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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 81
PROCEEDINGS OF THE TENTH INTERNATIONAL CONFERENCE ON CIVIL, STRUCTURAL AND ENVIRONMENTAL ENGINEERING COMPUTING
Edited by: B.H.V. Topping
Paper 153

Repair of Cracked Steel I-Beams: Fracture Demand versus Force and Stress Criteria

S.S.F. Mehanny, O.M.O. Ramadan and H.G.Z. Nasralla

Structural Engineering Department, Cairo University, Giza, Egypt

Full Bibliographic Reference for this paper
S.S.F. Mehanny, O.M.O. Ramadan, H.G.Z. Nasralla, "Repair of Cracked Steel I-Beams: Fracture Demand versus Force and Stress Criteria", in B.H.V. Topping, (Editor), "Proceedings of the Tenth International Conference on Civil, Structural and Environmental Engineering Computing", Civil-Comp Press, Stirlingshire, UK, Paper 153, 2005. doi:10.4203/ccp.81.153
Keywords: cracking, fracture demand, linear elastic fracture mechanics, stress, force, steel I-beams.

Summary
Cracking of tension flange in steel I-beams at locations of high stresses (e.g., extreme tensile fibres of bottom flange at mid span of simply supported beams) may be encountered in practice. Cracks may initiate due to some flaws or imperfections during manufacturing process or during installation and welding procedures for built-up sections. Unload/reload cycles in some special structures (e.g., bridges) may also generate such cracks constituting critical stress risers. Such cracks negatively affect the carrying capacity of the section at their location and may even cause progressive damage leading to global failure or collapse.

The main objective of this paper is to predict the performance of simply supported steel I-beams under uniformly distributed loads with a major crack in the bottom flange at mid span. Different crack lengths as a ratio of the flange thickness have been investigated. A repair scheme is introduced by welding a bottom plate to the cracked flange to restore the original carrying capacity of the cross section. Another major goal of the current study is to determine the most economic dimensions of the repair plate for different crack lengths to efficiently re-achieve the capacity of the uncracked section [1].

The study has adopted several criteria to evaluate repair efficiency: (1) Fracture demand versus (2) Force and Stress criteria. Linear Elastic Fracture Mechanics principles [2]] are applied through a numerical finite element analysis using special fracture software - FRANC2D [3].

The first criterion looking at the fracture demand in terms of the stress intensity factor KI monitored at the crack tip is found to be a non-controlling criterion for the repair adequacy. However, it has been shown that longer repair plates are needed for longer cracks to reduce KI. But to render such longer plates efficient enough in reducing fracture demand, their axial stiffness shall be increased to limit crack mouth opening in the fractured tension flange. The plate axial stiffness is amplified by increasing its cross-sectional area; the latter being achieved by using either a "thicker" or a "wider" plate.

As designers usually care more for actual versus allowable stresses (or forces) than for fracture mechanics measures, the other criteria are thus introduced using force and stress analysis results of the cracked beam - with and without repair plate - at and around the crack location. Repair plates with different lengths and thickness are tried to ensure the same, or greater, "force" carrying capacity of the original uncracked beam with "stresses" not exceeding allowable values specified in design codes. It has been found that the "force" criterion is a necessary but not always sufficient criterion for an inclusive repair, and that it shall be used as a complementary part to the "stress" criterion. In other words, the force capacity of the originally uncracked flange may be guaranteed at the crack location through an adequate repair plate - with suitable length, thickness, and width - and the remaining uncracked ligament of the damaged tension flange, whereas the stress might still be exceeding allowable values. The "stress" criterion shall thus be also enforced, beside the "force" criterion, for an inclusive and complete repair.

However, as per the sound judgment of the designer or the engineer in charge, stresses above allowable values may be sometimes accepted. Such high stresses may even get amplified through the continuous usage of the structure due to progressive stress concentration at the crack tip. This may lead to probable crack propagation towards the web and eventually full cracking of the fractured flange, i.e., separation into two parts, even in the presence of the repair plate. In the limit, this may be looked at as the case of a clear field splice of the tension flange under consideration. The efficiency and safety of such splice is then guaranteed through the enforced "force" criterion previously satisfied to fully transfer the force flowing in the cracked flange through the repair plate adequately dimensioned for that purpose, provided that the crack does not propagate through the web of the I-beam.

As a general conclusion, the study proves that both "force" and "stress" criteria are closely inter-related and that for inclusive repair the two measures shall be enforced. To complete the full picture, economic dimensions of repair plates for each investigated crack length are also determined.

References
1
H.G.Z. Nasralla, "Effect of Cracks on Structural Steel Elements", M.Sc. Thesis, Cairo University, Giza, Egypt, September, 2004.
2
T.L. Anderson, "Fracture Mechanics - Fundamentals and Applications", CRC Press, USA, 1995.
3
FRANC2D User's Guide, "A Two-Dimensional Crack Propagation Simulator", Version 3.1, Cornell Fracture Group, Theory Center, Ithaca, NY, USA, 2002.

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