Perry Adebar

A GENERAL SHEAR DESIGN METHOD

The authors present a simple, unified method for the shear design of both prestressed concrete members and nonprestressed concrete members. The method can treat members subjected to axial tension or axial compression and treats members with and without web reinforcement. The derivation of the method is summarized and the predictions of the method are compared with those of the current American Concrete Institute (ACI) Code.

Design of Deep Pile Caps by Strut-and-Tie Models

Comparisons with results from 48 pile cap tests demonstrate that the one-way shear design provisions of the present ACI Building Code are excessively conservative for deep pile caps, and that the traditional flexural design procedures for beams and two-way slabs are unconservative for pile caps. Flexural design can best be accomplished using a simple strut-and-tie model, and test results demonstrate that the longitudinal reinforcement should be concentrated over the piles as suggested by strut-and-tie models. A simple shear design procedure is proposed in which maximum bearing stress is considered the best indicator of shear strength for deep pile caps. The maximum bearing stress that can be applied without causing splitting of compression struts within pile caps depends on the amount of confinement, as well as the aspect ratio (height-to-width) of compression struts. The influence of confinement is more gradual than suggested by the ACI Code bearing strength provisions.

BEARING STRENGTH OF COMPRESSIVE STRUTS CONFINED BY PLAIN CONCRETE

To study transverse splitting of compresson struts due to spreading of compression, concrete cylinders of varying diameters and of varying heights were loaded over a constant size bearing area. The travel time of an ultrasonic pulse was used to indicate internal cracking, and measured cracking loads were compared to finite element predictions. It was found that when compression struts are confined by plain concrete, the maximum bearing stress to cause transverse splittting depends on the amount of confinement, as well as the aspect ratio of the compression strut. Design recommendations are given for the maximum nodal zone bearing stress to prevent diagonal tension failures in deep memebers with unreinforced compression struts. A simple example is provided to illustrate the suggested nodal stress limits for the design of a pile cap using a strut-and-tie model.

SIDE-FACE REINFORCEMENT FOR FLEXURAL AND DIAGONAL CRACKING IN LARGE CONCRETE BEAMS

The current American Concrete Institute (ACI) Building Code and American Association of State Highway and Transportation Officials (AASHTO) Bridge Code requirements for side-face reinforcement are meant to control flexural cracking in the webs of large concrete beams and may not provide adequate diagonal crack control for certain exposure conditions. Twenty-one large concrete beam elements with 1,200-mm deep webs were tested in a specially constructed apparatus to study the influence of amount and arrangement of side-face reinforcement in controlling both flexural and diagonal cracking in large concrete beams. The amount of side-face reinforcement was varied from 50% to 300% of what is required by the current ACI Building Code and AASHTO Bridge Code. Deformed reinforcing bars, welded wire fabric, and hooked steel fiber were included in the study. Over 11,000 crack widths were measured with a microscope on the 21 specimens, and an analysis of the crack data revealed the relationship between crack width and average strain and the ratio of maximum to average crack widths. A procedure is presented for estimating diagonal crack widths in the webs of large beams caused by service level shear stresses, and a general design procedure is presented for the amount of side-face reinforcement needed to control both flexural and diagonal cracking in the webs of large concrete beams. The required spacing of side-face longitudinal reinforcing bars depends on the maximum acceptable crack width, strain of the longitudinal reinforcement on the flexural tension side, magnitude of the applied shear stress, amount of transverse reinforcement, and the diameter of and cover to the side-face reinforcing bars. A design example illustrates the proposal.

Drift Capacity of Walls Accounting for Shear: The 2004 Canadian Code Provisions

This paper presents the new provisions in the 2004 Canadian code for flexural displacement capacity of concrete walls, and the new provisions for seismic shear design of slender concrete walls. In order to facilitate explanation of the seismic shear provisions, general expressions for shear design are presented first, and the non-seismic shear design provisions in the Canadian and ACI 318 building codes are briefly reviewed. According to the new seismic shear design provisions presented here, the maximum shear force and concrete contribution depend on the inelastic rotation demand in the plastic hinge, and the compression stress (critical crack) angle used to determine the quantity of horizontal reinforcement depends on the axial compression stress applied on the wall. The 2004 Canadian code provisions generally require more horizontal reinforcement than the ACI 318 provisions except when inelastic rotational demand is small and axial compression stress is large; however, the Canadian provisions permit significantly higher shear stress for high-strength concrete walls. The new provisions can be used to design concrete walls given the expected level of drift demand or, as demonstrated in this paper, can be used to estimate drift capacity of walls accounting for the significant influence of shear.

Nonlinear Rotation of Capacity-Protected Foundations: The 2015 Canadian Building Code

When foundations are capacity-protected, inelastic deformations will occur primarily in the seismic force-resisting system. Soil flexibility can be ignored when determining seismic loads, but footings will rotate when subjected to the maximum overturning moment, and this may increase building drifts, particularly in lower stories where gravity-load columns are less flexible. A “hand calculation” method is presented for estimating rotation of a footing from the uniform bearing stress distribution required to resist the applied overturning moment. The method, which has been adopted in the 2015 Canadian building code, accounts for initial linear rotation of footings and additional nonlinear rotation due to footing uplift and nonlinearity of soil. A quick, safe estimate can be made using approximate equations, or a more accurate estimate can be made by determining two parameters from figures. Design examples demonstrate how the method can be used to design foundations for improved performance at a small additional cost.

Interstory Drifts from Shear Strains at Base of High-Rise Concrete Shear Walls

AbstractShear strains may have negligible influence on maximum displacements at the top of slender shear walls, but may significantly increase interstory drift ratios at lower levels where gravity-load columns are often less flexible. A nonlinear finite-element (FE) model calibrated with experimental results confirmed that large shear strains occur in flexural tension regions of concrete walls due to vertical tension strains in the presence of diagonal cracks and in the absence of demand on the horizontal shear reinforcement. A fan of diagonal cracks will form at the base of flexurally hinging walls independent of the shear stress level. A parametric study confirmed that a principal strain angle of 75° can be used to estimate shear strains from vertical tension strains. Thus interstory drift ratios due to shear strains can be estimated from the easily calculated flexural demands. A simple and safe estimate of interstory drift ratio due to shear strains is 60% of the global drift ratio. Interstory drift ra...

Repair of an 18-Story Shear Wall Building Damaged in the 2010 Chile Earthquake

Three transverse shear walls at one corner of an 18-story building in Santiago failed in flexural compression just below grade, causing the ground floor to drop 75 mm and the corner of the roof to displace laterally 185 mm. Cracking of walls and floor slabs caused significant building distortions. Nonlinear analyses of shear walls and floor slabs were used to understand the measured residual displacements and determine effective stiffnesses needed for a three-dimensional (3-D) model of the building. This model was used to estimate jacking forces needed to reposition the building. Existing cracks in shear walls were injected with epoxy, and fiber reinforced plastic (FRP) fabric was used to control new cracks. Instrumentation was used during jacking to monitor building movements, inclinations of walls and slabs, maximum compression and tension strains in walls, and crack widths. The building was repaired for less than 25% of the replacement cost and with much less impact on building habitants and the surrounding community.

Shear Strength Evaluation of Concrete Bridge Girders

This paper presents a shear strength evaluation procedure for structural concrete girders that contain at least minimum transverse reinforcement. This procedure is similar to the 2008 American Association of State Highway and Transportation Officials Load & Resistance Factor Design (AASHTO LRFD) shear design method except that it does not require trial-and-error for shear strength evaluation and provides more insight by providing information about different shear failure modes involving stirrups yielding, diagonal crushing of concrete, and longitudinal reinforcement yielding. The procedure is simple enough to be implemented into a small computer program for checking numerous sections along a bridge girder. To validate the proposed method, shear strength predictions are compared with results from strength tests on reinforced and prestressed concrete beams, as well as predictions from shear design methods in AASHTO LRFD and American Concrete Institute 318. The proposed procedure is used to evaluate the shear strength of girders in three existing bridges with prestressed concrete I-girders, prestressed concrete box-girders, and reinforced concrete channel-girders. Results from the validation tests and demonstrations compare well with results from previous tests and models.