Dissecting a Load Combination

[jheidt2543]

From the IBC 2000, Formula 16-10 load combination:

D + (Lr or S or R) + L + (W or .7E)

Given: D = 20 psf, Lr = 20 psf, S = 20 psf,

An industrial building with a cab operated overhead crane. When looking at the load combinations that include seismic loads, we take the building weight and we take the weight of the bridge crane mechanism (trolley & bridge, but no lifted load) to figure out the seismic shear force, vertical and horizontal for the building as a whole.

Now, look at an individual column supporting the building and the crane (where in this case L = the crane loading on the column; vertical, lateral and transverse).

First, I consider the entire crane load as live load, the trolley, the bridge and the lifted capacity + impact from the single column’s point of view for vertical, lateral and transverse loads.

Secondly, when considering seismic, it seems that the column sees it’s portion of the building seismic load and, when the crane is on the column line in question, ALL of the bridge crane seismic load (trolley & bridge, but no lifted load) divided between the two columns in the line (assuming a clear span crane bay).

So, the combination for the individual column would look like:
D + .75*(Lr + S + L) + .7(E building + E crane) where E crane = total crane mechanism weight used in the seimic base shear equation.

Is this how you all look at it too?

 

[kslee1000]

I agree, for light duty crane it (dynamic analysis) is not warrant. However, for heavy duty crane tied to the building envelop, dynamic analysis would yield more realistic responses as suggested. But down to the bottom line, building code assumption is controlling and conservative no matter what type of analysis was employed. So, you were correct in this sense.

 

[DaveAtkins]

jheidt2543,

Sorry I confused you. You are applying the 0.75 factor correctly. My point is the 2000 IBC has an error in it, which says you cannot apply the 0.75 factor to the 0.7E. The 2006 IBC corrects this, and the load combination becomes D + 0.75(Lr + S + L) + 0.75(0.7E).

DaveAtkins

 

[jheidt2543]

Dave:

OK, I got it, thanks again. In my region, seimic rarely controls over wind, but with .75*.70 = .525 it is even less important - the Building Inspectors still want to see it though.