Practice Periodical on Structural Design and Construction

Woodframe (light-frame wood) structures make up the vast majority of the United States building stock including those in areas of high seismicity. Their life cycle is unique in that when they are damaged, repair is often nonstructural in nature consisting of drywall repair, and possibly wall replacement. The main reason for this is that most woodframe structures are residential and life safety is typically implied through prescriptive means. Thus, one important question that has yet to be answered is: What is the effect of previous earthquakes on the performance or integrity of a woodframe structure during the next earthquake? Because the primary lateral force resisting system in western style woodframe construction is the wood shearwall, an attempt is made herein to answer this question, albeit in part, based on a series of assembly-level tests conducted at Colorado State University on (1) isolated wood shearwall assemblies; and (2) corner assemblies consisting of a shearwall and short transverse wall. Both types of assemblies were tested with and without drywall and subjected to a series of six earthquakes of increasing intensity. As expected, the effect of the drywall was quite significant, reducing the peak drift of the isolated shearwalls by 30% and the corner assemblies by over 50%. Interestingly, the corner assembly without drywall was most severely damaged by the repetitive simulated earthquakes. In contrast, the corner assembly with drywall suffered very little damage simply because of the added stiffness and strength, so peak displacements were below 1% wall height. Woodframe assemblies appear to perform quite well, only suffering moderate degradation, when subjected to subsequent earthquakes provided peak drifts are kept below 1.5%.

Precast segmental construction offers many advantages, such as construction economy, simplicity of construction, and reduced maintenance cost. Repeated loading due to moving vehicular loads on the bridges may soften the concrete in due course of time and cause early cracks and larger deflections. This loading in segmental bridges may also cause openings of the joints between the segments. This paper presents the results of an experimental study of a single cell precast posttensioned segmental box bridge system subjected to cyclic loading and temperature changes. The test bridge system is a 1:3.5 scale model of the Long Key Bridge in the Florida Keys. The behavior of the bridge model was examined under fatigue loading, with emphasis on joint performance and flexural stiffness. The temperature distribution of the bridge system was also measured, and compared with analytical predictions. The performance of the bridge model was evaluated in terms of deflections, strains in the concrete and strains across the joints, and cracking behavior of joints between the segments with an increasing number of cycles of fatigue loading. The studies indicated that the behavior of joints between the segments under static and fatigue loadings was satisfactory up to two million cycles. No joint openings or cracks were observed at the joints under the loading. The temperature distribution of the bridge model was also studied using finite-element analysis and the predicted temperatures across the cross section were compared with the observed values.

Temporary structures such as scaffolds, shelters, tents, and facilities used during the reconstruction or repair of buildings and bridges, etc., are usually constructed for a limited-time use. Although the design of such structures to dead and live loads usually does not impose any particular challenge, their design for potential seismic or wind load requires more careful investigation. This is due to the fact that the service life of a temporary structure is much shorter than a “permanent structure,” and as such, the probability of load exposure to the temporary structure is substantially less. Designing a temporary structure for the same level of seismic load exposure probability than that required for a permanent structure will not be economically viable. Seemingly, it will make sense to use a reduced design load—a level proportional to the intended service life of the structure. Using a reduced load may be a reasonable assumption for a structure that is used once. However, if the decision is to reuse the structure and to keep it for a longer time than was originally intended, a question arises as to whether, with the extended usage, the safety of the structure will be compromised. This paper provides a review of available studies on seismic and wind loads for temporary structures. Further, the use of a modified risk level, estimated based on the performance record of the structure in its initial usage cycle, is suggested. This modified risk can then be employed in making a decision on whether to allow the structure to be used longer. A simple illustrative example and several conclusions are then presented in an effort to open discussions among the structural engineering community to further elaborate on the issue of design loads for temporary structures.

The objective of this research was to use a three-dimensional (3D) analysis method to evaluate “system effects” in light-frame roof truss assemblies. The goal of this study was to develop an improved and practical design method for 3D roof truss assemblies used in residential construction. A truss plate manufacturer (TPM) design software was used to lay out assemblies and to design individual trusses. The TPM software used a conventional design procedure (CDP) by analyzing one truss at a time in two dimensions. A commercially available structural analysis program was utilized to model and analyze 3D truss assemblies as a system. This system design procedure (SDP) is being proposed as a tool to analyze and design 3D roof truss assemblies. Three truss assemblies, L shape, T shape, and a complex assembly, were analyzed. The structural responses including combined stress index (CSI), truss deflections, and reactions from both CDP and SDP were compared and the system effects were evaluated. From this investigation, it is concluded that there are three system effects observed by the SDP, but not accounted for by CDP. These are: reduced applied load effect, truss-to-truss support effect, and stiff truss effect. Based on this investigation, the maximum CSI for most trusses in all three assemblies reduced by 6–60% because of system effects. SDP can help to improve the design of truss assemblies by directly including system effects that are not accounted for by the CDP.

An overview of an earthquake resisting system (ERS) strategy for the design and retrofit of bridges in Illinois is presented in the paper. Initial development of this ERS strategy by the Illinois Dept. of Transportation (IDOT) began in mid-2005. It was initially implemented in late 2006 and revised in late 2007 to reflect increased design accelerations adopted into the American Association of State Highway and Transportation Official’s bridge design code and a new seismic bridge design guide specification in mid-2007. The primary objective of IDOT’s ERS strategy is to prevent span loss. This is achieved through three levels or tiers of seismic structural redundancy, which allow for certain levels of damage at planned locations in bridges. Controlled damage at strategic locations in structures allows for the mitigation of seismic energy. Sacrificial or fuse-like connections between superstructures and substructures, conservative beam support lengths, and plastic hinging at planned locations in substructures are the primary areas of focus for Levels 1–3 of seismic structural redundancy, respectively, in Illinois’ developed ERS strategy.