Carbon Fiber-Reinforced Polymer for Continuity in Existing Reinforced Concrete Buildings Vulnerable to Collapse

ACI Structural Journal, Sep/Oct 2009 by Orton, Sarah, Jirsa, James O, Bayrak, Oguzhan

Carbon fiber-reinforced polymer (CFRP) can provide continuity in reinforced concrete beams and thereby reduce the likelihood of progressive collapse if a supporting column were lost due to extreme loading (blast or impact). Seven half-scale specimens representing two spans of a reinforced concrete (RC) frame with a center supporting column removed were tested. The capacity of vulnerable RC building beams with discontinuous reinforcing steel were evaluated and compared with beams using CFRP to provide continuity, and with beams designed with continuous reinforcing steel. It was found that CFRP is capable of providing sufficient continuity to withstand the loss of a supporting column through either catenary (or cable) action, which reduces material usage, or flexural action, which reduces deflections. Furthermore, beams with continuous reinforcement may not be able to withstand the loss of a center-supporting column due to limited rotational ductility of the beam.

Keywords: carbon fiber-reinforced polymer; catenary action; continuity; progressive collapse; structural integrity.

INTRODUCTION

Progressive collapse is defined by the U.S. General Services Administration (GSA)1 as "a situation where local failure of a primary structural component leads to the collapse of adjoining members which, in turn, leads to additional collapse. Hence, the total damage is disproportionate to the original cause." Generally, buildings are not designed for abnormal loading conditions that may result in progressive collapse.2 When an unlikely loading event occurs, however, the injuries and loss of life due to progressive collapse can be severe.

To resist progressive collapse, some governmental agencies (GSA and Department of Defense) have begun implementing guidelines to address the issue.1,3 These guidelines apply three types of approaches:

* Indirect design-Emphasize strength, continuity, redundancy, and ductility; relies on integrated system of tie forces.

* Direct design-alternate load path: Analyze structure for instantaneous loss of a vertical load-bearing member, provide redundant or alternate load path to bridge over failed member; analysis can account for plastic or large deformations including catenary or membrane action.

* Direct design-specific local resistance: Each member is designed to resist a specific threat.

The alternate load path is the procedure used in this research. To resist progressive collapse, a structure must survive the loss of a load-bearing member with a load of 2� (dead load 0.25 live load) applied in the tributary area surrounding the removed member.1 The 2-times factor accounts for the dynamic nature of the falling load due to the lost member. While the quantification of the dynamic amplification factor is beyond the scope of this study, some studies indicate that the amplification factor could be reduced to 1.5.4

Loss of a supporting column may leave a beam unable to resist gravity loads and may lead to collapse of the spans on either side of the lost column. The collapse can continue to progress to higher floors or to adjacent spans, leading to even more damage. Examples of poor design that lead to progressive collapse include the Ronan Point Building in the UK and the Murrah Building and L'Ambiance Plaza in the U.S.2,5-7 Conversely, buildings with good design (continuity of reinforcement, redundancy, alternate load paths, and ties between structural elements), such as the Pentagon Building and Khobar Towers, have been able to withstand abnormal loads without progressive collapse.2,8

Reinforced concrete (RC) buildings may be vulnerable to progressive collapse due to a lack of continuity in their reinforcement (or reinforcement cutoffs). Discontinuous reinforcement is typical of many structures designed before the implementation of requirements for structural integrity. The GSA guidelines state that "providing continuous bottom reinforcement is essential to accommodating the double span condition."1 The ACI Code first implemented structural integrity provisions that required continuity of reinforcement in 1989.9 To provide the ability to survive progressive collapse, existing reinforced concrete beams may need modification to provide continuity of reinforcement. Carbon fiber-reinforced polymer (CFRP) can be used to enhance strength by providing the missing continuity and thereby reduces the likelihood of progressive collapse if a supporting column was lost due to extreme loading (blast or impact). In this study, methods to apply the CFRP to provide enough continuity to resist progressive collapse are investigated. The intent of the CFRP is to give occupants a chance to escape from the building using a system that does not require extensive changes in the configuration of the building or in the dimensions of the members. This study does not consider the effects of fire that may occur after an abnormal load and may be detrimental to the CFRP's integrity. The purpose of the CFRP is to reduce the risk of progressive collapse immediately after the extreme event occurs so that building occupants are allowed to escape before the effects of fire become significant. If the engineer decides that fireproofing is an important issue (to allow sufficient time for the occupant to vacate the building after an explosion/column failure due to another cause), such systems exist and should be used with the FRP repair. Furthermore, the study did not consider the effects of Vierendeel truss action, which may allow upper stories to help support lower damaged stories.