1、砼相对界限受压区高度

1、相对界限受压区高度 ξb

εcu = 0.0033 - (f cu,k - 50)×10-5 = 0.0033 - (25 - 50)×10-5 = 0.0036 > 0.0033 取 （只要混凝土强度等级不超过C50）εcu = 0.0033; β1= 0.8； 按规范公式（6.2.7-1）：

1）HPB300；f y = 270.00 N/mm 2；E s = 2.1×105 N/mm 2 ξb = β1

1 + f y E s εcu = 0.801 + 2702.1×105×0.0033

=0.58 2）HRB335；f y = 300.00 N/mm 2 ；E s = 2.0×105 N/mm 2 ξb = β1

1 + f y E s εcu = 0.801 + 3002.0×105×0.0033

=0.55 3）HRB400；f y = 360.00 N/mm 2 ；E s = 2.0×105 N/mm 2 ξb = β11 + f y E s εcu = 0.801 + 3602.0×105×0.0033

=0.52 4）HRB500；f y = 435.00 N/mm 2 ；E s = 2.0×105 N/mm 2 ξb =

β11 + f y E s εcu = 0.801 + 4352.0×105×0.0033 =0.48