only to welds with internal soundness meeting the acceptance requirements of Section 6.12.2 or 6.13.2 of AWS D1.1. No evaluation of fatigue resistance is required if the live load stress range is less than the threshold stress range, See Table A-K3.1. No evaluation of fatigue resistance is required if the number of cycles of application of live load is less than 2 x 104. The cyclic load resistance determined by the provisions of this appendix is applicable to structures with suitable corrosion protection or subject only to mildly corrosive atmospheres, such as normal atmospheric conditions. The cyclic load resistance determined by the provisions of this appendix is applicable only to structures subject to temperatures not exceeding 300° F (150° C). The Engineer of Record shall provide either complete details including weld sizes or shall specify the planned cycle life and the maximum range of moments, shears and reactions for the connections. 2.

Calculation of Maximum Stresses and Stress Ranges Calculated stresses shall be based upon elastic analysis. Stresses shall not be amplified by stress concentration factors for geometrical discontinuities. For bolts and threaded rods subject to axial tension, the calculated stresses shall include the effects of prying action, if any.

In the case of axial stress combined with bending, the maximum stresses, of each kind, shall be those determined for concurrent arrangements of the applied load.

11/09/99

APPENDICES

164

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DRAFT

For members having symmetric cross sections, the fasteners and welds shall be arranged symmetrically about the axis of the member, or the total stresses including those due to eccentricity shall be included in the calculation of the stress range. For axially loaded angle members where the center of gravity of the connecting welds lies between the line of the center of gravity of the angle cross section and the center of the connected leg, the effects of eccentricity shall be ignored. If the center of gravity of the connecting welds lies outside this zone, the total stresses, including those due to joint eccentricity, shall be included in the calculation of stress range. 3.

Design Stress Range

The range of stress at service loads shall not exceed the stress range computed as follows. (a) For stress categories except category A, B, B', C, D, E and E' the design stress range, shall be determined by Equation A-K3.1.

(A-K3.1)

Metric:

(A-K3.1M)

where

Design stress range, ksi (MPa) Constant from Table A-K3.1 for category Number of stress range fluctuations in design life Number of stress range fluctuations per day x 365 x years of design life Threshold fatigue stress range, maximum stress range for indefinite design life from Table A-K3.1, ksi (b) For stress category F, the design stress range, by Equation A-K3.2.

shall be determined

(A-K3.2)

Metric:

(A-K3.2M)

(c) For tension-loaded plate elements at their end by cruciform, T or corner details with complete joint penetration welds or partial joint penetration welds, fillet welds, or combinations of the preceding, transverse to the direction of stress, the maximum stress range on the cross section of the tension-loaded plate element at the toe of the weld shall be determined as follows:

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APPENDICES

165 13-22

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT

Based upon crack initiation from the toe of the weld on the tension

loaded plate element the design stress range, shall be determined by Equation A-K3.1, for Category C which is equal to

Metric: Based upon crack initiation from the root of the weld the design stress range, on the tension loaded plate element using transverse partialjoint- penetration welds, with or without reinforcing or contouring fillet welds, the design stress range on the cross section at the toe of the weld shall be determined by Equation A-K3.3, Category C' as follows: (A-K3.3)

Metric:

(A-K3.3M)

where: reduction factor for reinforced or non-reinforced transverse PJP joints

the length of the non-welded root face in the direction of the thickness of the tension-loaded plate, in. (mm) the leg size of the reinforcing or contouring fillet, if any, in the direction of the thickness of the tension-loaded plate, in. (mm) thickness of tension loaded plate, in. (mm) Based upon crack initiation from the roots of a pair of transverse fillet welds on opposite sides of the tension loaded plate element the design stress range, on the cross section at the toe of the welds shall be determined by Equation A-K3.4, Category C" as follows:

(A-K3.4)

Metric:

4.

(A-K3.4M)

Bolts and Threaded Parts

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APPENDICES

166 13-23

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT

(a) For mechanically fastened connections loaded in shear, the maximum range of stress in the connected material at service loads shall not exceed the design stress range computed using Equation A-K3.1 where and are taken from Section 2 of Table A-K3.1. (b) For not-fully-tightened high-strength bolts, common bolts, and threaded anchor rods with cut, ground or rolled threads, the maximum range of tensile stress on the net tensile area from applied axial load and moment plus load due to prying action shall not exceed the design stress range computed using Equation A-K3.1 or A-K3.1M. The factor shall be taken as 3.9 x 10 8 (as for category E'). The threshold stress, shall be taken as 7 ksi (as for category D). The net tensile area is given by Equation A-K3.5.

(A-K3.5)

Metric:

(A-K3.5M)

where

pitch, mm per thread the nominal diameter (body or shank diameter), in. (mm) threads per in. In joints that are not fabricated and installed to satisfy all of the requirements for slip-critical connections (Section J3.8), except the requirements for faying surface condition, all axial load and moment applied to the joint plus effects of prying action (if any) shall be assumed to be carried exclusively by the bolts or rods. In joints that are fabricated and installed to satisfy all of the requirements for slip-critical connections, except requirements for faying surface condition, an analysis of the relative stiffness of the connected parts and bolts shall be permitted to be used to determine the tensile stress range in the pretensioned bolts due to the total service live load and moment plus effects of prying action. Alternatively, the stress range in the bolts shall be assumed to be equal to the stress on the net tensile area due to 20 percent of the absolute value of the service load axial load and moment from dead, live and other loads.

5.

Special Fabrication and Erection Requirements

Longitudinal backing bars are permitted to remain in place, and if used, shall be continuous. If splicing is necessary for long joints, the bar shall be joined with complete penetration butt joints and the reinforcement ground prior to assembly in the joint. In transverse joints subject to tension, backing bars, if used, shall be removed and the joint back gouged and welded. In transverse complete joint penetration tee and corner joints, a single pass reinforcing fillet weld, not less than ¼ in. (6 mm) in size shall be added at reentrant corners.

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APPENDICES

167 13-24

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT

The surface roughness of flame cut edges subject to significant cyclic stress ranges shall not exceed 1 000 µin. (25 µm), where ASME B46.1 is the

reference standard. Re-entrant corners at cuts, copes and weld access holes shall form a radius of not less than 3/8 in. (10 mm) by pre-drilling or sub-punching and reaming a hole, or by thermal cutting to form the radius of the cut. If the radius portion

is formed by thermal cutting, the cut surface shall be ground to a bright metal surface with a surface roughness value not more than 1 000 µin. (25 µm) (ASME B46.1). For transverse butt joints in regions of high tensile stress, run-off tabs shall be used to provide for cascading the weld termination outside the finished joint. End dams shall not be used. Run-off tabs shall be removed and the end of the weld finished flush with the edge of the member. See Section J2.2b Fillet Weld Terminations for requirements for end returns on certain fillet welds subject to cyclic service loading.

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APPENDICES

168 13-25

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1

Fatigue Design Parameters Description

Stress

Constant

Category

C1

Threshold FTH

Potential Crack Initiation Point

ksi

SECTION 1 - PLAIN MATERIAL AWAY FROM ANY WELDING 1.1 Base metal, except non-coated weathering steel, with rolled or cleaned surface Flame-cut edges with surface roughness value of 1 000 µin (25 µm) or less, but without re-entrant corners. 1.2 Non-coated weathering steel base metal with rolled or cleaned surface. Flame-cut edges with surface roughness value of 1 000 µin (25 µm) or less, but without re-entrant corners 1.3 Member with drilled or reamed holes. Member with re-entrant corners at copes, cuts, block-outs or other geometrical discontinuities made to requirements of Appendix K3 5, except weld access holes, with surface roughness value of 1 000 µin (25 µm) or less 1.4 Rolled cross sections with weld access holes made to requirements of Section J1.6 and Appendix K3.5. Members with drilled or reamed holes containing bolts for attachment of light bracing where there is a small longitudinal component of brace force.

A

250x10

8

24

Away from all welds or structural connections

B

120 x10 8

24

Away from all welds or structural connections

B

120 x10 8

16

At any external edge or at hole perimeter

C

44x10

8

10

At reentrant corner of weld access hole or at any small hole (may contain bolt for minor connections)

SECTION 2 - CONNECTED MATERIAL IN MECHANICALLY FASTENED JOINTS 2.1 Gross area of base metal in lap joints connected by high-strength bolts in joints satisfying all requirements for slip-critical connections 2.2 Base metal at net section of highstrength bolted joints, designed on the basis of bearing resistance, but fabricated and installed to all requirements for slip-critical connections 2.3 Base metal at the net section of other mechanically fastened joints except eye bars and pin plates. 2.4 Base metal at net section of eyebar head or pin plate.

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B

120x10

8

16

Through gross section near hole

B

120x10 8

16

In net section originating at side of hole

D

22x10 8

7

E

11 x10 8

4.5

In net originating of hole In net originating of hole

section at side section at side

169

APPENDICES

13-26 © 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (cont'd)

Fatigue Design Parameters

Illustrative Typical Examples SECTION 1 - PLAIN MATERIAL AWAY FROM ANY WELDING

SECTION 2 - CONNECTED MATERIAL IN MECHANICALLY FASTENED JOINTS

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170

APPENDICES 13-27

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters Description

Stress Category

Constant

cf1

Threshold FTH

Potential Crack Initiation Point

ksi

SECTION 3 - WELDED JOINTS JOINING COMPONENTS OR BUILT-UP MEMBERS

3.1 Base metal and filler metal in members without attachments built-up of plates or shapes connected by continuous longitudinal complete penetration groove welds, back gouged and welded from second side, or by continuous fillet welds.

From surface or

internal

B

120x10 8

16

3.2 Base metal and filler metal in

members without attachments built-up of plates or shapes, connected by continuous longitudinal complete penetration groove welds with backing

discontinuities in weld away from end of weld

From surface or internal

B'

61 x 108

12

discontinuities weld,

in

including

weld attaching backing bars

bars not removed, or by continuous partial joint penetration groove welds. 3.3

Base

metal

and weld

metal

termination of longitudinal welds at weld access holes in connected built-up members. 3.4 Base metal at ends of longitudinal intermittent fillet weld segments.

D

22x10 8

7

E

11 x108

4.5

3.5 Base metal at ends of partial length

From the termination

weld into

the web or flange In connected material at start and stop locations of any weld deposit

welded coverplates narrower than the

In flange at toe of end weld or in

flange having square or tapered ends,

flange

with or without welds across the ends of coverplates wider than the flange with welds across the ends.

termination of longitudinal weld or in edge of flange with wide

Flange thickness < 0.8 in. (20 mm)

E

11 x10 8

4.5

Flange thickness > 0.8 in. (20 mm)

E'

3.9 x10 8

2.6

3.6 Base metal at ends of partial length

E'

3.9 x 108

2.6

welded coverplates wider than the

flange without welds across the ends.

at

coverplates

In edge of flange at end of coverplate weld

SECTION 4 - LONGITUDINAL FILLET WELDED END CONNECTIONS

4.1 Base metal at junction of axially loaded members with longitudinally welded end connections. Welds shall be on each side of the axis of the

Initiating from end of any weld termination extending into the

member to balance weld stresses.

base metal

t<½-in. (13mm)

E

11x10 8

4.5

t>½-in. (13mm)

E'

3.9x10 8

2.6

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171

APPENDICES

13-28 © 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters Illustrative Typical Examples SECTION 3 - WELDED JOINTS JOINING COMPONENTS OF BUILT-UP MEMBERS

SECTION 4 - LONGITUDINAL FILLET WELDED END CONNECTIONS

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172

APPENDICES 13-29

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters Description

Stress Category

Constant

Threshold

Potential Crack Initiation Point

SECTION 5 - WELDED JOINTS TRANSVERSE TO DIRECTION OF STRESS 5.1 Base metal and filler metal in or adjacent to complete joint penetration groove welded splices in rolled or welded cross sections with welds ground essentially parallel to the direction of stress. 5.2 Base metal and filler metal in or

From

internal

discontinuities B

8

120 x 10

16

in

filler metal or along the fusion boundary

adjacent to complete joint penetration

From

groove welded splices with welds ground essentially parallel to the

discontinuities in filler metal or along fusion boundary or at start of transition when

direction of stress at transitions in thickness or width made on a slope no greater than 8 to 20%. B

120 x10 8

B'

61 x10

16

internal

(620 MPa)

5.3 Base metal with equal to or greater than 90 ksi (620 MPa) and filler metal in or adjacent to complete joint penetration groove welded splices with welds ground essentially parallel to the direction of stress at transitions in width

B

8

120 x10 8

12

16

made on a radius of not less than 2 ft.

From internal discontinuities in filler metal or discontinuities along the fusion

boundary

(600 mm) with the point of tangency at the end of the groove weld. 5.4 Base metal and filler metal in or

adjacent to the toe of complete joint penetration T or corner joints or splices, with or without transitions in thickness

having slopes no greater than 8 to 20%, when weld reinforcement is not removed. 5.5 Base metal and weld metal at transverse end connections of tensionloaded plate elements using partial joint

C

44x10 8

10

From surface discontinuity at toe of weld extending into base metal or along fusion

boundary. Initiating geometrical

from

penetration butt or T or corner joints,

discontinuity at toe

with reinforcing or contouring fillets, shall be the smaller of the toe crack or root crack stress range. Crack initiating from weld toe:

of weld extending

Crack initiating from weld root:

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C

44x10

C'

Eqn. (A-K3.3)

8

10

into base metal or, initiating at weld root subject to

None provided

tension extending up and then out through weld

APPENDICES

173 13-30

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters Illustrative Examples

SECTION 6 - WELDED JOINTS TRANSVERSE TO DIRECTION OF STRESS

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APPENDICES

174 13-31

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters Description

Stress

Constant

Threshold

Category

Potential Crack Initiation Point

SECTION 5 - WELDED JOINTS TRANSVERSE TO DIRECTION OF STRESS (cont'd) Initiating from geometrical discontinuity at toe of weld extending

5.6 Base metal and filler metal at transverse end connections of tensionloaded plate elements using a pair of fillet welds on opposite sides of the plate. shall be the smaller of the toe crack or root crack stress range.

into base metal or, initiating at weld 8

Crack initiating from weld toe:

C

44 x 10

10

Crack initiating from weld root:

C"

Eqn. (A-K3.4)

provided

44x10 8

10

5.7 Base metal of tension loaded plate elements and on girders and rolled beam webs or flanges at toe of transverse fillet welds adjacent to welded transverse stiffeners.

C

None

root subject to tension extending up and then out through weld From geometrical discontinuity at toe of fillet extending into base metal

SECTION 6 - BASE METAL AT WELDED TRANSVERSE MEMBER CONNECTIONS 6.1

Base metal at details attached by

complete joint penetration groove welds

subject to longitudinal loading only when the detail embodies a transition

Near point of tangency of radius at edge of member

radius R with the weld termination ground smooth.

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B

120 x 108

C

44x10

D

22x10

E

11 x 108

8

8

16 10 7

4.5

175

APPENDICES 13-32

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT

SECTION 5 - WELDED JOINTS TRANSVERSE TO DIRECTION OF STRESS (cont'd)

SECTION 6 - BASE METAL AT WELDED TRANSVERSE MEMBER CONNECTIONS

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176

APPENDICES 13-33

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters Stress Category

Description

Constant

Threshold

Potential Crack Initiation Point

SECTION 6 - BASE METAL AT WELDED TRANSVERSE MEMBER CONNECTIONS (cont'd) 6.2

Base metal at details of equal

thickness attached by complete joint penetration groove welds subject to

transverse loading with or without longitudinal loading when the detail embodies a transition radius R with the weld termination ground smooth: When weld reinforcement is removed:

(600 mm > 150 mm)

B

120 x10 8

16

C

44x10 8

10

D

22x10 8

7

E

11 x 108

4.5

Near points of tangency of radius or in the weld or at fusion boundary or member or attachment

2 in. (50 mm) > R

When weld removed:

reinforcement

is

not

C

44x10 8

10

C

44x10 8

10

D

22x10 8

7

E

11 x 108

4.5

At toe of the weld either along edge of member or the attachment

2 in. (50 mm) > R

6.3 Base metal at details of unequal thickness attached by complete joint

penetration groove welds subject to transverse loading with or without longitudinal loading when the detail

embodies a transition radius R with the weld termination ground smooth. At toe

When weld reinforcement is removed:

along

D

22x10 8

E

11 x 10

E

11 X 108

7 4.5

8

of

weld

edge

of

thinner material In weld termination in small radius

When reinforcement is not removed: 4.5 Any radius

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At toe of weld along edge of thinner material

177

APPENDICES

13-34 © 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT SECTION 6 - BASE METAL AT WELDED TRANSVERSE MEMBER CONNECTIONS (cont'd)

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178

APPENDICES 13-35

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters Description

Stress

Constant

Threshold

Potential Crack Initiation Point

Category

SECTION 6 - BASE METAL AT WELDED TRANSVERSE MEMBER CONNECTIONS (cont'd) 6.4

Base metal subject to longitudinal

stress at transverse members, with or without transverse stress, attached by fillet or partial penetration groove welds

In weld termination

parallel to direction of stress when the detail embodies a transition radius, R,

or from the toe of the weld extending

into member

with weld termination ground smooth: 8

D

22x10

E

11 x10

7 4.5

8

SECTION 7 - BASE METAL AT SHORT ATTACHMENTS 7.1

Base metal subject to longitudinal

loading at details attached by complete penetration groove welds parallel to

direction of stress where the detail embodies a transition radius, R, less than 2 in. (50 mm), and with detail length in direction of stress, a, and attachment height normal to surface of member, to:

a < 2 in. (50 mm) or 4 in (100mm)

a> 12b or 4in. (100mm) when b is > 1 in. (25 mm)

In the member at the end of the weld

C

44 x 108

D

22 x 10

7

E

11x10 8

4.5 2.6

E'

3.9 x10

8

10

8

7.2 Base metal subject to longitudinal stress at details attached by fillet or partial joint penetration groove welds, with or without transverse load on detail, when the detail embodies a

transition

radius,

R,

with

In weld termination extending into member

weld

termination ground smooth:

R > 2 in. (50 mm)

D

22 x 108

7

E

11 x108

4.5

1

"Attachment" as used herein, is defined as any steel detail welded to a member which, by its mere presence and independent of its loading, causes a discontinuity in the stress flow in the member and thus

reduces the fatigue resistance.

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179

APPENDICES

13-36 © 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT SECTION 6 - BASE METAL AT WELDED TRANSVERSE MEMBER CONNECTIONS (cont'd)

SECTION 7 - BASE METAL AT SHORT ATTACHMENTS

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180

APPENDICES 13-37

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters Description

Stress Category

Constant

Threshold

Potential Crack Initiation Point

10

At toe of weld in base metal

8

In throat of weld

SECTION 8 - MISCELLANEOUS 8.1 Base metal at stud-type shear connectors attached by fillet or electric stud welding.

C

8.2 Shear on throat of continuous or intermittent longitudinal or transverse fillet welds.

F

8.3 Base metal at plug or slot welds

E

11 x 108

8.4 Shear on plug or slot welds

F

50X10

44x108

50x10

10

(Formula A-K3.2)

10

4.5

At end of weld in base metal

8

At faying surface

7

At the root of the threads extending into the tensile stress area

(Formula A-F3.2) 8.5 Not fully-tightened high-strength bolts, common bolts, threaded anchor rods and hanger rods with cut, ground or rolled threads. Stress range on tensile stress area due to live load plus prying action when applicable.

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E'

3.9 x10 8

APPENDICES

181 13-38

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.

DRAFT TABLE A-K3.1 (Cont'd)

Fatigue Design Parameters

SECTION 8 - MISCELLANEOUS

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182

APPENDICES 13-39

© 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without permission of the publisher.