Concrete Ultimate Limit States

[ncoon]

I have a concrete deadman reference using the following formula for area of steel:
As = M / 1.44 d
M = moment(ft-k)
d = section depth(inch)

Where the 1.44 comes from is not explained. Not being familiar with ultimate design, is this a common coefficient?

Thanks.

 

[Taro]

The usual relationship is:

As=Mu/4d

where:
As=steel area, in^2
Mu=factored moment, k-ft
d=rebar depth, in

You can derive this by assuming the internal moment arm (d-a/2) is 0.9d and fy is 60 ksi in the following equation:

Mu=0.9*As*fy*(d-a/2)/12

That said, I would strongly advise against using any equations you don't understand. Especially if you are "not familiar with ultimate design".

 

[UcfSE]

Notice when you use that equation you need also check bd²=23M_u at f'_c = 4000 psi. The coefficient 23 will vary with different concrete strength.

These equations are approximate and are good for quick checks or estimations. With the calculator and computer power we have available to us I recommend always just using proper equations. At least understand the assumptions that went into your two quick equations and verify them at the end.

 

[ncoon]

Thanks guys,
I'm just trying to make sense of someone's previous work.

I appreciate your replies.