压力和弯矩共同作用下的截面受力性能

◆ Then, the crack will be wider and the
neutral axis depth will be reduced.
◆Finally the bar A's in the compression zone
fyAs f'yA's
will yield and the concrete is crushed.
N N
As 太 多
σsAs
f'yA's
σsAs
f'yA's
8.2 偏心受压构件的承载力计算
2、受压破坏 产生受压破坏的条件有两种情况： ⑴当相对偏心距e0/h0较小 ⑵或虽然相对偏心距e0/h0较大，但受拉侧纵向钢筋配置较多时
N N
As 太 多
σsAs
f'yA's
σsAs
f'yA's
8.2 偏心受压构件的承载力计算
8.2 偏心受压构件的承载力计算
一、Failure character
The mode of failure is related to the eccentricity e0 and the longitudinal bar ratioρ 1、Tensile failure
N M
N
fyAs
f'yA's
fyAs
f'yA's
M较大，N较小 As配筋合适
偏心距e0较大
8.2 偏心受压构件的承载力计算
1、 Tensile failure
N
◆ Cracks will first appear in the tension
zone ,the stress of As will improve quickly with increase of load and will yield first.
◆ 形成这种破坏的条件是：偏心距e0较
大，且受拉侧纵向钢筋配筋率合适， 通常称为大偏心受压。
8.2 偏心受压构件的承载力计算
2、Compressive failure
There are two conditions for a compression failure ： ⑴Relative eccentricity e0/h0 is small. ⑵ Although relative eccentricity e0/h0 is large, the amount of tension reinforcing bar is big.
◆The calculation of member with an eccentricity load is the same
with the one under moment, we also take the assumption of plane section.
◆ According
to the relation of strain and stress between concrete and steel ,we can analyze the process under compression and flexure.
8.2 偏心受压构件的承载力计算
N
N
As 太 多
σsAs
f'yA's
σsAs
f'yA's
◆ 截面最后是由于受压区混凝土首先压碎而达到破坏， ◆ 承载力主要取决于压区混凝土和受压侧钢筋，破坏时受压
区高度较大，受拉侧钢筋未达到受拉屈服，破坏具有脆性 性质。 ◆ 第二种情况在设计应予避免，因此受压破坏一般为偏心距 较小的情况，故常称为小偏心受压。
8.2 偏心受压构件的承载力计算
2、受压破坏
⑴当相对偏心距e0/h0较小
N
N
⑵或虽然相对偏心距e0/h0较大，但受 拉侧纵向钢筋配置较多时
As 太 多
σsAs
f'yA's
σsAs
f'yA's
◆ 截面受压侧混凝土和钢筋的受力较大， ◆ 而受拉侧钢筋应力较小， ◆ 当相对偏心距e0/h0很小时，‘受拉侧’还可能出现受压情况。
fyAs
f'yA's
M is big and N is small
eccentricity e0is big
8.2 偏心受压构件的承载力计算
As is suitable
一、破坏特征
偏心受压构件的破坏形态与偏心距e0和纵向钢筋配筋率有关 1、受拉破坏 tensile failure
N M
N
fyAs
f'yA's
◆ There be visible warning of failure and have large deflection ,it likes the beam with suitable reinforcement. Its bearing capacity depends on the steel in the tension zone. ◆ Condition is that the eccentricity e0 is large and the longitudinal bar ratio is suitable.
受拉破坏
受压破坏
8.2 偏心受压构件的承载力计算
第七章
受压构件
受拉和受压破坏的共同点： 靠近轴向力一侧的混凝土均能 达到极限压应变； 远离轴向力一侧的钢筋能达到 抗拉极限强度的为受拉破坏， 不能达到抗拉极限强度（钢筋 受拉或受压，但一般达不到设 计强度）的为受压破坏。 P.138
受拉破坏
受压破坏
8.2 偏心受压构件的承载力计算
8.2 偏心受压构件的承载力计算
2、正截面承载力计算
◆ 偏心受压正截面受力分析方法与受弯情况是相同
的，即仍采用以平截面假定为基础的计算理论，
◆ 根据混凝土和钢筋的应力-应变关系，即可分析截面
在压力和弯矩共同作用下受力全过程。
◆ 对于正截面承载力的计算，同样可按受弯情况，对
受压区混凝土采用等效矩形应力图，
(压力和弯矩共同作用下的截面受力性能)
N M=N e0 As
′ As
e0
N
a
′ As
a'
=
As
As
h0
′ As
b
压弯构件 e0=0？ When e0→∞，N=0，？
偏心受压构件
Failure mode of an eccentrically member with compression load is between axial loaded and flexure member.
ξb =
β
1+ fy
ε cu E s
8.2 偏心受压构件的承载力计算
受拉破坏和受压破坏的界限
◆ We can also take the equivalent rectangular stress block in the
calculation of bearing load.
◆ The
of equivalent depth to neutral axis depth is β 。
strength of equivalent rectangular block is α fc，the ratio
N
N
As 太 多
σsAs
f'yA's
σsAs
f'yA's
◆ The section is destroyed due to the concrete is crushed. ◆ Load carrying capacity is decided by the concrete and bars
in compression zone, and the steel does not reach the tensile yield strength，the section fails in brittle. strength ◆ This case is called small eccentricity compression column due to the eccentricity is usually small. We should avoid the second case in our design.
8.2 偏心受压构件的承载力计算
1、受拉破坏
N
◆ 截面受拉侧混凝土较早出现裂缝，As的
应力随荷载增加发展较快，首先达到 屈服。
◆ 此后，裂缝迅速开展，受压区高度减小 ◆ 最后受压侧钢筋A's 受压屈服，压区混
凝土压碎而达到破坏。
fyAs f'yA's
◆ 这种破坏具有明显预兆，变形能力较
大，破坏特征与配有受压钢筋的适筋 梁相似，承载力主要取决于受拉侧钢 筋。
8.2 偏心受压构件的承载力计算
8.2 Behaviors under flexure and axial load
压力和弯矩共同作用下的截面受力性能)
N M=N e0 As
′ As
e0
N
a
′ As
a'
=
As
As
h0
′ As
b
压弯构件 偏心距e0=0时？ 当e0→∞时，即N=0，？
偏心受压构件
偏心受压构件的受力性能和破坏形态界于轴心受压构件和受弯 构件。
大偏压： ⎧ N = 560kN ⎨ ⎩M = 280kN .m ⎧ N = 1560kN ⎨ ⎩M = 280kN .m ⎧ N = 1560kN ⎨ ⎩M = 200kN .m ； ； ；