Dead Load assumption

[yaroosh]

Hello Everyone,

I have a silly question but it is sort of driving me crazy which is determining roof dead load of steel structure.

Back in college, they will say DL= 12 psf for example. However in my case, I am trying to determine this dead load when I have steel deck and steel frame. My steel deck weights about 3 psf, how can I calculate how much the framing will be if I don't know what are the beam sizes yet? How can I make a reasonable assumption?

Thanks.

 

[dik]

You arbitrarily pick some value 3psf or 5psf... do the calcs and check back to see that your assumption was OK.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[phamENG]

Don't forget about rigid insulation, roof protection board, membranes, gravel (maybe...rare in my part of the world now), ductwork, sprinkler piping, ceiling, roof top HVAC equipment, etc.

 

[dik]

Sorry... I thought he was only referring to the structural steel... typical roof design loads in these environs are about 20 psf TDL and 8 psf for MECS (mech, elect, ceiling and sprinklers.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BridgeSmith]

The beam selfweight should be a minor percentage of the total design loading, and is therefore typically ignored or added as a small percentage increase in the total load in preliminary sizing. If it's not, then your beam is much too shallow and very inefficient.

S required = w*L^2 / (8*Fs)

I would start by finding a beam that has an S about 10% greater than S required for the superimposed load, and try that.

For steel bridge "I" girders, we usually start at a depth to span ratio of about 0.04, since that usually results in a fairly efficient section for our loads. Not sure what's considered an efficient depth to span ratio for roof beams.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[JLNJ]

Roughly size a couple of beams and girders and get yourself in the ballpark. It’s a 15 minute exercise - less time than it took you to create this thread on eng-tips.