Load Combination Question/Poll

[MarcbSE]

A question for the structural engineering community: The ASCE/IBC load combinations include a combination under the ASD Basic Load Combinations for D+(W or E)+L+(Lr or S or R). When two or more transient loads are combined in this section a 0.75 factor on the live loads may be used. Now here is the question, how many engineers use this load combination for C&C design of say a rafter or bar joist?

Here is an example that would be typical for my part of the country. Assume a roof snow load of 20 psf (unreducible) and the wind load minimum inward pressure of 10 psf.

S = 20 psf
W = 10 psf
0.75(20 psf + 10 psf) = 22.5 psf > 20 psf for snow

However, I don't know personally of any engineers who would design for 22.5 psf in this case as nobody combines the roof live load with wind.

The typical interpretation I hear is that this load combination is intended for design of elements such as columns where axial load results from the lateral loads.

Do you agree or disagree???

 

[JAE]

Technically you'd use the 22.5 psf. For a bar joist, the wind+snow would be checked (for the typical joist tables) as a live load.

 

[DaveAtkins]

On a bar joist roof, I don't think wind and snow would ever be additive--the wind would typically be uplift.

DaveAtkins

 

[JAE]

Depends on the roof - there are wind combinations in ASCE 7 that require a downward wind pressure - primarily on windward slopes.

 

[woodengineer]

The multiple tranient live load reduction is used quite often in designing roof trusses. You can see the advantages especially when designing attic trusses. It make it quite difficult to properly load girder trusses with the common trusses taking advantage of the live load reduction. Metal-plate connected roof trusses may also be designed using a hybrid method: the component is designed for C&C and the uplift reactions are MWFRS.

woodengineer

 

[MarcbSE]

Thanks for the comments. The ASCE wind loads have both positive and negative pressures that must be checked. I think many engineers tend to "forget" or "neglect" the positive pressures, but according to the required load combinations these should be combined with the roof live load (Snow, Live, etc.). Does anyone other than JAE actually use this load combination?

 

[Lion06]

I believe the positive wind pressure on roofs is the minimum of 10 psf unless teh roof slope exceeds (or equals) 45 degrees. I would check it for positive wind pressure (not a positive value of the negative wind pressure).

 

[MarcbSE]

StructuralEIT:
Yes, that is what I meant. In the example I used in the orginal post, I combined the 10 psf positive wind pressure with the assumed 20 psf roof snow. Thanks for clarifying that.

 

[DaveAtkins]

Well, you learn something new every day. I see that there is a GCp coefficient for positive pressure on a flat roof. I did a quick example--under 15' eave height, Exposure B, 90 mph wind, Iw = 1.0, and found the positive pressure to be 5.92 psf.

Here is where engineering judgment comes in--when COMBINING wind load with live load, I think you could use 3/4 of the 5.92 psf plus 3/4 of the 20 psf, which is 19.4 psf. And so 20 psf live load would still control. I think wind acting ALONE with dead load would need to be the 10 psf minimum, but then that wouldn't control either.

I suppose if you have a higher eave height, Exposure C, or Iw > 1.0, live load plus wind load would control.

BUT--isn't the 20 psf an ARBITRARY minimum roof live load? If you are in a five psf snow load area, for example, could you combine 3/4 of the snow load based on five psf with 3/4 of the 5.92 psf wind load, and THEN compare that to 20 psf?

Just some food for thought.

DaveAtkins

 

[MarcbSE]

Wouldn't the wind load used in the combination would still have to be 10 psf minimum? Section 6.1.4.2 of ASCE7-05 states that a minimum design wind pressure for C&C shall not be less than 10 psf acting in either direction.

DaveAtkins is correct in that for areas of the country with small roof snow loads this particular load combination would not control. It would also not control for large snow loads as the 25% reduction would more than account for the (net) 7.5 psf wind.

I've always said that the day I stop learning will be the day I die.

 

[UcfSE]

I usually check the positive wind load along with live at the roof. In my area the positive wind load tends to be a bit larger than 10 psf so it becomes important. In the end the difference is not a lot, but it counts for being thorough. I don't agree that truss uplift reactions are necessarily MW, but that can be in another thread, and has been in several.