写给加拿大水泥协会关于混凝土设计手册双筋截面设计例题的电邮及附件

 about Concrete Design Handbook's Example 2.3e

此处省略

I have some question with Example 2.3 of Concrete Design Handbook, 3rd edition, fourth printing December 2010.

In page 2-13, Example 2.3, it is assumed that the compression steel is yielding, and the moment resistance is calculated accordingly. I disagree with the assumption. I find that the compression steel does not yield, and with more compression reinforcement, compression steel's stress is getting lower and further from yielding stress. The attached is my calculation. Would you please do me a favor to give me some advice about my calculation?

此处省略

下面是电邮附件（计算过程）

Concrete Design Handbook, 3rd edition, fourth printing December 2010

Page 2-13, Example 2.3 – Analysis of a Rectangular Beam with Tension and Compression Reinforcement

Calculate the flexural resistance of the beam shown in the figure.

   

d = 330mm, = 65mm, = 30MPa, fy = 400Mpa, Es = 2 X105 MPa, Ec =  4500 = 4500 = 24647 MPa

= 0.85 – 0.0015 = 0.85 – 0.0015 30 = 0.805 0.67

= 0.97 – 0.0025 30 = 0.895 0.67

= , 

= MPa

Compression force of compression rebars plus compression concrete is equal to tension force of tension rebars. Therefore,

+ =

Among above equation, is compression rebars’ area deducting the concrete area where the compression rebars occupy. is used because the elastic moduli between steel and concrete are different.  

Hence,

0.85=

0.85,

and =108.46mm is got.

= = = 324.4MPa  <  fy = 400MPa

Therefore, the compression rebars do not yield.
If there are three 30M compression rebars in this example, is calculated 282.2MPa. The trend is that with more compression rebars, becomes lower, and the compression rebars are getting further from yielding.

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