A number of studies have been carried on to assess the impact of LRFD Specifications on bridge design. A study by Shahawy et al. indicates that from shear considerations AASHTO Standard specifications (1989) is superior to LRFD (1994). Detailed studies by Zokaie et al. (2003) and Richard et al. (2002) suggests that LRFD design is more conservative and requires higher prestress or reinforcement as compared to the design by Standard Specifications, due to various factors as described by them.
2.7.2 Shear Design of Prestressed Concrete Girders
Shahawy et al. (1996) compared the shear provisions in the AASHTO Standard Specifications (1989) and LRFD Specifications using laboratory tests on AASHTO Type II prestressed concrete girders. The AASHTO Standard Specifications are based on constant 45-degree truss analogy for shear, whereas LRFD adopts variable truss analogy based on modified compression field theory for its shear provisions. As a part of laboratory testing 20 full-scale prestressed concrete girders were used with variable span, amount of shear reinforcement, shear span and strand diameter. Three of the girders were tested without any shear reinforcement in order to figure out the contribution of concrete to shear strength, Vc.
Shahawy et al. (1996) found that the AASHTO Standard Specifications gives a good estimate of the shear strength of the girders and is conservative regardless of the shear reinforcement ratio, whereas the LRFD Specifications overestimates the shear strength of girders having high reinforcement ratios. The shear provisions of AASHTO Standard Specifications were found to agree with the test results in almost all the cases,
whereas for a/d ratios less than 1.5, LRFD (1994) overestimates the shear strength and for a/d more than 2.0 LRFD underestimates the shear strength. The predictions of AASHTO Standard Specifications for Vc are also found to be better than that of LRFD,
both being conservative as compared to test results. The overall results for shear indicate the superiority of AASHTO Standard Specifications (1989) over LRFD Specifications (1994).
2.7.3 AASHTO Type III Girder Bridge
Richard et al. (2002) compared the design of AASHTO Type III Girder Bridge using the AASHTO Standard Specifications for Bridges, 16th Edition, and the AASHTO LRFD Bridge Design Specifications. The authors found the bridge design to be same in most respects irrespective of the Specifications used. The most significant changes observed by them were in the shear design where the skew factor and reinforcement requirements in LRFD Specifications required increased concrete strength and reinforcement. An increase in reinforcement in deck overhang and in wing wall was also observed by the authors, due to increased collision force. The design of bridges using LRFD specifications was found to be more calculation-intensive and complex. The design experience and conclusions were limited to a single span AASHTO Type III girder bridges.
The LRFD Specifications allows the distribution of permanent loads of and on the deck to be distributed uniformly among the beams and/or stringers (LRFD Specifications Article 4.6.2.2.1) which is a significant change from the Standard Specifications design practice where the dead loads due to parapets, sidewalks, railings were applied only to the exterior girder. An increase in non-composite dead load by 9% and decrease in composite dead load by 50% on the exterior girder, while a decrease in non-composite dead load by 4% and an increase in composite dead load by 97% on the interior girder were observed when LRFD Specifications is followed, as compared to the design by Standard Specifications (Richard et al. 2002). The Standard Specifications required the bridge to be designed for HS-25 loading, which is 125% of the AASHTO
HS-20 truck load or a design lane load comprising of 800 plf distributed load plus 22.5 kip or 32.5 kip point load for flexure or shear design cases, respectively. The LRFD Specifications adopts HL-93 load case for bridge design, which consists of a 36 ton design truck or design tandem and a 640 plf design lane load. The shear and bending moment after load distribution for both load cases were found to be roughly comparable. Richard et al. (2002) found that LRFD design requires same number of prestressing strands as that of standard Specifications design but a higher concrete strength was required which could be explained as an effect of changes in live loads, load distribution factors, impact factors, skew factors and prestressing losses. The LRFD design effected the shear design significantly as the requirement of shear reinforcement went up substantially which was a result of increase in live load distribution factor for shear and a constant skew factor. LRFD design of the overhang is significantly different from that of standard design and it requires more reinforcement.
2.7.4 Post-Tensioned Girder Bridges
Zokaie et al. (2003) reviewed the impact of LRFD specifications on the design post tensioned concrete box girder bridges and highlighted the changes in the Specifications which lead to the requirement of higher post tensioning. Although the present study deals with prestressed concrete girder bridges, it may be of interest to find out the cause of the requirement of higher post tensioning which also may cause an increase in required prestress force.
The change in design live load was found to be one of the factors. The “Dual Truck” loading in LRFD Specifications increases the negative moment at interior supports which require additional negative reinforcement. The major changes in the load distribution factors were another factor which influenced the design. The load factors for different limit states are different in LRFD Specifications as compared to the fixed load factors in Standard Specifications however the allowable stresses are almost same in both the Specifications. The prestress loss equations are slightly changed in the LRFD Specifications and are more conservative as compared to Standard ones. Zokaie et al.
(2003) carried on detailed design for two different cases and found that self weight is nearly the same irrespective of the Specifications used. The live load response in LRFD case was much higher than LFD. The impact factor was higher but the load distribution factor for moment went down for LRFD design case. Service limit state-III which checks the tensile stresses in bottom fiber governed in both the cases and required 13% additional post tensioning for LRFD based design. Zokaie et al. (2003) did not considered shear in their design.