# Finding the allowable uniform load

1. Dec 14, 2009

### mustang1988

1. Determine the allowable uniform load w if the allowable bending stress is 24ksi and
P=15w
Given: I=12,800in^4; c=16.965in

2. I know that Mmax=$$\sigma$$max/c(I); And Mmax=wL$$^{2}$$/8

3. I solved for Mmax=18,107.869 k in; but I dont understand how to solve for w with the two P forces also. If somone could help me with this part I would greatly appreciate it. Thanks

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2. Dec 14, 2009

### pongo38

some of the moment capacity is used up by the moment of 3P in the central portion of the beam, thus leaving less available for the uniform load. I suggest you first draw the M diagram for the whole beam.

3. Dec 14, 2009

### mustang1988

Thats supposed to be a 3ft from P to the edge, sorry my drawing skills are not great. I already did the M diagram that is where i came up the max of 575. I just dont know how to solve for the w.

4. Dec 15, 2009

### pongo38

Maybe you are missing the point that p=15w. Therefore M isn't just wL2/8 but also 36x15w=540w k.in at the centre

5. Dec 15, 2009

### mustang1988

O ok, so M would be equal to wL2/8+540w?

6. Dec 15, 2009

### pongo38

Yes, but be careful to use consistent units throughout. Like kip inches, or whatever.

7. Dec 15, 2009

### mustang1988

ok thanks for the help