Journal of Structural Engineering

This study revisits the anchoring mechanism believed to be the source of the postbuckling strength of plate girder web panels. In classical tension field theories, it has been implicitly assumed that a tension field cannot develop in the web panel without recourse to the anchoring mechanism of the flanges and/or adjacent panels. The contribution of the flange anchoring mechanism in practical plate girders differs from theory to theory. It has been found during this study that the anchoring mechanism hardly develops, no matter how heavy the flanges are, without transverse stiffeners that have sufficient axial stiffness necessary to keep the flanges from moving the web panel inwards during the anchoring action. In practical plate girders, the contribution of the flange anchoring mechanism to the postbuckling strength is negligibly small not because the flanges are too flexible to function as anchors but because the transverse stiffeners are axially too flexible to support the flanges. Taking advantage of the flange anchoring mechanism in practical designs is beyond the realm of possibility because it requires an inconceivably high axial stiffness of the transverse stiffeners.

This study brings light to the source of plastic hinge-like failure modes often observed in tests. It is found that such failure modes, accompanied by severe flange deformations, are not due to the anchoring action of the flanges, but are due to direct shear force acting on the flange cross sections. This study also examines why the horizontal anchoring mechanism cannot develop in interior panels or in end panels with a heavy end stiffener. The results obtained in this study, along with the findings reported in the companion paper (Part I), reveal that the anchoring mechanism is virtually nonexistent in normal plate girders. That is to say, the postbuckling strength of the web panel in normal plate girders is mostly attributable to the postbuckling mechanism developed by means of lateral supports provided by the flanges and the transverse stiffeners. It is also found that a tension field can develop in the end panel and it is recommended, therefore, that any restriction contrary to this observation found in current design codes be repealed.

Results from 18 large-scale tests of steel bracing members are presented to examine their inelastic buckling and fracture behavior as related to the seismic design of concentrically braced frames. The brace specimens include square hollow structural shapes (HSS), pipe, and wide-flange sections. The effect of various parameters, including width–thickness and slenderness ratios, cross-section shape, loading history, loading rate, and grout fill on the performance of these braces is investigated. Among these parameters, loading history, width–thickness ratio and slenderness ratio are shown to have the largest influence on brace ductility. The test data suggest that for some HSS and pipe specimens, current seismic design provision limits on maximum width–thickness ratios may not provide sufficient ductility for seismic design. Effects of loading rate are found to be insignificant, and the grout fill is shown to provide a modest improvement in cyclic ductility. Measurements of brace stiffness, tensile strength and compressive strength compare well with design formulas. Future analytical studies to simulate brace buckling and fracture are outlined as a way to generalize the findings of the physical tests.

In ductile braced frames, column seismic demands depend on the strength of ductile elements and yielding patterns that occur during earthquake loading. Column demands were investigated in buckling restrained braced frames (BRBFs), special concentrically braced frames (SCBFs), and eccentrically braced frames (EBFs) with different heights (3, 9, and 18 story) and strength levels using nonlinear time history analysis. For columns at the base of 9- and 18-story BRBFs and EBFs, axial demands observed from the analyses were 55–70% of demands commonly used in design, indicating potential cost savings on columns, anchor rods, base plates, and foundations in tall buildings; however, in low-rise SCBFs with braces in the 2-story X-configuration, column axial demands were up to 100% greater than those commonly used in design because of force redistribution that occurs after brace buckling. Column rotations in all frames were less than $0.03\mathrm{rad}$ . These rotation demands are lower than rotation capacities that have been demonstrated in other work.

The influence of residual stresses on structural members is to cause premature yielding and loss of stiffness, often leading to deterioration of load carrying capacity. Knowledge of their magnitude and distribution is therefore important for both structural design and finite-element simulations, and hence extensive studies have been performed on structural carbon steel components. With greater emphasis now being placed on durability and reducing consumption of resources, the use of stainless steel in construction is growing, heralding the need for a more precise understanding of its structural response. Stainless steel exhibits differing physical and thermal properties from carbon steel, both of which influence the formation of residual stresses, and it cannot simply be assumed that residual stress models for carbon steel are also appropriate for stainless steel. This paper examines all existing data on residual stresses in stainless steel sections, including data generated from a recent experimental program carried out at Imperial College, London and summarized herein. The collated residual stress data have been used to develop models for predicting the magnitude and distribution of residual stresses in press braked, cold rolled, hot rolled, and fabricated stainless steel structural sections.

The Alfred Zampa Memorial Bridge (AZMB), a newly built long-span suspension bridge, is located $32\mathrm{km}$ northeast of San Francisco on interstate Highway I-80. A set of dynamic field tests were conducted on the AZMB in November 2003, just before the bridge opening to traffic. These tests provided a unique opportunity to identify the modal properties of the bridge in its as-built condition with no previous traffic loads or seismic excitation. A benchmark study on modal identification of the AZMB is performed using three different state-of-the-art system identification algorithms based on ambient, as well as forced vibration measurements. These system identification methods consist of: (1) the multiple-reference natural excitation technique combined with the eigensystem realization algorithm; (2) the data-driven stochastic subspace identification method; and (3) the enhanced frequency domain decomposition method. Overall, the modal parameters identified using these system identification methods are found to be in very good agreement for each type of tests (ambient and forced vibration tests). For most vibration modes, the natural frequencies and mode shapes identified using the two different types of test data also match very well. However, the modal damping ratios identified from forced vibration test data are, in general, higher than those estimated from ambient vibration data. The identified natural frequencies and mode shapes are finally compared with their analytical counterparts from a three-dimensional finite-element model of the AZMB. The modal properties of the AZMB presented in this paper can be used as baseline in future health monitoring studies of this bridge.

The East Pagoda of Yongzuo Monastery (Twin Pagodas Monastery), built in Taiyuan city during the Chinese Wanli Period (1573–1619 AC) of the Ming Dynasty, has suffered serious movement and damage. A series of maintenance and refurbishment projects, including foundation reinstatement (Phase I), foundation strengthening (Phase II), structural reinforcement and interior/exterior repairs (Phase III), were completed between 1992 and 1997. The dynamic characteristics of the East Pagoda were measured consistently before Phase I, after Phase II, and after Phase III. These measurements were used to obtain the change of the stiffness of each story of the pagoda in order to assess the effects of each of the phases of work on the condition of the pagoda. It has been shown that this vibration-based inspection method offers an easy and useful means to identify the weakness of a structure and assess the effects of strengthening and repairs of a historic building.

Sectional shear models for slender reinforced concrete members developed by several research groups differ in the treatment of longitudinal reinforcement details when evaluating one-way shear capacity. Some models relate the shear capacity directly to the geometric reinforcement ratio while others consider the influence of reinforcement strain or stress. Further, only some models also consider the influence of member depth. The results from six new experimental tests on slender shear-critical members of varying depth, reinforcement ratio, and reinforcement strain at the time of failure are presented and compared with published data. The one-way shear stress at failure was accurately predicted using a recent modified compression field theory based capacity model that incorporates a “size effect” factor related to the member depth, and a “strain effect” factor related to the longitudinal reinforcement demands. Design implications for shear associated with the flexural reinforcement design strength are identified.

The addition of adhesives to resist uplift wind forces at the roof framing-to-sheathing connection was tested. Two specimen configurations were used to compare the tensile resistance of typical nail connections to that of connections that also included acrylic foam tape or construction adhesive. A set of small specimens had a single nail connecting the sheathing to the framing, and a set of larger specimens had three nails. Connections were tested using monotonic deflection rates and a cyclic loading protocol that was a modified version of the CUREE protocol. Specimens were tested at several monotonic deflection rates. In general, the faster deflection rates increased the connection capacity. The results demonstrated that the specimens constructed with the addition of adhesives provided an improvement over the ones with only a nail. The addition of construction adhesive resulted in the highest resistances for monotonic tests, whereas the addition of adhesive tape provided the most strength in the cyclic tests.

The load combinations in ASCE Standard 7 for load and resistance factor design (LRFD) and for allowable stress design (ASD) differ in their treatment of counteracting loads, where the permanent gravity load (dead load) stabilizes the structural system against the effect of uplift or lateral loads. In LRFD, the loads are factored prior to the analysis, and the load factor on dead load is less than unity. In ASD, one factor of safety (the allowable stress factor) had been applied to the difference between the nominal load effects through the 1995 edition of ASCE Standard 7. Beginning with the 1998 edition, the load factor on nominal dead load in the ASD combination was increased to 0.6 to achieve consistency between ASD and LRFD. This particular ASD load requirement has been controversial and has drawn criticism from certain segments of the structural engineering community. This technical note examines the treatment of counteracting loads in ASCE Standard 7 from a reliability viewpoint, and shows that failure to reduce the gravity load for design may lead to inadequate safety.