Page 163 - Failure Analysis Case Studies II

148

Fig. 1. Prestressed steel frame using two bars.

1. The tendons were high strength steel bars, 36 mm diameter, with a rupture load FR = 1300 kN.
The chemical composition and mechanical properties of the steel bars are shown in Tables 1 and 2,
respectively.
Two fractures happened during building erection when stressing the bars; one at 400 kN (about
0.3FR) and another at 600 kN (about 0.46FR); both failure loads were well below the scheduled
working load of 0.6FR.
Examination of the fracture surfaces revealed that in both cases the fracture was triggered by a
small surface crack. The morphology of the fracture surfaces was very similar in the two bars; a
representative picture is shown in Fig. 2. Observation by scanning electron microscopy revealed a
quasi-cleavage morphology.

Table 1. Chemical composition of the steel bars
Element C Si Mn P S V
Weight % 0.65 0.70 1.20 0.014 0.018 0.25

Table 2. Mechanical properties
Elastic modulus Yield stress Tensile stress Elongation under max. load
208 GPa 1143 MPa 1285 MPa 6.7%