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Point of View : Lessons Learnt from the Collapse of Footbridge in Miami, USA N. Subramanian

On March 15, 2018, a prestressed concrete truss footbridge in Miami, Florida, USA collapsed during construction. This bridge was designed as a ‘signature’ bridge with an unorthodox, non-redundant prestressed concrete truss bridge that was made to look like a cable-stayed bridge just for aesthetic appearance. The collapse of this footbridge caused several casualties and raised serious concerns about the design and construction of the bridge, in addition to the use of accelerated bridge construction (ABC). Recently the National Transportation Safety Board (NTSB), after conducting extensive investigations, released a report that attributed the collapse to flawed design, especially not providing enough shear transfer mechanism at the cold joint at the northern end of the bridge, and not considering the behaviour of the footbridge during all the construction stages. It also blamed all the involved agencies for their negligence of not proof checking the bridge design during all the construction stages. In addition it criticized the agencies for not considering the cracks (which were noticed near cold joints during the erection) as serious and life threatening even when the cracks were widening and subsequently lead to the collapse. As the failure of this bridge gives important lessons for bridge engineers, the features of the bridge and the reasons for its failure are described briefly.

Economical and Rational Design of ‘One-Way’ RC Beam-Slab Systems Anurag Singh, Bijily Balakrishnan, Devdas Menon

In the conventional design of reinforced concrete (RC) rectangular slabs in beam-slab systems subject to gravity loading, it is assumed that the code-specified moment coefficients (derived based on yield line theory, assuming non-deflecting supports at the edges) can be used, provided the beams provided at the edges are adequately stiff. Recent experimental and theoretical studies have established that such designs turn out to be over-conservative, as the assumed yield line mechanism of the slab does not occur. In general, a combined beam-slab collapse mechanism occurs at the limit state of collapse, in which the yield lines in the slab connect to plastic hinges in the supporting beams. With a proper understanding of possible collapse mechanisms and estimation of the lowest collapse load, using yield line theory, a more rational and economical design of the beam-slab system is possible. Considerable savings in steel can be achieved, while fully complying with the strength and serviceability requirements of the code.

Effect of Reinforcement Orientation on Ultimate Flexural Capacity of Solid and Biaxial Voided RC Slabs R. Sagadevan, B. N. Rao

A common observation is that flexural load carrying capacity of reinforced concrete slab from yield line analysis is lower than that from an experimental study. This capacity difference arises due to tensile membrane action developed at the post-yield stage. The orientation of reinforcement also plays a significant role in enhancing load carrying capacity, since the fracture moment exceeds moment capacity estimated using yield line analysis. It happens only when reinforcement in orthogonal directions is compelled to yield by diagonal fracture line. The present study investigates the load enhancement due to tensile membrane action and orientation of reinforcement in solid and biaxial voided slabs. Experimental results of 10 specimens (6 solid and 4 biaxial voided slab) were taken from literature. Two-way flexural capacity of those specimens was estimated by yield line analysis in conjunction with Indian code provisions (IS 456: 2000). Results from the experimental study are comparable with yield line analysis after considering the effects due to tensile membrane action and reinforcement orientation.

Influence of Stirrup Leg Spacing across Beam Width on Shear Strength of Wide Beams D. Sree Rama Chandra Murty, G. Papa Rao

Along with the very commonly occurring narrow normal beams, wide beams also occur in some situations in different kinds of structures. The shear strength of normal beams, with well distributed shear reinforcement can be safely predicted by the Indian code IS: 456 (2000) [1], like other national codes across the globe. But in wide beams, stirrup shear carrying capacity decreases significantly as the stirrup leg spacing across the width increases. Limited test data available to date, variously state the stirrup leg spacing limits, in member width, for realising the needed shear strength, implying the need for additional studies; no guidance is available to date, in any code on this aspect. This study investigates the behaviour of and evaluates the benefit accruing to, shear critical RC wide beams, subjected to high and low shear stresses provided with multiple vertical stirrup legs, across the width.

Flexural Behaviour of RC Beams Retrofitted with GFRP: An Experimental Approach Uttam R. Awari, Ajay R. Gajbhiye

Fiber Reinforced Polymer (FRP) has been recognized as an efficient retrofit material for structures worldwide. The present study involved casting and testing of twenty one RC beams strengthened with single, double and triples layers of externally bonded GFRP of varying gsm. The virgin and repaired reinforced concrete (RC) beams were tested up to ultimate load using four-point loading. The effects of number of layers and varying weights of Glass Fiber Reinforced Polymer (GFRP) on the flexural strength, deflection, failure modes are reported in this paper. The tests results were validated with analytical models as per ACI 440.2R-08 recommendation. The findings of this shows that the ultimate load carrying capacity of strengthened beams were enhanced up to 40% as compared to referenced specimens.

A Case Study on the Design of Fastening of External Prestressing Anchorage S. Saibabu, P. Srinivasan, Vimal Mohan

External post tensioning is a common technique to increase the structural member`s capacity due to its active strengthening systems. It induces external forces to the structural elements to counter act the external loads. A case study on the off shore structure has been undertaken in which girders shown distress due to corrosion of tendons. From the flexural analysis, it is found that the tensile stresses at the mid span exceeds permissible stresses. External post-tensioning, a best scheme to reduce the tensile stresses of the girder has been proposed as an immediate strengthening measure. Formula to find external prestressing force which is required to bring back the tensile stresses within the permissible stresses is predicted. Flexural stresses and deflections of the strengthened girders at mid span have been computed using the predicted external prestressing force and found to be within the corresponding permissble values. It is proposed to transfer the external prestressing force to the concrete principally by shear using the post-installed expansion type anchors. This method has advantage of not requiring lateral/transverse prestressing as necessary in case of conventional method and more suitable where ends of girders are not accessible for anchoring external prestressing. The bolts will not cause damage to the existing reinforcement at the anchor zone due to its smaller length. Design details for fastening of anchorage of external prestressing force using expansion type anchor bolts are presented in this paper.

Evaluation of Vibration-Based Damage Detection Techniques for RC Beams Shamsher Bahadur Singh, Shubham Aggarwal, Shreyas Pranav

This study aims at comprehensively exploring and comparing four different methods of vibration-based damage detection in reinforced-concrete beams with three different support conditions. The methods chosen are Change in Flexibility (FC), Curvature Mode Shape (CMS), Modal Strain Energy Change Ratio (MSECR) and Principal Eigenvector of Modal Flexibility Change (PE). The parameters considered for this study are damage detection, localization, and quantification, for both single and multiple damage cases. The objective is to propose the most efficient method for both these cases. A beam of length 3.15m is analyzed using a self-developed finite element model-based program on MATLAB™ and damage is simulated by reductions in effective flexural rigidity. Based on the aforementioned parameters, it is found that the FC Method is most suitable for single damage detection while the MSECR method is most suitable for multiple damage detection.

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