Roof Live Load
Roof Live Load
(OP)
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.
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.






RE: Roof Live Load
If you look at it simply as the direction that loads are acting, then gravity loads are globally vertical and downward.
Pressures act normal to the surface.
so, say for a purlin design on a sloped roof, gravity loads would cause bi-axial bending in the purlin, but pressure loads would cause only uni-axial bending.
agreed?
RE: Roof Live Load
RE: Roof Live Load
RE: Roof Live Load
RE: Roof Live Load
don't do it man, I'm serious!
If BARetired starts making diagrams about torsion, I'm outta here!!!!
RE: Roof Live Load
You would multiply the live load by cosine of the rise to run angle to get the vertical load acting along the slope.
You multiply that value by cosine of the rise to run angle to get the component of load acting perpendicular to the member, or by sine of the angle to get the component acting parallel (axial force) to the member.
So...in summary:
Component perpendicular to member: LL(cos theta)^2
Component parallel to member: LL(cos theta)(sine theta).
where theta = rise to run angle in degrees.
RE: Roof Live Load
Say you have:
A truss with a 4:12 pitch, a 10 ft tributary width, and live load of 20 psf.
Say you want to model this truss with a uniform distributed load acting in the Global Y (down) direction, then you are saying that load would be:
rise = 4'
run = 12'
hypot.= 12.65'
vertical load (12/12.65)* 20psf * 10' = 189.7 plf?
And, from here you'd break that into forces perpendicular and parallel to the top chord?
RE: Roof Live Load
RE: Roof Live Load
I honestly never interpreted in that way.
I guess it is a good thing most of the roofs I have done are 4:12 and under.
Even at 4:12 it seems to only represent about a 6% change.
RE: Roof Live Load
For a detailed examples of the Sloping Beam and Horizontal Plane Methods of analysis see pages 2.19 to 2.21 of the 6th edition of Design of Wood Structures (ASD/LFRD) by Donald E. Breyer, et al.
Basically the two approaches yield the same results (monents & shear).
RE: Roof Live Load
RE: Roof Live Load
It is particularly applicable in Hip and Valley member design.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof Live Load
with a 4:12 pitch and 10' trib width with 20 psf LL why wouldn't the line load simply be 20 psf * 10' = 200plf ; then use that to determine the load acting perpendicular to the beam (causing bending) and in line with the beam (causing axial load)? The 10' trib isn't sloped so the horizontal projection is 10'.
RE: Roof Live Load
RE: Roof Live Load
If you have a beam that spans horizontally 10' with a trib width of 10' and a LL of 20psf and is flat, the vertical reaction at each end is 1000#. Of you keep the horizontal span of 10', but offset the ends vertically by 3', the vertical reaction at each end is still 1000#, no? In both cases the horizontal projection is 10'.
RE: Roof Live Load
RE: Roof Live Load
You can get correct moments without dealing with this, but this is the only way to get correct axial forces.
RE: Roof Live Load
Dead load is also a gravity load, but if the dead weight of deck plus roofing materials weighs 'w' psf and the slope is θ then the dead weight on plan area A is w*A/cosθ.
BA
RE: Roof Live Load
Sorry to disappoint you. No diagrams about torsion.
BA
RE: Roof Live Load
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof Live Load
RE: Roof Live Load
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
RE: Roof Live Load
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
RE: Roof Live Load
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?
RE: Roof Live Load
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.
RE: Roof Live Load
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.
RE: Roof Live Load
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/cosθ per foot of span.
BA
RE: Roof Live Load
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.
RE: Roof Live Load
I think I see the point, too.
RE: Roof Live Load
RE: Roof Live Load
Sometimes my brain takes a few days to kick in.
BA