Journal of Structural Engineering

A new proposal for describing the hysteretic stress–strain behavior of high-strength concrete (HSC) under reversed cyclic loading is presented. The envelope curve is derived from the results of uniaxial, monotonic, compression loading tests of 108 large-scale specimens. It explicitly accounts for the effects of concrete compressive strength, volumetric ratio of transverse reinforcement, yield strength of ties, tie spacing, and tie pattern. The proposed model is implemented in the finite element program ADAPTIC, with a view to analyzing HSC members under reversed cyclic loading. This analysis accounts for energy dissipation through hysteretic behavior, stiffness degradation as damage progresses, and degree of confinement. Comparisons with test results showed that the proposed model provides a good fit to a wide range of experimentally established hysteresis loops. This model also keeps the computing costs of nonlinear analysis within acceptable limits.

A model that calculates both short- and long-term deflections (i.e., transverse deflections and section rotations) in reinforced, prestressed (or posttensioned) and composite concrete beams with generalized end conditions subjected to bending about any transverse axis proposed. The effects of creep, shrinkage, and tension stiffening in the concrete and flexural rotational restraints at the beam extremes are included in the proposed model. The model is limited to beams in which torsional moments or deformations along the member are not induced by the externally applied loads or by the prestressing or posttensioning. The moment–curvature $(M–\varphi )$ diagrams are first determined at each section in order to calculate both the transverse deflections and section rotations. The concrete stress–strain curve is modeled by: (1) a hyperbola for high-strength concrete (HSC) for the zone under compression and (2) a straight line combined with a hyperbola for the zone under tension. The stress–strain curves of the reinforcements are modeled using a modified Ramberg-Osgood function for the prestressed steel (bonded or unbonded) and a multilinear function for regular rebars and composites (like fiber-reinforced polymers (FRP) and carbon fiber-reinforced polymers (CFRP)). Five examples are included that show the effectiveness of the proposed method to predict the load–deflection behavior of reinforced, prestressed (or posttensioned), composite HSC beams, and steel-concrete composite beams.

To fully exploit the structural potential of prestressed masonry in flexure, it is essential to know its postcracking behavior. For this, a nonlinear finite element model has been developed to predict the complete response of prestressed masonry structural elements subjected to a monotonically increasing load. The present model incorporates the effects of geometric as well as material nonlinearity caused by nonlinear stress-strain relationship, cracking and crushing of masonry, yielding of prestressing steel, and predicts deflections, stresses and strains at different points in masonry and in steel. The model is suitable for predicting behavior of both bonded and unbonded in-plane prestressed masonry structures. The masonry super element has been employed to make such an analysis economically viable for prediction of the behavior at macro level. For the purpose of validation of the proposed model, the experimental test results of prestressed masonry beams and slab specimens have been used. The theoretically predicted results using the proposed model and the reported test results are found to be in good agreement both in pre- and postcracking regimes.

To reduce the floor-to-floor height, the technical, economical, and convenient (TEC) beam is developed as a new composite beam composed of structural tee, horizontal top stud, precast concrete, stirrup, and cast-in-place concrete slab. The preliminary testing of the proposed system was performed for simple beams, and the results indicated good structural behavior. However, application of the system in the relevant field requires the development of a moment-resisting connection using steel brackets on which upper rebars of the TEC beam are anchored. In this paper, three types of the proposed system are experimentally investigated under cyclic loadings. The parameters of the test are the spacing of transverse bars in the slab and the ratio of width of rebar spacing to bracket length. The structural behaviors such as load-displacement relation, ductility, variation of neutral axis, and failure pattern, were explored. Based on test results, specimens were classified as semirigid full strength by the Eurocode 4. It could be concluded that the proposed moment-resisting system shows good structural behavior and may be applicable in the field.

This paper investigates the behavior of simply supported prestressed steel–concrete composite beams through experimental and analytical studies. Eight specimens were instrumented and tested in the experimental program considering different parameters. The slip effect between the steel and concrete interface and the increase of the prestressing tendon force with the increase of loading were accounted for in the analytical model. A reduced stiffness method was then proposed for calculating the deflection of prestressed composite beams. Formulas for calculating the yield and ultimate moments were also derived. It was found that including the slip effect on the deflection and yield moment can significantly improve the accuracy of analytical predictions. As the calculated values showed good agreements with the test results, the proposed formulas can be reliably applied to the analysis and design of prestressed steel–concrete composite beams.

To investigate monitoring of concrete members by integrating fiber optic technology, a series of static and dynamic loading tests have been conducted on a posttensioned concrete girder with a span of $16.8\mathrm{m}$ and a total depth of $0.8\mathrm{m}$ . For strain measurements optical fiber strain sensors, based on Bragg gratings, have been used. In literature only few papers deal with changes of dynamic parameters in posttensioned concrete at increasing damage levels. The obtained test results demonstrate the feasibility of fiber Bragg grating strain sensors, also for dynamic monitoring. On the dynamic behavior of the tested girder, it is found that, although a drop in eigenfrequencies is noticed with increasing load, significant shifts are only measured when the reinforcement starts yielding. Also with respect to damping values, no clear indication of damage in an early stage is observed. Further results show that the curvature of the mode shapes changes with increasing load and is related to the formation of a plastic hinge. An interesting observation is the change in phase angle between the modal strains and the vertical acceleration, which already occurred at an early stage.

This paper presents results of a combined experimental and analytical approach to investigate modal properties of a lively open-plan office floor. It is based on state-of-the-art finite-element (FE) modeling, FRF-based shaker modal testing, FE model correlation, manual model tuning, and sensitivity-based automatic model updating of a detailed FE model of this composite floor structure. The floor studied accommodates a fully furnished office. Such environments can be problematic regarding their vibration serviceability. However, there is a lack of reliable information about their as-built modal properties and the ability of designers to predict them. Therefore this paper has two aims: (1) to assess the ability to both predict and measure as accurately as possible the fundamental and higher modes of floor vibration, and (2) to correlate and update the initially developed FE model of the floor, so that its modes match as accurately as possible their measured counterparts. It was found that even a very detailed FE model, the development of which was based on best engineering judgment, missed the natural frequencies by 10–15% in some of the first four modes of vibration which possibly could be excited by walking. The key reasons for this were both over- and underestimation of the stiffness of the main composite beams, depending on the beam location. This was probably caused by uncertainties due to visible cracking of the lightweight concrete in the zone above the beams, effects of nonstructural elements, such as false flooring, and the inevitable uneven distribution of mass and stiffness in the real-life floor in operation. All of these factors are difficult to model explicitly in the floor, so their aggregate effect was taken into account via changes in the beam stiffness. This was found by performing a sensitivity-based FE model updating in which the first four vertical bending modes of the floor were successfully updated. Such updating was possible only after all perimeter walls were explicitly modeled. The obtained updated properties are by no means a unique solution which minimizes the difference between natural frequencies and maximizes the MAC values between the experimental and analytical mode shapes. Rather, it is a reasonable set of modeling parameters which quantifies the possible uncertainty when specifying FE modeling parameters for an open-plan floor structure like the one described in this paper.

The present paper addresses a foundation problem in seismic resistant buildings braced by structural walls. In particular, the box-type foundation for an internal shear wall is investigated. This kind of foundation can be an alternative solution for direct foundations. It solves the problem where oversized ordinary raft foundations are needed. In fact, the box foundation gathers the gravity loads resting on the peripheral walls, so that the global soil reaction reduces its eccentricity. In most cases the elements constituting the box structure, such as peripheral walls, raft, and ground floors, are already part of any actual building; they only need a proper seismic design, which is herein proposed. A simplified theoretical approach is presented to allow a correct proportioning and design of each structural element, such as reinforcing ribs and panels constituting the box. Moreover, the solution given by this analytical procedure can be useful to check the reliability of any numerical analysis. Since the analytical approach is based on equilibrium conditions, it can be easily extended to the ultimate loading state. A practical application is finally presented for a seven floor building where both the shear walls and the box foundation are subjected to a severe seismic action.

A procedure is presented to develop probabilistic constant-strength ductility demand spectra based on statistical analyses of the results of comprehensive dynamic time history simulations of bilinear single-degree-freedom systems. The procedure offers an approach to estimate both the mean and standard deviation of the ductility demand, given the normalized strength and fundamental system period. The probabilistic distribution of the ductility demands conditional on normalized strength and natural period is validated through goodness-of-fit test. The effects of post-yield stiffness ratio on ductility demand are also investigated. The proposed spectra can allow the treatment of the frequency content and peak intensity measure of the ground motion separately, and describe the state variation of a structure from elastic to inelastic as a function of the peak ground acceleration, which enables its application in stochastic seismic displacement analysis and probabilistic seismic demand analysis. An illustrative example of a seven-story planar frame structure is presented to demonstrate the applicability and utility of the proposed methodology.

A detailed seismic performance assessment procedure has been developed for reinforced concrete (RC) frame buildings with masonry infill walls. The procedure is based on member damage functions, in terms of interstory drift ratios. These functions are developed for the primary components, namely, columns, beams, and infill walls. For developing these functions, analytical investigations to determine the influence of several parameters on the damageability of components were combined with experimental data. A new approach has been developed to synthesize component damage states to determine the story and eventually the building level performance states. The procedure has been calibrated and compared with other procedures by predicting the observed performance of three buildings exposed to recent earthquakes in Turkey. It is concluded that the developed procedure can satisfactorily predict the damage experienced by most of the components of the studied buildings. Moreover, the procedure reasonably captured the observed building damage states. Accordingly, the developed procedure can be used for a reliable performance assessment as well as performance-based design of RC frame structures.

The design of a large proportion of the low-rise metal buildings in the United States is based on the ASCE 7-93 Standard and the use of allowable stress design (ASD). In this paper we examine, for a typical case, the question of whether such buildings satisfy requirements implicit in more recent versions of the ASCE Standard. We base our investigation into this question on all three wind loading conditions specified in the ASCE 7-02 Standard. In particular, we use the nonlinear database-assisted design technique to assess the degree of safety of a typical portal frame belonging to the wind-force resisting system of a low-rise industrial structure designed in accordance with ASCE 7-93 and ASD. The conclusion of our assessment is that the frame being considered satisfies all ASCE 7-02 requirements with respect to wind loading, but that its safety level is relatively low, and could be improved substantially at very low cost.

This paper proposes a new planar reinforced concrete (RC) formulation capable of modeling geometric and material nonlinearity. A new co-rotational sub-parametric formulation consisting of a four-noded element with quadratic hierarchic shape functions is presented. The novelty of this large-displacement-small strain element lies in the choice of the local coordinate system and the local degrees of freedom, which is mainly responsible for its simplicity and computational efficiency. For material nonlinearity, the Kotsovos and Pavlovic concrete model is used, with the addition of some numerical modifications including: (1) an accurate tangent stiffness for the nonlinear behavior of uncracked concrete; (2) a nonlinear uniaxial constitutive relationship for cracked concrete; and (3) exact cracking stress evaluation. Assuming perfect-bond, two layers of steel with only axial stiffness are smeared over the element according to their angles of orientation. Verification studies are presented to highlight the significance, limitations, and applicability of the proposed element in large displacement analysis of planar structures.