F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

Stress/strain characteristics of steel and concrete 69
defines the yield stress of such bars as the stress at 0.43% strain (point P in
Fig. 3.2-1 (a».
For design purpose, BS 8110 idealizes the stress/strain curves for
reinforcement to that shown in Fig. 3.2-1 (b), which applies to both
tension and compression. The partial safety factor Ym for the ultimate limit
state is taken as 1.15 (see Table 1.5-2). Therefore the design strengths, in
tension and compression, are
/i5 = 0.87fy (3.2-1)
The design yield strains are the strains at 0.87fy and are hence calculated as
0.87fy1Es = 0.002 for fy = 460 N/mm 2 and 0.0011 for fy = 250 N/mm 2 •
(Note: Es = 200 kN/mm 2 from Fig. 3.2-1(b).)
The stress/strain characteristics of concrete were briefly referred to in
Section 2.5(d). The exact shape of the stress/strain curve is much
dependent on the concrete strength. Figure 3.2-2(a) shows typical curves
500
"'~ 400
E
E
Z 300
1/1
1/1
~ 200
-VI
Cold-worked high yield bars
p
Mild steel bars
0·0043
(lo002 (lo004 (lo006
Strain
(a)Adual stress/strain curves
1/1
1/1
CII
...
-VI
Strain
(b) Design stress/strain curves (858110)
Fig. 3.2-1 Stress/strain curves for reinforcement