Page 162 - Failure Analysis Case Studies II

Failure Analysis Case Studies N
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 147

PREMATURE FAILURE OF PRESTRESSED STEEL BARS

A. VALIENTE and M. ELICES*
Departamento de Ciencia de Materiales, E.T.S.I. Caminos, Universidad Polithica de Madrid,
Ciudad Universitaria s/n, 2804&Madrid, Spain
(Received 28 November 1997)

Abstract-The details of two failures of prestressed steel bars are reported and a failure analysis is carried
out using Linear Elastic Fracture Mechanics: the fracture toughness was measured using two standardized
procedures and the pertinent stress intensity factor computed by numerical methods. The analysis of the
failures show that the usual design requirements for prestressing bars fail to warn against brittle failures if
damage exists. Some procedures, based on the concept of damage tolerance, are suggested to avoid similar
accidents. 0 1998 Elsevier Science Ltd. All rights reserved.
Keywords: Damage tolerance, fracture toughness, prestressing, stress intensity factors, surface cracks

1. INTRODUCTION
Prestressing, a powerful technique in Civil Engineering, can be defined as the application of a
predetermined internal force to a structural member in such a manner that the combined internal
stresses in the member, resulting from this force and from external loading, will be confined within
specific limits. Whether prestressing is applied to steel or concrete, its ultimate purpose is twofold:
first, to induce desirable strains and stresses in the structure and second, to counterbalance unde-
sirable strains and stresses.
The term pre-tensioning is used to describe any method of prestressing in which the tendons are
tensioned before the concrete is placed. In contrast,post-tensioning is a method of prestressing where
the tendon is tensioned after the concrete has hardened or-as in this case-study-the steel structural
element is built. Post-tensioning offers a means of prestressing on the job site.
When post-tensioning short members, the elongation of the tendonduring the stressing-is
small, and requires very precise measurement by the workmen. This is why bars are preferred to
cables as short tendons for post-tensioning. In this example, some frames of a steel structure were
post-tensioned with high-strength alloy steel bars. .I .
The bars, once stressed, remain as such forever. Under these high stresses-about 780 MPa or
0.6 uts (ultimate tensile stressethey are very susceptible to small damage. Surface scratches, cracks,
pits or notches, that will not usually affect the performance of bars of concrete reinforcement may
become dangerous when present in prestressed bars. In fact, the fractures reported in this paper
happened at loads as low as 0.3 uts and were triggered by very small surface cracks.
The purpose of this paper is to illustrate this fact. Two failures of post-tensioning bars with
surface damage are analyzed using the tools of Fracture Mechanics and some procedures, based on
the concept of damage tolerance, are suggested to avoid similar accidents.

2. CASE STUDY FAILURE OF PRESTRESSED BARS
2.1. Background
The reported failures occurred in a prestressed steel structure. To improve the transversal stiffness
in some steel frames, post-tensioning with a couple of bars was applied diagonally, as shown in Fig.

* Corresponding author.
Reprinted from Engineering Failure Analysis 5 (3), 219-227 (1998)