Roof Live Load 1

[hubba]

I don't work in this stuff everyday, so please forgive me for the elementary nature of this question. ARE ROOF LIVE LOADS APPLIED VERTICALLY (GLOBAL -Y) TO SLOPED ROOF MEMBERS? (i.e. not perpendicular to sloped member?) Seems like they should bc big part of roof live is weight (i.e. vertical load) of various construction incidentals.

However, I get confused bc codes/stds/packages make point of distinguishing horizontal projection vs real length. If it's vertically applied, isn't it always the horz projection? Thanks.

 

[msquared48]

Careful BA. He gets torqued easily.

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

 

[Lion06]

Just a side question, but under most circumstances doesn't a sloped roof member need to be sheathed in which case the longitudinal force goes into the diaphragm, not as an axial force in the member, right?

 

[msquared48]

It can, yes.

But if there are rafters each side of a ridge beam, and a the rafters are tied to the ridge in tension, the longitudinal force is never seen, either to the purlin or the diaphragm.

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

 

[BAretired]

A rafter with a birdsmouth joint at the wall plate and a joist hanger at the ridge beam has a vertical reaction at each end. There are no horizontal forces involved, so the bending moment is the same as a horizontal member of the same span. Reactions can be resolved into components parallel and perpendicular to the rafters to find shear and axial loads. I wouldn't say that the axial load goes into the sheathing, although some might.

Tied rafters do not require a ridge beam. Often, a ridge plate is placed between opposing rafters. Rafters can have a birdsmouth joint at the walls. Ceiling joists must be continuous across the entire span and must be spliced to carry tension from wall to wall. The rafters feel more compression in this case and some of that compression may be transferred to the sheathing, but it may not be prudent to rely on that.

Sometimes, rafters are tied half way up the rafter. This changes the statics considerably.

BA

 

[ToadJones]

I think the confusion here is mostly just semantics.
The code simply states that the live load shall be taken as the horizontal projection.
After some thought, it makes sense. Dead load is there, can't do anything about that. But in the case of a steep roof (say 12:12) it would certainly be more difficult to "load the roof" with live loads especially during construction.
Thoughts?

 

[steellion]

Don't overthink it. Gravity loads always act down (-Y). They have to, the earth is pulling them that way.

Roof live loads and snow act downward on the projected area. Dead loads also act downward, but you need to apply the whole sloped area because that is all actual material with weight. If you've got a 45-degree roof with 10' of rise and 10' of run, on the rafters you would apply [plf weight]*[2/sqrt(2)] on the horizontal projected area.

 

[dhengr]

Hubba:
MiTek and others hold their cards pretty close to their vest when it comes to their design and analysis programs, and it is becoming more and more difficult to do some simple calcs. to verify their controlling reactions and member stresses and sizing, etc. What with all the load combinations, variable wind loadings and nonuniform snow and drift loading, and then add earthquake loadings, and who knows but that a different load combination doesn’t control every single member in the truss. I expect that, in another generation or so of code changes a simple building will be impossible to design and analyze by what we used to call rational engineering methods. You won’t ever get past trying to summerize the potential different load combinations, let alone know how to factor them up or down, and in wood, don’t forget all those adjustment factors also. And, then if you use different programs, you most likely will get different answers, because the various programmers interpreted an un-interpretable bunch of probabilistic babel differently, and none of them is really wrong; but their solutions are maybe more exacter (?) than slide rule solutions were 50 years ago, they just need sixty more pages of printout to get there. And, you needed more time to compile the load combinations now than I did to design the whole roof system. They have ten decimal place answers, that must be gooder, but my building is still standing after 50 years, so who is righter? If that roof structure knew what contortions you and they went through to prove it might stand up under load, it would fall down from sheer exhaustion or sheer exasperation. That’s different than horiz. shear stress, but is going to be included in the next version of the NDS. And, LEED says it’s greener too, when you use more trees to produce the paper for the paperwork and computer printouts than you do to produce the actual roof trusses.

 

[BAretired]

Finally, I think I see what WillisV is saying. If the snow load is 20 psf, the angle is [θ] and the spacing is b, then the load is 20*b per foot of horizontal span, but it is only 20*b*cos[θ] per foot of slope length.

If the dead load of the deck and roofing is 10 psf, then the load is 10*b per foot of slope length and 10*b/cos[θ] per foot of span.

BA

 

[dhengr]

TJ:
Well..... the confusion is actually a good deal more than just semantics. The designer/engineer has to be smart enough to know when he/she needs to finesse the problem or when they can use the simplification implied by the code and our common practice for normal slopes. The confusion has more to do with the proper interpretation of vertical loads ON a horiz. projection (not, “taken as the horiz. projection”), vs. load components perpendicular (bending) and/or parallel (axial) to the axis of the rafter; and the rafter’s horiz. projected length and/or its actual sloped length. You correctly saw that for normal pitch roofs the correction (or error) isn’t too great, only about 6% for a 4:12 pitch. We understand how to deal with floor joists where the DL and LL are both vertical loads (globally) which also happen to be perpendicular to the axis of the beam and the axial loading is zero. That’s right out of our Strength of Materials understanding of beams. I believe that for normal pitches if you use all loads ON a horiz. projection and the horiz. projected length of the beam to calc. moments and shears you get the same results as if using the sloped length of the beam and the loads converted to components perpendicular to and parallel to the sloped axis of the beam, they’ve both been converted appropriately by the cos of the slope angle. Except the math is tedious so we usually avoid it, but it does work out to give the correct reactions, etc., as long as you don’t screw up the trig. conversions, but then we never know what to do with that axial loading since it’s only 6%.

But, what happens when you have a 12:12 pitch (45° slope) or 20.78:12 pitch (60° slope) roof. The snow load which is based on local ground snow loading, on a horiz. projection, slides off the roof; but the DL hangs on the rafter through its connection to the rafter, and the roof diaphragm, and the rafter starts acting as a column member with a fairly small bending component from these loads. Now you are designing a completely different member, not a beam, but a beam/column which might buckle and isn’t even braced along the tension flange. The point is that at some point we are designing a column type member rather than predominantly a beam type member. On heavy enough DL’s you would want to pay attention to this cos conversion, it’s important, as WillisV and BA suggest. Fact is, we add up the DL’s wrt the plane of the roof and then generally apply them directly to the horiz. projection, but I’m not sure I know my loads within +/- 6%.

Long experienced framers seem to know what to do with hips and valley, through years of trial and error (failures). Engineers scratch their heads for days trying to do an exact analysis and finally say close enough, go ahead and nail er down.

 

[Lion06]

BA-

I think I see the point, too.

 

[WillisV]

BAretired - bingo - glad you and SEIT have seen the light =)

 

[BAretired]

WillisV,

Sometimes my brain takes a few days to kick in.

BA