Doubly reinforced beams?

DDP · Apr 22, 2003

How do you determine the moment capacity of a beam with equal tension and compression reinforcement? All the examples I've seen have more tension than compression steel and the beams are broken into two separate beams, but there is no "second" beam here as there is no excess rebar.

(Please disregard why there is equal reiforcement, the beam already exists)

Thanks

whymrg · Apr 23, 2003

You will need to use analysis based on stress and strain compatibility. You will need to go through several iterations to determine location of neutral axis.

If you can yse PCACOL, it will show design strength moment based on your section and reinforcement for P=0 kips.

Good luck.

SRO · Apr 29, 2003

Simplified design:

We know the F(t) of steel (real safe asumption I use is 20ksi, I wouldn't go more than 30ksi). So with the rebars' area of steel we can get the the tension of each each bar, but more important, all the bars in tension.

Tension = A(steel) * F(t)

With a symmetric beam the rebar acts as a moment couple (actually the concrete takes the compression stress from bending) but for simplicity

T<--/---
|
|
X M(max) = T * X (watch your units)
|
|
---/-->T

boo1 · Apr 29, 2003

See:

http://www.strunet.com/concrete_beam/beam05.html

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Doubly reinforced beams?

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