Roof Live load (Lr) vs. Live Load (L) in ASCE 7 load combinations, cranes supported from roof

[fne]

Existing structure has bridge cranes suspended from the roof structure (joists). In looking at this to evaluate the joists would you consider the crane load a roof live load (Lr) or could it be considered as a live load (L)?

Reference also the definitions of the two in the ICC building codes, chapter 2.

 

[fne]

To help in understanding question, we are looking at where the crane load should be in the ASCE-7 load combinations.

 

[waytsh]

I treat crane loads as live load (L). Are you using ASD or LRFD combinations? IBC 2012 1605.3.1 Exception 1 states that "Crane hook loads need not be combined with roof live load or with more that three-fourths of the snow load or one-half of the wind load."

 

[fne]

waytsh - being an old f**t, I would use ASD, but the young engineer working on it will probably use LRFD. The discussion was whether you could use the load combination which includes 0.75 Live (putting the crane load here) when combined with the snow and other loads.

We are aware of the exception you have noted in ICC building codes. Based on that we felt the full load (ASD) of the crane would be used in combination with 0.75 snow, but do not need to check the crane load with roof live load Lr. We were looking for any possible way to reduce the crane load.

 

[waytsh]

The IBC commentary indicates that the crane load can be treated as live load so I would interpret this to mean that you can reduce the crane live load per the load combination factors for (L) when used in combination with snow and wind. Keep in mind that this is only the crane lifting capacity that can be reduced. Dead load of the crane, runways, rails, etc. will still need to be treated as dead load.

 

[dhengr]

Fne:
I don’t have the last few eds. of ASCE-7 and its multitude of load combinations, so I’m not thinking of a specific formula, condition or load combination. We have so many minor variations on LL’s, be they wet, dry, cold, hot, with subscripts, superscripts, etc. etc. that it takes almost as much time and computer power to determine the controlling load combinations as it used to take to design the whole roof system. With only a little more of today’s thinking, effort and complexification they could, no doubt, show that every bolt in a given connection was controlled by a different load combination on the greater structure. And then, because of all that complexity, we never even see the forest for all those trees. Things have gotten so probabilistically and statistically manipulated that some guy farting on the other side of the world should be figured into your final bldg. loading, to keep the world energy and mass balance in order.

We have always guessed at design LL’s, with varying degrees of accuracy and certainty and then used factors of safety, etc. to account for the uncertainty embedded in our certainty. The original intent of LL reductions, was the reasonable assumption that a large enough contributing area to the loading of a member would not all likely be loaded to that max. assumed design loading. And, these reductions primarily involved columns and large girders, large structural elements, etc. which gathered their loading from many smaller members over are larger contrib. area; not individual smaller members which didn’t encompass a large enough or diverse enough contrib. area.

Now, you have a bridge crane, hung from stl. roof joists (?, not a common condition), every jst. or every third jst. who knows, you don’t say? Let’s hope the jsts. knew of this loading when they were designed and fabed and erected. And, the sign on the crane shows 10 tons, or whatever, who knows, you don’t say? The crane load, as a LL on the roof system, can not be reduced unless you change the cap’y. limit sign on the crane, because they may try to lift to that cap’y. load. And, this has to do with every loaded stl. jst., their bearing support beams or jst. girders, and the bldg. columns which may be loaded by a crane load in their immediate area. However the load combinations are parsed, and whether the snow is wet or dry or drifted or what-not, or what other roof LL’s you might conjure you can’t reduce this one. That is a total misunderstanding of the entire concept, in part because of the complexity of the cookbook, and then blindly following the recipes.

 

[fne]

dhengr - I feel we are of the same generation. I look at the size of the first ASCE-7 I owned compared to the latest and ....