International Concrete Abstracts Portal

The optimum thickness of a concrete flat plate is the best or the most favourable for an objective. The most common objective is often miuimum cost; however, it can also be noise insulation, least vibration or plane soffit, with or without beams or drop panels. The minimum cost is often achieved by the smallest thickness that avoid excessive deflection in service. With small thickness, reinforcement is commonly needed for safety against shear failure. Part II: Strength, presents the design of shear and flexural reinforcements to resist punching shear of slabs whose thickness has been decided. Slabs directly supported on columns without beams or column heads are considered. Punching shear is an inaccurate term used for shear failure, at inclined surface, adjacent to a column. Headed Stud Shear Reinforcement (HSSR) is used in current practice to resist shear failure. The presented equations apply to HSSR and stirrups, except where stated otherwise.

Optimum thickness of a concrete flat plate is the best or the most favourable for an objective. The most common objective is often miuimum cost; however, it can also be noise insulation, least vibration or plane soffit, with or without beams or drop panels. The minimum cost is often achieved by the smallest thickness that avoid excessive deflection in service. With small thickness, reinforcement is commonly needed for safety against shear failure. Part I: Serviceability, presents a procedure to predict long-term deflection of floors and bridge decks considering the effects of cracking, creep and shrinkage of concrete and relaxation of prestressing reinforcement. The system consists of a solid slab with or without drop panels and/or beams. For analysis, the system is idealized as grid of rigidly connected short beam elements. Strain distributions at end sections are determined, assuming linear elasticity and that plane cross sections remain plane. The analysis is based on compatibility and equilibrium principles, combined with time-dependent parameters for concrete and prestressed reinforcement. The displacements – translations and rotations – are determined from strain parameters by virtual work. Part II is concerned with design of slabs for shear strength.

The current AASHTO LRFD provisions for live load distribution factors (LLDF) for composite prestressed concrete I-girder bridges depend on various factors, such as span length, spacing and stiffness of girders. The approach assumes 100% composite action between the deck and the supporting girders and does not consider the effect of any possible loss of composite action due to construction issues or time-dependent deterioration. The current study evaluated the actual bending moment LLDFs in a recently constructed partially composite prestressed concrete I-girder bridge on SH-75 in Dallas, TX that has excessive vibration issues. In-situ load testing with loaded trucks and instrumentation (strain gages and rotational tiltmeters) showed only 15% remaining deck-girder composite action. Two load tests at an interval of about one year were employed to determine any time-dependent loss of composite action. The interior girder LLDF values obtained from load tests varied between 0.26 to 0.43, while the AASHTO LRFD approach yielded a constant value of 0.52 for all girders. A calibrated numerical model showed good agreement with load test results and may be used for evaluation of any retrofitting methods to increase the composite action.

Traditionally, a time-varying mass system is viewed as the motion of moving bodies exiting or colliding with the system, such as rockets. A standing structure is not typically considered a time-varying mass system, but a silo during discharge of the infill is a subtle time-varying mass structure. Slender silos and silos with insufficiently stiffened supports are vulnerable to excessive vibration (silo quaking) and loud disruptive noises (silo honking) caused by the flow of the exiting masses. Using principles of mechanics and conservation of momentum, the equation of motion of such systems can be formulated to incorporate the discharge rate, material properties and the time-dependent characteristics of the system (mass, damping and stiffness). In this paper, the acceleration and mass flow of granular fill in a perspex tubing during discharge have been reproduced to simulate silo honking. By controlling the majority of influential factors, the replication of a small-scale silo design was possible with the repeatability of silo honking achieved in a controlled environment. A comparative study between discharge testing results of the sand fill with 0% (control), 5% and 10% moisture content shows that increasing the moisture content of the fill reduces the vibrational effect on the silo walls, and in turn reduces the magnitude of silo honking. Further, the effect of the sudden mass loss on a system of reinforced concrete columns depicting that of silo supports is investigated. The results show the exponential changes in the acceleration and velocity responses of the structure when subjected to a sudden mass loss. Finally, notes on how to consider the system of the forces in the silo structure based on the existing silo theory are provided.

The “Notional Pile” formulation is developed for modeling a group of piles in a foundation. It is a new procedure for foundation modeling for dynamic analysis in conformance with ACI 351.3R. It is an augmentation of the well-known Novak procedure. Foundation stiffness is represented as a set of notional pile elements. This differs from conventional procedures in which the pile group stiffness is represented by a set of springs lumped at one point. With notional piles and finite element modeling of the cap, flexible-cap modes of vibration can be extracted. With conventional procedures, only lower-frequency rigid body modes can be extracted. Notional piles distribute stiffness more realistically and enable cap-pile interaction. A specific case is used to illustrate the new procedure. For that case, the cap did not have a regular distribution of mass or stiffness. Dynamic loads were applied with considerable eccentricity, at multiple locations and with multiple frequencies. Notional piles accommodated these irregularities. The notional pile formulation was validated by comparing measured to computed foundation responses. The comparison was good but not great. The foundation was to be reconfigured for new machinery. The retrofit design was modeled using notional piles. Responses were computed and compared to applicable limits.