بررسی عرض موثر شاه‌تیرهای مرکب فولادی-بتنی

Steel-concrete composite beams have been widely used in building and bridge construction since about a century ago. Construction with composite system provides some advantages including: reduction in the weight of steel (saving of 20 to 30%), increasing floor stiffness and increasing span length.In order to analyze such composite beams using the ordinary beam theory, the concept of effective width is used instead of their actual width. Effective width is one of the most influencing factors which plays an important role in determining the ultimate load bearing capacity of steel-concrete composite beams. There are a number of influencing parameters which affect the value of composite beams’ effective width including the ratio between the half width of the slab and the beam length (S/L), the number and rigidity of the shear connectors, the steel section yield strength and the concerte slab strength. There are numerous studies in the literature which have been conducted on the effective width of composite I-beams. However, the steel-concrete composite girders are generally analyzed and designed in current practice (on the safe side) as non-composite steel girders without taking into account the effect of concrete slab, yet. This procedure may lead to an uneconomic design. Although such girders have been widely used in the construction of residential steel building frames, however, their effective with has not been investigated, yet. This paper develops a three dimensional finite-element model using ABAQUS for the elastic and inelastic nonlinear static analysis of steel-concret copmposite girders and uses it to investigate the real effective width of concret slab in these kinds of girders under two types of loadings (a concentrated point load and a uniform distributed load) on their moment gradient factor in different behavioral zones. The concrete slab and steel section were modeled using the three-dimensional eight-node solid element (C3D8). However, the reinforcements were modeled using the Truss one-dimensional elements. To simulate the shear connectors implicitly, the concrete slab and steel girder were joined together with the Tie command available in ABAQUS. To achieve this end, some simply-supported with differet ratios of S/L, and symmetric/unsymmetric adjacent concrete panels were designed first taking into account the effects of secondary perpendicular composite I-beams. Then, considering the actual distribution curve of axial elastic compressive stress within the concrete slab in each model and estimating the area under the curves, their effective width was calculated in different locations along the girders lengths. Furthermore, in order to investigate the effects of concrete slab cracking and damage on the effective width of such composite girders, some of the above mentioned girders were analyzed nonlinearly and their effective widths were recalculated. Comparison of the calculated effective width values with the corresponding values proposed in the Iranian National Building Codes (INBC- Part 10) showed that such composite girders are capable of providing the effective width given in INBC. This indicates that despite the effects of perpendicular composite I-beams as well as bi-directional action of concret slab, the investigated steel-concrete composite girders can be analyzed and designed as common composite beams.