NCHRP-20-07-244-FR.pdf
5-i 5 Modifications for AASHTO LRFD Bridge Design Specifications to Incorporate or Update the Guide Specifications for Design of Pedestrian Bridges Requested by: American Association of State Highway and Transportation Officials (AASHTO) Standing Committee on Highways Prepared by: Thomas P. Murphy, Ph.D., P.E., Associate and Modjeski and Masters, Inc John M. Kulicki, Ph.D., P.E., Chairman/CEO 4909 Louise Drive, Suite 201 Mechanicsburg, PA 17055 January 2009 The information contained in this report was prepared as part of NCHRP Project 20-07, Task 244, National Cooperative Highway Research Program, Transportation Research Board. LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 5-II Acknowledgements This study was requested by the American Association of State Highway and Transportation Officials (AASHTO), and conducted as part of National Cooperative Highway Research Program (NCHRP) Project 20-70. The NCHRP is supported by annual voluntary contributions from the state Departments of Transportation. Project 20-07 is intended to fund quick response studies on behalf of AASHTO Subcommittee on Bridges and Structures. The report was prepared by Thomas P. Murphy, Modjeski and Masters, Inc. The work was guided by a task group which included Dr. Lian Duan, Susan E. Hida, Thomas P. Macioce, Jiten Soneji, Jonathan VanHook, Kevin Western, and Dr. Bojidar Yanev. The project was managed by David Beal, NCHRP Senior Program Officer. Disclaimer The opinions and conclusions expressed or implied are those of the research agency that performed the research and are not necessarily those of the Transportation Research Board or its sponsors. This report has not been reviewed or accepted by the Transportation Research Board's Executive Committee or the Governing Board of the National Research Council. LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 5-III FINAL DRAFT LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES TABLE OF CONTENTS 5-iv 1GENERAL . 1 1.1SCOPE 1 1.2PROPRIETARY SYSTEMS. 1 1.3COLLISION MITIGATION . 1 2PHILOSOPHY . 2 3LOADS 2 3.1PEDESTRIAN LOADING (PL) . 2 3.2VEHICLE LOAD (LL) . 3 3.3EQUESTRIAN LOAD (LL) . 4 3.4WIND LOAD (WS) 4 3.5FATIGUE LOAD (LL) . 5 3.6APPLICATION OF LOADS 5 3.7COMBINATION OF LOADS 5 4FATIGUE 5 4.1RESISTANCE 5 4.2FRACTURE 6 5DEFLECTIONS . 6 6VIBRATIONS . 6 7STABILITY . 7 7.1HALF-THROUGH TRUSSES . 7 7.1.1Lateral Frame Design Force 7 7.1.2Top Chord Stability 7 7.1.3Alternative Analysis Procedures . 9 7.2STEEL TWIN I-GIRDER AND SINGLE TUB GIRDER SYSTEMS 10 7.2.1General 10 7.2.2Lateral Torsional Buckling Resistance - Twin I-Girder 10 7.2.3Lateral-Torsional Buckling Resistance-Singly Symmetric Sections 11 8TYPE SPECIFIC PROVISIONS . 11 8.1ARCHES . 11 8.2STEEL TUBULAR MEMBERS . 11 8.2.1General 11 8.2.2Detailing . 12 8.2.3Tubular Fracture Critical Members . 12 8.3FIBER REINFORCED POLYMER (FRP) MEMBERS . 13 LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 1 1GENERAL 1.1SCOPE These Guide Specifications address the design and construction of typical pedestrian bridges which are designed for, and intended to carry, primarily pedestrians, bicyclists, equestrian riders and light maintenance vehicles, but not designed and intended to carry typical highway traffic. Pedestrian bridges with cable supports or atypical structural systems are not specifically addressed. These Guide Specifications provide additional guidance on the design and construction of pedestrian bridges in supplement to that available in the AASHTO LRFD Bridge Design Specifications (AASHTO LRFD). Only those issues requiring additional or different treatment due to the nature of pedestrian bridges and their loadings are addressed. Aluminum and wood structures are adequately covered in AASHTO LRFD, and as such are not specifically addressed herein. Where two letter abbreviations are used in Article 3, they relate to the loads and load combinations given in Table 3.4.1-1 of AASHTO LRFD. Implementation of the wind loading and fatigue loading provisions require reference to the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals (AASHTO Signs). C1.1 This edition of the Guide Specifications was developed from the previous Allowable Stress Design (ASD) and Load Factor Design (LFD)-based, edition (AASHTO 1997). An evaluation of available foreign specifications covering pedestrian bridges, and failure investigation reports as well as research results related to the behavior and performance of pedestrian bridges was performed during the development of the LRFD Guide Specifications. 1.2PROPRIETARY SYSTEMS Where proprietary systems are used for a pedestrian bridge crossing, the engineer responsible for the design of the system shall submit sealed calculations prepared by a licensed Professional Engineer for that system. C1.2 It is important to clearly delineate the responsibilities of each party when proprietary bridge systems are used. All portions of the design must be supported by sealed calculations, whether from the bridge manufacturer, or the specifying engineer. The interface between the proprietary system and the project-specific substructures and foundations needs careful attention. 1.3COLLISION MITIGATION AASHTO LRFD Article 2.3.3.2 specifies an increased vertical clearance for pedestrian bridges 1.0 ft. higher than for highway bridges, in order to mitigate the risk from vehicle collisions with the superstructure. Should the owner desire additional mitigation, the following steps may be taken: Increasing vertical clearance in addition to that contained in AASHTO LRFD Providing structural continuity of the superstructure, either between spans or with the substructures C1.3 In most cases increasing vertical clearance is the most cost effective method of risk mitigation. LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 2 Increasing the mass of the superstructure Increasing the lateral resistance of the superstructure 2PHILOSOPHY Pedestrian bridges shall be designed for specified limit states to achieve the objectives of safety, serviceability and constructability, with due regard to issues of inspectability, economy, and aesthetics, as specified in the AASHTO LRFD. These Guide Specifications are based on the LRFD philosophy. Mixing provisions from specifications other than those referenced herein, even if LRFD based, should be avoided. 3LOADS 3.1PEDESTRIAN LOADING (PL) Pedestrian bridges shall be designed for a uniform pedestrian loading of 90 psf. This loading shall be patterned to produce the maximum load effects in accordance with AASHTO LRFD Article 3.4. Consideration of dynamic load allowance is not required with this loading. C3.1 The previous edition of these Guide Specifications used a base nominal loading of 85 psf, reducible to 65 psf based on influence area for the pedestrian load. With the LFD load factors, this results in factored loads of 2.17(85) = 184 psf and 2.17(65) = 141 psf. The Fourth Edition of AASHTO LRFD specified a constant 85 psf regardless of influence area. Multiplying by the load factor, this results in 1.75(85) = 149 psf. This falls within the range of the previous factored loading, albeit toward the lower end. European codes appear to start with a higher nominal load (approx 105 psf), but then allow reductions based on loaded length. Additionally, the load factor applied is 1.5, resulting in a maximum factored load of (1.5)105 = 158 psf. For a long loaded length, this load can be reduced to as low as 50 psf, resulting in a factored load of (1.5)50 = 75 psf. The effect of resistance factors has not been accounted for in the above discussion of the European codes. There are, however, warnings to the designer that a reduction in the load based on loaded length may not be appropriate for structures likely to see significant crowd loadings, such as bridges near stadiums. Consideration might be given to the maximum credible pedestrian loading. There is a physical limit on how much load can be applied to a bridge from the static weight of pedestrians. It appears that this load is around 150 psf, based on work done by Nowak (2000) from where Figures C1 through C3 were taken. Although there does not appear to be any available information relating to the probabilistic distribution of pedestrian live loading, knowing the maximum credible load helps to define the limits of the upper tail of the distribution of load. The use of a 90 psf nominal live load in combination with a load factor of 1.75 results in a loading of 158 psf, which provides a marginal, but sufficient, LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 3 reserve compared with the maximum credible load of 150 psf. Figure C3.1-1Live Load of 50 psf Figure C3.1-2Live Load of 100 psf Figure C3.1-3Live Load of 150 psf 3.2VEHICLE LOAD (LL) Where vehicular access is not prevented by fixed C3.2 The vehicle loading specified are equivalent to the LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 4 physical methods, pedestrian bridges shall be designed for a maintenance vehicle load specified in Figure 1 and Table 1 for the Strength I Load Combination unless otherwise specified by the Owner. A single truck shall be placed to produce the maximum load effects and shall not be placed in combinations with the pedestrian load. The dynamic load allowance need not be considered for this loading. Table 3.2-1Design Vehicle Clear Deck With Design Vehicle 7 to 10 feet H5 Over 10 feet H10 W = TOTAL WEIGHT OF TRUCK AND LOAD 14'-0“ 0.1W0.1W 0.1W0.1W 0.2W 0.8W Figure 3.2-1Maintenance Vehicle Configurations. H-trucks shown in Article 3.6.1.6 of AASHTO LRFD at the time of this writing (2009) and contained in previous versions of the AASHTO Standard Specifications for Highway Bridges. 3.3EQUESTRIAN LOAD (LL) Decks intended to carry equestrian loading shall be designed for a patch load of 1.00 kips over a square area measuring 4.0 inches on a side. C3.3 The equestrian load is a live load and intended to ensure adequate punching shear capacity of pedestrian bridge decks where horses are expected. The loading was derived from hoof pressure measurements reported in Roland et. al. (2005). The worst loading occurs during a canter where the loading on one hoof approaches 100% of the total weight of the horse. The total factored load of 1.75 kips is approximately the maximum credible weight of a draft horse. 3.4WIND LOAD (WS) Pedestrian bridges shall be designed for wind loads as specified in the AASHTO Signs, Articles 3.8 and 3.9. Unless otherwise directed by the Owner, the Wind Importance Factor, Ir , shall be taken as 1.15. The C3.4 The wind loading is taken from AASHTO Signs specification rather than from AASHTO LRFD due to the potentially flexible nature of pedestrian bridges, and also due to the potential for traffic signs to be mounted on LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 5 loading shall be applied over the exposed area in front elevation including enclosures. Wind load on signs supported by the pedestrian bridge shall be included. In addition to the wind load specified above, a vertical uplift line load as specified in AASHTO LRFD Article 3.8.2 and determined as the force caused by a pressure of 0.020 ksf over the full deck width, shall be applied concurrently. This loading shall be applied at the windward quarter point of the deck width. them. For porous wind enclosures, the wind pressure may be reduced but pressures less than 85% of the pressure on a solid enclosure are not recommended. 3.5FATIGUE LOAD (LL) The fatigue loading used for the fatigue and fracture limit state (Fatigue I) shall be as specified in Section 11 of the AASHTO Signs. The Natural Wind Gust specified in Article 11.7.3 and the Truck-Induced Gust specified in Article 11.7.4 of that specification need only be considered, as appropriate. C3.5 For vehicular bridges, wind loads are not part of the Fatigue I load combination. Note that since this article designates wind as a live load for pedestrian bridges, via the designation LL, this confirms that wind should be considered a fatigue live load. Neither the pedestrian live load nor the maintenance vehicle load used for strength and serviceability is appropriate as a fatigue design loading due to the very infrequent nature of this loading. The fatigue loading specified is consistent with the treatment of sign support structures. For bridges crossing roadways, the truck- induced gust loading should be considered. The other loadings specified in AASHTO Signs are not applicable to pedestrian bridges due to their decreased susceptibility to galloping or vortex shedding vibrations. 3.6APPLICATION OF LOADS When determining the pattern of pedestrian live loading which maximizes or minimizes the load effect on a given member, the least dimension of the loaded area shall be greater than or equal to 2.0 ft. C3.6 The dimension given is meant to represent a single line of pedestrians; any width less than this would not represent a practical loading scenario. 3.7COMBINATION OF LOADS The types of bridges identified in Article 1.1 shall be designed for the load combinations and load factors specified in AASHTO LRFD Table 3.4.1-1, with the following exceptions: Load combinations Strength II, Strength IV, and Strength V need not be considered. The load factor for the Fatigue I load combination shall be taken as 1.0, and the Fatigue II load combination need not be considered. C3.7 Load combination Strength II is meant for special permit trucks, which is not applicable to pedestrian bridges. Strength IV is for dead load dominant structures such as long span trusses, and would not likely apply to pedestrian bridges. Strength V addresses the case of strong wind combined with reduced live loading, which is not likely to occur for pedestrian bridges. For unusual cases where the excluded load combinations have a reasonable chance of occurring, they should be considered in the design. The fatigue loading specified in AASHTO Signs and referenced herein was calibrated for a load factor of 1.0 and the design condition of infinite life. 4FATIGUE 4.1RESISTANCE The fatigue resistance for steel components and details shall be as specified in the AASHTO LRFD, LRFD GUIDE SPECIFICATIONS FOR THE DESIGN OF PEDESTRIAN BRIDGES 6 Article 6.6.1.2.5 for the Fatigue I load combination. For those components and details not covered in AASHTO LRFD, the nominal fatigue resistance may be taken from Table 11.3 of AASHTO Signs or Figure 2.13 of AWS D1.1 Structural Welding Code Steel. The fatigue resistance for steel reinforcement in concrete structures shall be as specified in the AASHTO LRFD Article 5.5.3. 4.2FRACTURE Except as specified herein, all of the provisions specified in Article 6.6.2 of the AASHTO LRFD relating to Charpy V-notch (CVN) fracture toughness req
