Is there an "industry standard" of 20 psf superimposed load? 1

[Iasonasx]

I just read a comment that there is an "industry standard" of applying 20 psf superimposed load on roofs of steel structures. Is that true? Isn't the whole combination of those factors given in the LRFD system adequate? And if we add another 20 psf, we add it as what type of load? Is it Dead Load, Live Load, Wind Load, Rain Load? Under what category is that superimposed load supposed to fall and how is it factorized? I have to admit that I never heard of it.

 

[phamENG]

Standard roof live load is 20psf, and it is superimposed, so perhaps that's what they were referring to?

 

[HTURKAK]

I do not know any industry standard but i can say that , the superimposed loads for roofs of steel structures are , hvac ducts , cable trays , piping etc.
The following para. from METAL BUILDING SYSTEMS ( by A.NEWMAN )

(Collateral or superimposed dead load is a specific type of dead load that includes the weight of
any materials other than the permanent construction. It may account for the weight of mechanical
ducts, pipes, sprinklers, electrical work, future ceilings, and reroofing.)
...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[Iasonasx]

@ HTURKAK, but these are the live loads that we anticipate on the roof and there is a process in ASCE 7 for the method of calculating Roof live load reduction based on tributary area for Roof Live Loads. So if someone calls them "industry standard" I guess we are getting into this new culture of identifying as something else. Sorry, I am not trying to be sarcastic toward you by any chance. I just find it funny that it was presented to me as "industry standard" or "standard practice" for steel roofs if it really is just part of the most important code we have to follow for every type of roof whether that is steel, concrete or whatever material. So thank you for your input.

 

[HTURKAK]

This is copy and paste from ASCE 7-16 The definitions;

SERVICE LOADS: Loads imparted on a building or other structure because of (1) self-weight and superimposed dead load,
(2) live loads assumed to be present during normal occupancy or use of the building or other structure, (3) environmental loads that are expected to occur during the defined service life of a building or other structure, and (4) self-straining forces and effects.

When you say dead load ,in my zone it will understood as the own weight of the structure . The finishes, HVAC ducts , Elect. trays , fire piping , suspended ceiling etc are SUPERIMPOSED DEAD LOADS .

GOOD LUCK.
....

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[lexpatrie]

To me I see "superimposed" more in precast as dead loads that are applied beyond the weight of the plank, so topping, Carpet and pads, things like that.

To some extent a lot of dead loads are superimposed but the terminology just hasn't drifted into, say, steel joists. Pre engineered, sure. Wood trusses, sometimes but it's not often I see it called superimposed.

 

[DaveAtkins]

My guess is whoever wrote that is referring to the 20 psf minimum roof live load that is used when snow load does not govern.

DaveAtkins

 

[RobertHale]

When I first started working in industry, most of the engineers I worked with would apply a uniform DL of 20PSF and an unreduced live load of 20PSF on roofs. It worked well with gravity design as it was pretty conservative for most light roofs. These engineers also tended not to look at the wind uplift case. Coincidentally, they would also apply a uniform 20PSF wind load to the face of the building and design the lateral system for that resultant force. They would also put on the drawings that wind load controlled the design and the seismic load was effectively ignored. I stopped using a blanket 20 PSF DL after I was witness to a few projects going sideways, and I realized that while it worked well for gravity load, it tended to significantly underestimate the uplift loading on the roof. Since then, I have calculated the dead load based on the specified system components and trusted in the load combinations to provide the necessary conservatism.