Live Load Reduction

[OHIOMatt]

I have a project were I am designing steel trusses that span 80'. The trusses are spaced at 16' o.c. so the total tributary area for the truss is 1280 square feet. The trusses support purlins which will be placed at 2'-0" o.c. The pulins have a tributary area of only 32 square feet meaning that they are designe for 20 psf.

For webs that are perpendicular to the chords, should they be designed for 20 psf or can they be designed for 12 psf based upon the total area of the truss? It seems logical that they should be able to support the reaction of the pulin but I just wanted to get confirmation that I am not over thinking this.

 

[ToadJones]

I dont't think using live load reduction on one part of the truss and not on the whole thing is a good idea.
Trusses have a lot of hard-to-determine secondary forces relating to the relative stiffness of truss....stiffening a member here and there, can drastically change load paths in a second order analysis.
Of course you are probably using software to model these effects so I'll shut the hell up

 

[steellion]

I agree with most here in saying that while it's pretty clear that ASCE allows you to design roof live load members below 20 psf, I wouldn't. No way I could justify the construction live load being less than 20 psf, so unless you're planning to shore the roof trusses or you're going to be on site 24/7 to make sure proper construction protocol is followed, stick with 20 psf and sleep a little easier at night.

 

[JAE]

Well...should I be nervous? I've designed a 9.5 acre roof at 12 psf.

 

[woodman88]

JAE, I would not be nervous (I have used the live load reduction many times, for floors and roofs). The building codes do not consider construction loads (See the IBC 2009 Section 1602 and ASCE 7-05 section 4.1 “Live Loads...do not include construction or...”). The contractor is responsible to build the structure per the design loads or hire an engineer to design support for increase construction loads. The most typical example from me is contractors using a higher strength concrete so they could start the walls and floors framing early. But that was when construction was booming.

Garth Dreger PE
AZ Phoenix area

 

[hokie66]

Don't you guys have wind to consider? Where I am, we never have to worry about roof live load, as the wind cases always control.

 

[msquared48]

No comment...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[hokie66]

So if it is not construction load, just what constitutes a roof live load of 20 psf? Hail? If so, I wouldn't reduce that.

 

[paddingtongreen]

The LL provisions are provided on the basis that there will be pockets of high loading in a general area of zero loading. The loading on any element must be considered "real" because the local high loading can be concentrated upon it. The immediate supports for it must be capable of supporting it.

Any internal truss member that solely supports a low tributary area member, has that same tributary area by definition.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[woodman88]

hokie66, Hail is a environmental load. Per the IBC 2009 and ASCE 7-05 "LIVE LOADS (ROOF). Those loads produced (1) during maintenance by workers, equipment and materials; and (2) during the life of the structure by movable objects such as planters and by people."

Garth Dreger PE
AZ Phoenix area

 

[hokie66]

So for those of us in other countries not familiar with the IBC and ASCE documents, I wish someone would please explain a bit more about this.

1) Hail being an "environmental load", how much hail do you have to design for? Does that vary with location? Generally, areas without snow have the most hail.

2) For live loads, I understand the maintenance requirements, but planters and people? Would this not be only for a trafficable roof?

For comparison, in Australia the prescribed roof load for members supporting large inaccessible roof areas is only 5 psf. The loading goes up as the area goes down.

 

[asixth]

Live load reduction is a reduction in live load that contributes to the actions on a member. If the roof design load is 20psf and you are considering the compression in a web member that is located near the mid-span than I would not use live load reduction for the compression in that member. As you say the purlin reaction on the truss would be the correct load.

For web members that are located near a support which have much higher compression forces due to the accumulation of roof load that I would see it applicable to use area reduction.

I am overseas so will not comment on the use of UBC/ICC/ASCE or whatever code it is that governs this design.

I am actually surprised by the magnitude of the live load (20psf). As hokie mentioned, in Australia roofs which cannot be used for floor-like activities have a minimum design load specified in the codes as 5psf. Generally wind will be several times larger than this.