Sherif El-Tawil

Progressive Collapse Resistance of Steel-Concrete Composite Floors

This paper discusses the progressive collapse resistance of steel-concrete composite floors in which steel beams are attached to columns through shear tabs. This is a common type of system used for the gravity bay portions of steel buildings. The study is conducted using computational simulation models validated through extensive comparisons to disparate test data. The models are used to investigate key parameters influencing the robustness of generic composite floors subjected to the removal of a center column. In particular, the effects of deck thickness, steel reinforcement, and the numbers of bolts in the shear tab connection on the behavior of the system are studied as a function of the loading scheme. The simulation results show that the majority of collapse resistance comes from the steel deck and that, for the system considered, increasing connection strength by adding more bolts might not be that beneficial in increasing overall collapse strength. The dynamic impact factor, which is widely used to account for dynamic effects within a static design framework, is also computed and the DIF value recommended in existing design guidelines is evaluated.

Approximations in Progressive Collapse Modeling

Assumptions must necessarily be made when the collapse response of structures is investigated using simulation models. The type and extent of modeling assumptions depend on the computational resources available, modeling expertise, and results sought. Modeling choices that are commonly made include planar versus three-dimensional (3D) representation, simplification of member response for modeling purposes, and the use of macroelements to mimic behavior instead of using elements that are based on fundamental constitutive relationships. Using four different types of models, this paper sheds light on the effect of some commonly employed approximations in collapse modeling. The models represent a 10-story seismically designed steel building and encompass computationally expedient planar and 3D macromodels as well as continuum models of individual frames and the full 3D structural system. After a validation exercise, the simulation models are exercised to investigate system collapse response when columns are f...

Loading Rate Effect on Pullout Behavior of Deformed Steel Fibers

Single-fiber pullout test results under loading rates ranging from seismic to static level are described in this paper. A basis for better understanding of the effect of strain rate on fiber-reinforced cement composite tensile properties is provided through loading rate effect investigation on single-fiber pullout behavior. There is evaluation of two high-strength deformed steel fiber types (twisted and hooked fibers) known, under static pullout loading, to have slip-hardening behavior. That twisted steel fiber pullout response shows rate sensitivity that is matrix compressive strength dependent is revealed by experimental results. Rate sensitivity under pullout for various matrixes tested was not shown, on the other hand, by high-strength hooked fibers. That twisted fiber pullout energy can be up to five times that of hooked fibers and increases with the compressive strength matrix was also shown in the test results.

Evaluation of ACI 318 and AISC (LRFD) strength provisions for composite beam-columns

Abstract The development of an interactive computer program for modeling biaxial bending of encased composite steel-concrete columns and its application to design are presented. Included is a description of an analytical procedure for modeling inelastic behavior based on the fiber element method. Results of fiber element analyses are used to evaluate nominal uni- and biaxial bending strengths of composite columns calculated according to the ACI-318 and AISC-LRFD specifications. Slender column behavior and the effect of residual stresses and built-in forces (resulting from the construction sequence) in the steel sections are included. In addition to providing insight into behavior and assessing current design provisions for composite columns, the implementation of the fiber element method demonstrates the feasibility of using inelastic simulation programs in design.

Seismic Design of Hybrid Coupled Wall Systems: State of the Art

Hybrid coupled walls (HCWs) are comprised of two or more reinforced concrete wall piers connected with steel coupling beams distributed over the height of the structure. Extensive research over the past several decades suggests that such systems are particularly well suited for use in regions of moderate to high seismic risk. This paper reviews the state of the art in seismic modeling, analysis, and design of HCW systems. Design methodologies are presented in both prescriptive and performance-based design formats and a discussion of alterative types of hybrid wall systems is provided.

Inelastic analyses of a 17-story steel framed building damaged during Northridge

A series of two- and three-dimensional static and dynamic inelastic frame analyses are performed for a 17-story steel moment frame building damaged by the 1994 Northridge earthquake. The primary objectives of the study are to: (1) exercise state-of-the-art inelastic static and dynamic analyses for the evaluation and design of steel buildings; (2) establish to what degree frame analyses can be used to predict the types of brittle connection damage that occurred during the Northridge earthquake; and (3) investigate the reliability of the analyses and the influence of modeling parameters on computed performance indices. In general, this study shows that calculated interstory drift ratios and curvature demands obtained from inelastic time history analyses correlate reasonably well with the pattern of connection damage observed in the building. However, there is significant scatter in the computed deformation demands that are strongly dependent on the degree to which three-dimensional torsion, secondary structural elements and strength/stiffness degradation (associated with connection fractures) are modeled in the analyses. Further, comparisons of static and dynamic analyses indicate that for this building static pushover analyses do not capture higher vibration modes that are significant.

Three-dimensional effects and collapse resistance mechanisms in steel frame buildings

AbstractThis study investigates the robustness of a seismically designed steel moment frame building using 3-D nonlinear models. Computational simulations are carried out using validated models to investigate building response to loss of internal and external columns as well as columns in lower, middle and upper floors. Studies are conducted to gain insight into the sources of collapse resistance of the prototype building, focusing in particular on the role of composite action between the slab and underlying steel beams and slab membrane action. The simulation results suggest that the prototype building can be more vulnerable to loss of columns in the upper stories than in the lower ones and is particularly vulnerable to loss of interior gravity columns at all floor levels. The role of the slab is quantified and it is shown that it can contribute significantly to the robustness of the structure, but it is a double edge sword that can also be detrimental, especially in the final stages of collapse.

Performance of Steel Moment Connections under a Column Removal Scenario. II: Analysis

AbstractThis paper presents a computational investigation of the response of steel beam-column assemblies with moment connections under monotonic loading conditions simulating a column removal scenario. Two beam-column assemblies are analyzed, which incorporate (1) welded unreinforced flange bolted web connections, and (2) reduced beam section connections. Detailed models of the assemblies are developed, which use highly refined solid and shell elements to represent nonlinear material behavior and fracture. Reduced models are also developed, which use a much smaller number of beam and spring elements and are intended for use in future studies to assess the vulnerability of complete structural systems to disproportionate collapse. The two modeling approaches are described, and computational results are compared with the results of the full-scale tests described in the companion paper. Good agreement is observed, demonstrating that both the detailed and reduced models are capable of capturing the predominan...

Inhibiting Steel Brace Buckling Using Carbon Fiber-Reinforced Polymers : Large-Scale Tests

A new method is proposed for inhibiting the buckling response of steel braces. The strengthening technique involves attaching a core comprised of mortar blocks to the braces and then wrapping the entire system with carbon fiber-reinforced polymer (CFRP) sheets. An advantage of this method is that the technology can be applied in situ by a single individual with minimal training. Test results of seven specimens subjected to reversed axial loading show that it is feasible to achieve buckling restrained response up to 2% interstory drift. Other than the number of longitudinal fiber layers, it is observed that performance of the strengthening scheme depends upon the size of the core material, the presence of bond between the steel plate and core material, the existence of extra stitch plates for double angle members, and the presence of transverse CFRP layers at the member ends. It is also observed that double angle braces benefit more from the strengthening scheme than single angle braces, which are prone to premature out-of-plane buckling.

High Performance Fiber Reinforced Cement Composites with Innovative Slip Hardending Twisted Steel Fibers

This paper provides a brief summary of the performance of an innovative slip hardening twisted steel fiber in comparison with other fibers including straight steel smooth fiber, high strength steel hooked fiber, SPECTRA (high molecular weight polyethylene) fiber and PVA fiber. First the pull-out of a single fiber is compared under static loading conditions, and slip ratesensitivity is evaluated. The unique large slip capacity of T-fiber during pullout is based on its untwisting fiber pullout mechanism, which leads to high equivalent bond strength and composites with high ductility. Due to this large slip capacity a smaller amount of T-fibers is needed to obtain strain hardening tensile behavior of fiber reinforced cementitious composites. Second, the performance of different composites using T-fibers and other fibers subjected to tensile and flexural loadings is described and compared. Third, strain rate effect on the behavior of composites reinforced with different types and amounts of fibers is presented to clarify the potential application of HPFRCC for seismic, impact and blast loadings.