The research addresses the effect of diagonal and horizontal stiffeners on shear transfer in rigid frame square knees. Rigid frame square knees are an integral component of pre-manufactured metal building systems. This paper examined a more efficient design of the rigid frame square knee. Five full-scale laboratory tests on square knee joints were performed to verify the effects of a diagonal stiffener on shear transfer. Two frames were fabricated without diagonal stiffeners, and three frames were fabricated with 1/8- inch- thick diagonal stiffeners, not extending the full diagonal length of the knee web. Experimental results, coupled with a finite element analysis, are compared to AISC provisions in Section F4: shear yielding and shear buckling, and in Section G3: tension field action. This research shows that if diagonal stiffeners are needed, then thin, shortened diagonal stiffeners are sufficient to restrain shear buckling of the knee web until shear yielding occurs.

Plate girders are fabricated in situations where standard structural shapes do not possess the required strength necessary to carry applied loads. In many instances, plate girders are tapered so that the resistance to bending is proportional to the bending moment, creating cost effective, aesthetically pleasing structures. The AISC 2010 Specifications accurately predict the flexural capacity of tapered plate girders but recent research has suggested that the required shear strength is overly conservative. The researchers postulate that the required shear strength is overly conservative due to an effect known as modified shear that has been neglected from the AISC 2010 Specifications but has been suggested by several authors. This research investigates both analytically and experimentally, tapered member ultimate shear strength considering a “modified” and “unmodified” applied shear approach. A new design formula introduced by Lee et al. (2008) will be used in conjunction with the AISC 2010 Specification in making ultimate shear strength comparisons. A total of 12 specimens are tested to failure, ten tapered and two prismatic built-up plate girders.

AASHTO LRFD requires the use of longitudinal stiffeners in plate girder webs when the web slenderness D/tw is greater than 150. This practice is intended to limit the lateral flexing of the web plate during construction and at service conditions. AASHTO accounts for an increase in the web bend buckling resistance due to the presence of a longitudinal stiffener. However, when the theoretical bend buckling capacity of the stiffened web is exceeded under strength load conditions, the Specifications do not consider any contribution from the longitudinal stiffener to the girder resistance. That is, the AASHTO LRFD web bend buckling strength reduction factor Rb applied in these cases is based on an idealization of the web neglecting the longitudinal stiffener. This deficiency can have significant impact on girder resistance in regions of negative flexure. This research is aimed at evaluating the improvements that may be achieved by fully considering the contribution of web longitudinal stiffeners to the girder flexural resistance. Based on refined FE test simulations, this research establishes that minimum size longitudinal stiffeners, per current AASHTO LRFD requirements, contribute significantly to the post buckling flexural resistance of plate girders, and can bring as much as a 60% increase in the flexural strength of the girder. A simple cross-section Rb model is proposed that can be used to calculate the girder flexural resistance at the yield limit state. This model is developed based on test simulations of straight homogenous girders subjected to pure bending, and is tested extensively and validated for hybrid girders and other limit states. It is found that there is a substantial deviation between the AISC/AASHTO LTB resistance equations and common FE test simulations. Research is conducted to determine the appropriate parameters to use in FE test simulations. Recommended parameters are identified that provide a best fit to the mean of experimental data. Based on FE simulations on unstiffened girders using these recommended parameters, a modified LTB resistance equation is proposed. This equation, used in conjunction with the proposed Rb model also provides an improved handling of combined web buckling and LTB of longitudinally stiffened plate girders. It is observed that the noncompact web slenderness limit in the Specifications, which is an approximation based on nearly rigid edge conditions for the buckling of the web plate in flexure is optimistic for certain cross-sections with narrow flanges. This research establishes that the degree of restraint at the edges of the web depend largely on the relative areas of the adjoining flanges and the area of the web. An improved equation for the noncompact web slenderness limit is proposed which leads to a better understanding and representation of the behavior of these types of members.