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Finite Element Modeling of Bridge Deck Connection Details Anthony H DePiero1 Robert K Paasch2 and Steven C Lovejoy3 Abstract Many steel bridges built prior to 1960 have bridge deck connections that are subject to high cycle fatigue These connections may be nearing their fatigue limit and will require increased inspection and repair over the next 10 20 years The Winchester Bridge on Interstate 5 in Roseburg Ore required the extensive replacement of connection details because of fatigue crack growth This report describes the results of a study to assess the loading conditions for the connection details on the Winchester Bridge Finite element modeling methods were used to characterize the structure on both a global and local level The global model provided the boundary conditions for the local model of the connection details The local model included the effects of rivet preload and friction Finite element analysis results were validated by hand calculation The analysis showed signifi cant variation in connection detail stress range depending on the detail s longitudinal and lateral location DOI 10 1061 ASCE 1084 0702 2002 7 4 229 CE Database keywords Finite element method Bridge decks Connections Bridges steel Introduction The Oregon Department of Transportation ODOT has 549 struc tures in its highway steel bridge inventory Some of these bridges were built prior to 1960 and have fl ooring system connection details that are nearing the end of their design life These struc tures will require increased inspection and repair over the next 10 30 years as a result of accumulated fatigue damage Bridges on major routes will require added attention since they can ex perience as many as 1 5 million signifi cant load cycles per year Some of these bridges have over 1 000 connection details making inspection and repair expensive To date fl ooring system connec tion details with fatigue cracks have been found in approximately 10 such structures As a result of a thorough inspection program these damaged connection details are identifi ed and repaired well before any critical load paths are lost The goal of this research was to accurately assess the loading conditions and stress ranges for the connection details of a spe cifi c bridge the Winchester Bridge on Interstate 5 north of Rose burg Ore crossing the North Umpqua River Using the analysis from one bridge there was an expectation that the procedure and to some degree the results could be applied to other bridges Problem Specifi cation The Winchester Bridge is a typical steel deck truss bridge in the inventory of ODOT The bridge was selected for this study be cause it has experienced high cycle fatigue problems in its fl oor ing system connection details The bridge has separate north and southbound structures that were constructed in 1953 and 1963 respectively The two structures are very similar in their construc tion Each structure is made of six 42 7 m 140 ft steel deck truss spans Fig 1 illustrates one span of the southbound superstructure with the reinforced concrete deck removed Each span is made up of a pair of steel trusses whose center lines are 6 1 m 20 ft apart Each pair of trusses supports nine laterally oriented fl oor beams that are 5 3 m 17 ft apart The sections between the fl oor beams are called panels The north bound structure has fi ve stringers in each panel running between the fl oor beams The southbound structure has seven stringers in each panel A 150 mm 6 in thick steel reinforced concrete deck lies on top and is supported by the fl oor beams and stringers The north and southbound structures have slightly different sized fl oor beams and stringers In the northbound structure the fl oor beams are W24376 and the stringers are W18350 wide fl ange steel beams In the southbound structure the fl oor beams are W27384 and the stringers are W18345 wide fl ange steel beams Fig 2 shows a typical connection detail between a fl oor beam and stringer The clip angles are connected to the stringers and fl oor beams using 22 mm 7 8 in diameter rivets Rivet holes are positioned 38 mm 1 5 in from the edges and spaced 75 mm 3 in apart on 1Lawrence Livermore National Laboratory Livermore CA 94550 2Oregon State University Corvallis OR 97331 3Oregon Department of Transportation Salem OR 97310 Note Discussion open until December 1 2002 Separate discussions must be submitted for individual papers To extend the closing date by one month a written request must be fi led with the ASCE Managing Editor The manuscript for this paper was submitted for review and pos sible publication on May 18 2000 approved on January 9 2001 This paper is part of the Journal of Bridge Engineering Vol 7 No 4 July 1 2002 ASCE ISSN 1084 0702 2002 4 229 235 8 001 50 per page Fig 1 Diagram of one span of southbound structure of Winchester Bridge without 6 in concrete deck JOURNAL OF BRIDGE ENGINEERING JULY AUGUST 2002 229 Downloaded 07 Nov 2010 to 113 240 233 8 Redistribution subject to ASCE license or copyright Visithttp www ascelibrary org center The clip angle s primary function is to transmit the shear from the stringer to the fl oor beam Since the angles are riveted to both the stringer and the fl oor beam they are subjected to fl exural stresses caused by the vertical defl ection of the stringer under wheel loads As the stringer de fl ects the rotation of the end of the stringer subjects the connec tion detail to a fl exural moment This fl exural moment causes the angle to distort which can lead to fatigue cracking Fatigue cracks as long as 100 mm 4 in have been found in some clip angles that connect the stringers to the fl oor beams on the Winchester Bridge The fatigue cracks were typically found in the clip angles connecting the stringers to the fl oor beams at the ends of the spans although some were found in interior clip angles The cracks were located at the corner of the clip angle running vertically from the top of the clip angle down The frac ture surface of the cracks was usually oriented at about a 45 angle to the legs of the clip angle Fig 3 illustrates a clip angle with a typical fatigue crack In 1994 repairs were conducted on both the north and south bound structures of the Winchester Bridge Thirteen cracked clip angles were replaced on the southbound structure at a total cost of 16 384 Similar work was performed on the northbound struc ture at a cost of 16 296 Previous Studies The problem of bridge fl ooring system fatigue has been under study at least as far back as the 1930s Several prior studies proved useful in guiding the work performed in this study and are noted below The connection angles examined in a railway bridge connec tion angle study performed by Wilson of the University of Illinois were very similar to the clip angles used on the Win chester Bridge Wilson 1940 In Colorado a fi nite element analysis and fi eld testing were performed on a bridge over the South Platte River near Com merce City Cao et al 1996 TheNationalCooperativeHighwayResearchProgram NCHRP Report No 299 Fatigue Evaluation Procedures for Steel Bridges contains comprehensive fatigue evaluation pro cedures developed to guide the fatigue evaluation of existing bridges Moses et al 1987 The Alabama Department of Transportation sponsored a study on the fatigue of diaphragm girder connections that incorpo rated both analytical and experimental research on bridge fl ooring connection details Stallings et al 1996 Loading Analysis Two analysis methods were used to calculate the distribution of live truck loads on the stringers The fi rst method called the stringer loading analysis is a linear elastic analysis hand cal culation The second method called the global fi nite element analysis FEA was performed using the fi nite element method A model validation analysis of the global FEA model is also discussed For both analysis methods the suggested standard fatigue truck outlined in the NCHRP Report No 299 was used for model loading Moses et al 1987 This truck was developed to represent the variety of different types and weights of trucks in actual traffi c It consists of two rear axles of 10 9 Mg 24 kip each and a front axle of 2 7 Mg 6 kip The rear axles are spaced 9 1 m 30 ft apart while the front and the fi rst rear axle are spaced 4 3 m 14 ft apart The width of each axle is 1 8 m 6 ft Stringer Loading Analysis The distribution of the truck loads through the deck onto the stringers is important in determining the loading on the clip angle The loads on each stringer were calculated with one rear axle of the fatigue truck positioned longitudinally in the center of a panel over the midlength of the stringers Laterally the axle was cen tered in the slow lane of traffi c For both the north and south bound structures three stringers were assumed to carry the entire weight of the axle Those stringers were the centerline stringer Fig 2 Typical stringer to fl oor beam connection detail assembly Fig 3 Clip angle with typical fatigue crack Fig 4 Location of strain gauges installed on three stringers and two fl oor beams on northbound structure of Winchester Bridge 230 JOURNAL OF BRIDGE ENGINEERING JULY AUGUST 2002 Downloaded 07 Nov 2010 to 113 240 233 8 Redistribution subject to ASCE license or copyright Visithttp www ascelibrary org the second from the centerline stringer and the third from the centerline stringer in the slow lane Fig 4 shows the location of the three stringers Each section of the deck between the three stringers was ana lyzed as an independent beam using beam tables Shigley and Mischke 1989 The stringer loads were calculated as the reaction forces at the ends of the beams Global Finite Element Analysis Model Finite element models for both the north and southbound struc tures were developed to determine the distribution of live loads on the stringers The fl oor beams stringers clip angles and the reinforced concrete deck of one panel were included in the model Three dimensional 3D beam elements were used to model the fl oor beams and stringers Orthotropic plate elements were used to model the reinforced concrete deck The properties of the orthotropic plate elements were determined by performing an analysis of the reinforced concrete deck Discussion of this analy sis is found in the section Steel Reinforced Concrete Deck Analysis Beam elements with a length of 2 5 mm 0 1 in were used to model the boundary conditions created by the clip angles and fl oor beams Since the boundary beam elements modeled the compliance of the fl oor beams the longitudinal rotation of the fl oor beams was fi xed The area moment of inertia of the bound ary beam elements was set so that the rotation at the end of the stringer beam elements matched the rotation of the clip angle from the clip angle defl ection analysis When results became available from the 3D FEA model see Results Section the properties of the boundary beam elements were adjusted Two boundary beam elements were developed from the results of the 3D FEA model One modeled the connection details in the inte rior of the span and the other modeled the connection details at the end of the span Models of an end panel and an interior panel were developed for the north and southbound structures One axle of the standard fatigue truck was used to load the models The primary interest was in the distribution of loads on the stringers It was observed that the properties of the boundary beam elements the area mo ment of inertia of the stringers and the longitudinal position of the axle did not play a signifi cant role in the loading of the string ers Individual loading on the stringers was found to be strongly dependent upon both the lateral position and the width of the load axle This fi nding indicates that detailed knowledge about the position of the stringers in relationship to the lanes of traffi c is important It also demonstrates the necessity of having a fatigue truck that accurately represents the actual characteristics of trucks The stringer loads calculated from the global FEA model are presented below Steel Reinforced Concrete Deck Analysis A 6 in thick steel reinforced concrete deck transmits the live load to the stringers and fl oor beams An analysis was performed to quantify the equivalent stiffness of the concrete deck During con struction steel rebar was placed in longitudinal and transverse directions The position and amount of rebar in each direction was different For this reason it was necessary to quantify the rein forced concrete deck stiffness properties in each direction The orthotropic properties of the deck were calculated by fol lowing the procedure outlined in Reinforced Concrete Design Everard and Tanner 1966 Area moments of inertia per unit width were calculated for the transverse and longitudinal direc tions The area moments of inertia were then used to fi nd equiva lent moduli of elasticity for a 152 mm 6 in thick uniform deck The resulting moduli of elasticity for the transverse and longitu dinal directions were 8 964 MPa 1 300 ksi and 3 765 MPa 546 ksi respectively Model Validation To quantify the live loading and to assist in validating the analy sis fi eld testing was performed on the Winchester Bridge Five strain gauges were installed on the top surfaces of the bottom fl anges at midspan on three stringers and on two fl oor beams of one span of the northbound structure Strain gauges were installed on the fi rst and second fl oor beams of the fi rst span Two stringers from the fi rst panel and one stringer from the second panel were fi tted with strain gauges Fig 4 shows the strain gauge location in relation to the stringers and fl oor beams Data were collected under normal traffi c fl ow conditions with both lanes open and under a known truck weight with the slow lane restricted to the test truck Table 1 shows the comparison of the measured stress ranges in the stringers to those calculated from the global FEA model for both the test truck weight and for random truck traffi c The root mean cubes of the measured stress ranges for the random truck traffi c are compared to the stress ranges calculated in the global FEA model loaded with the standard fatigue truck The measured stresses are much lower than those calculated from the global FEA model This can be partially explained by composite interaction between the deck and the stringers not modeled in the global FEA model If shear loads are transferred between the deck and the stringers the neutral axis is shifted upward and the area moment of inertia is increased This in creases the section modulus for the stringer resulting in a lower stress range The composite interaction between the deck and the stringers could be quantifi ed if strain data were available for the top and bottom fl anges The ratio of strain ranges could be used to calcu late the position of the neutral axis and the known load and the strain range of the bottom fl ange could be used to calculate the section modulus The effective area moment of inertia could be calculated from the new position of the neutral axis and the new section modulus Another possible reason for the difference in calculated and measured stress ranges is that the actual reinforced concrete deck is stiffer than calculated A stiffer deck would increase the distri bution of the axle load to other stringers Results of Global Finite Element Analysis Model results were analyzed to calculate the total vertical load on each stringer A signifi cant vertical load was considered to be one Table 1 Stringer Stress Ranges MPa from Global Finite Element Analysis Model Compared with Those Measured Experimentally for Known Truck Weight and Random Truck Traffi c Known truck weight Random truck traffi c ModelExperimentModelExperiment First panel centerline stringer24 913 319 810 5 First panel second stringer50 933 139 115 5 Second panel centerline stringer21 017 117 310 6 JOURNAL OF BRIDGE ENGINEERING JULY AUGUST 2002 231 Downloaded 07 Nov 2010 to 113 240 233 8 Redistribution subject to ASCE license or copyright Visithttp www ascelibrary org greater than 1 360 kg 3 000 lb based on previous hand calcula tions Two stringers in each panel of the northbound structure were loaded signifi cantly by this criterion They included the loads on the centerline stringer and the second from centerline stringer in the slow lane side Three stringers in each panel of the southbound structure were found to be loaded signifi cantly They included the centerline stringer second from centerline stringer and third from centerline stringer on the slow lane side Table 2 shows the stringer loads for both the northbound and southbound structures The results show that the two methods are in reasonable agree ment This is noteworthy because for the stringer loading analysis it was assumed that three stringers carry the entire axle load These results suggest that this assumption is reasonable for a 150 mm 6 in reinforced concrete deck Clip Angle Defl ection and Stress Analysis The clip angle creates a unique boundary condition for the stringer The compliance of the clip angle connection is between that of an ideal fi xed and an ideal pinned connection When the stringer is loaded there is a resulting end reaction moment M0 between the clip angle and stringer The clip angle defl ection dm the stringer end rotation ust and the level of stress in the clip angle are dependent upon M0 Since only live loading was con sidered the maximum level of stress in the clip angle translates to a stress range Three analysis techniques were used to investigate these relationships two of which are discussed in the following sections A closed form analysis was performed to bracket the FEA results but is not presented in this paper Two Dimensional Finite Element Analysis Model of Clip Angle A 2D FEA model of the top section of the clip angle was devel oped to determine the defl ections and stress ranges in the clip angles Plane stress plate elements of unit depth were used