Using a simple model for determining the probable moment strength of RC Structural Walls

ABSTRACT: In capacity design of RC structural walls the design shear force is obtained using the probable moment strength. This study proposes a simple procedure for determining the probable moment strength of RC structural walls.

This procedure is based on a simple model which has been used previously for determining the probable moment strength of RC columns. This model uses simple hypotheses calibrated with experimental results. Moment strengths measured in lateral load tests of a set of RC structural walls conducted by several authors are compared with predictionsof the probable moment strength of these test units.

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Nel progetto a capacità dei muri strutturali in c.a., il taglio di progetto si ottiene dalla resistenza a flessione probabile.

Questo studio propone una semplice procedura per determinare la resistenza probabile di tali muri ed è basata su un modello già adoperato per determinare il momento resistente probabile di pilastri. Tale modello impiega ipotesi semplici e calibrate su risultati sperimentali. La resistenza flessionale, misurata in prove sperimentali condotte da vari autori su muri in c.a. soggetti a forze laterali, viene confrontata con la previsione teorica della resistenza probabile.

USING A SIMPLE MODEL FOR DETERMINING THE PROBABLE MOMENT STRENGTH OF RC STRUCTURAL WALLS

IMPIEGO DI UN MODELLO SEMPLICE PER DETERMINARE LA RESISTENZA A FLESSIONE PROBABILE DI MURI IN C.A.

M. Rodriguez¹, J. Restrepo², M. Iñiguez¹

1 Universidad Nacional Autonoma de Mexico, Mexico City, Mexico

2 Structural Engineering Department, University of California San Diego, San Diego, USAC. Longo¹, T. Ciccone¹, M. Pasqualotto²

1 INTRODUCTION

1.1 Definitions of probable moment strength, credible moment strength, and maximum moment strength

The probable moment strength, M_pr, is the calculated maximum flexural strength for the critical section of a member subjected to bending and with or without axial load. The probable moment strength, M_pr, is determined from a flexural theory with the mean concrete compressive strengths, f_c , and mean steel reinforcement yield strength f_y and considering the effect of cyclic hardening in the reinforcement. The probable moment strength is used to capacity protect a member to reduce the potential of shear failure in a strong earthquake.

The credible moment strength, M_cd, is determined from a flexural theory with the measured concrete compressive strength f_c and the measured steel reinforcement yield strength f_y and considering the effect of cyclic hardening in the reinforcement. The main difference between M_pr and M_cd is that the former is determined with mean strengths of both concrete and steel reinforcement and the later with measured strengths of these materials.

The maximum moment strength, M_MAX, is the measured bending moment resisted at a member end in a reversed cyclic load test.

1.2 ACI 318-14 procedure for computing probable moment strength

According to ACI 318-14 (ACI 318, 2014), the probable moment strength in a member is determined with the flexural theory stated in ACI 318 and using the properties of the member end assuming a tensile stress in the longitudinal bars of at least 1.25f_y and a strength reduction factor ϕ of 1.0. The credible moment strength, M_cd, is not used in ACI 318. This strength is used for example for determining maximum flexural strength in members tested in a laboratory, where measured material strengths are known.

The current ACI 318 (ACI 318-14) uses capacity design for ductile structural elements in RC moment frames but not for structural walls. However, this approach will be included for RC walls in a future version of ACI 318.

For the case of RC columns, it has been shown that the Mcd values computed with the ACI 318 procedure underestimate the valued of M_MAX (Restrepo & Rodriguez, 2013).

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L'ARTICOLO COMPLETO E' DISPONIBILE IN ALLEGATO

KEYWORDS: Reinforced Concrete walls; probable moment strength; capacity design Muri in c.a., Resistenza flessionale probabile, progetto a capacità.

Articolo presentato in occasione degli Italian Concrete Days 2018 di aicap e CTE

Dal 14 aprile al 17 aprile 2021 si terrà la terza edizione degli Italian Concrete Days. Ulteriore informazione sull'evento a questo LINK.