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Another Truss Reaction Question
4

Another Truss Reaction Question

Another Truss Reaction Question

(OP)
Truss reactions provided are as shown. What is the RL? I am thinking lateral, but not 100%. W=5"8 is that the bearing width?(2x6 top plate)

RE: Another Truss Reaction Question

R is applied load ?
U is distributed load ?
what are the two 129 lbs loads ?

another day in paradise, or is paradise one day closer ?

RE: Another Truss Reaction Question

(OP)
R is the total reaction in my mind.
U is the total uplift in my mind.
RL is what I am asking
The 129# load is another issue/question i will have to figure out.

RE: Another Truss Reaction Question

can you make sense of the two numbers ? 251, -263 ??

the other "R" is the RH reaction, and uplift ?

what does the 3rd digit on the dim'ns mean ? ... 9'9"12 ... nine feet, nine inches, ... ? a reference number ??

W is ?

another day in paradise, or is paradise one day closer ?

RE: Another Truss Reaction Question

(OP)
rb1967, that is what i am asking.
R i am taking as the full reaction downwards, DL+LL+WL. Left one is left reaction, right one is right reaction.
The dims are in 16ths. So 9'-9" and 12/16"
W I am taking as the bearing width below. 5"8=5.5"

RL is what I am trying to confirm. Makes sense a left acting and right acting lateral load.

I hate truss drawings.

RE: Another Truss Reaction Question

Usually will list total reaction at each support. In my experience, R would be the the left bearing, U might be a lateral load at bearing #1, and RL would be reactions at bearing #2. The truss diagram should be labeled with each node having a label.

RE: Another Truss Reaction Question

(OP)
@ ChorasDen, see the picture above, thats what I got. If you look, there are two sets, one set not having the RL values. So I confident its the R=Vertical Reactions, U=Uplift, but RL would make sense to be the lateral, acting left and right when looking at the truss.

RE: Another Truss Reaction Question

(OP)
Found this from a Alpine truss drawing, so hopefully applies to mine. Says Alpine Job Designer at the top.

Maximum Reactions
• R = Maximum vertical reaction from a gravity load case.
• U = Maximum uplift reaction from a wind load case.
• Rw = Maximum downward reaction from a non-gravity load case (e.g. Wind or Drag load).
• Rh = Maximum horizontal reaction from a gravity load case.
• RL = Maximum horizontal reaction from a non-gravity load case (e.g. Wind or Drag load).
• W = Bearing Width

RE: Another Truss Reaction Question

is it saying -263<RL<251 ? It sort of makes sense to have only one lateral reaction(other side roller) though I suspect in reality both sides react 1/2 the load.

another day in paradise, or is paradise one day closer ?

RE: Another Truss Reaction Question

(OP)
I agree, I think its based on wind, so with geometry it could differ.

RE: Another Truss Reaction Question

Depends on direction of wind load, you'll have a peak load of either -263lbs or of +251 lbs. These occur non-cocurrently.

RE: Another Truss Reaction Question

So the RH side (unhelpfully dim'ned on the LH side of the sketch !?) is 126" *2 = 252 (close to 251)
and the LH side is 136.625" *2 = 273 (sort of close to 263 ?)
ie if they are lateral loads, then they're sort of a ratio if the respective heights.

another day in paradise, or is paradise one day closer ?

RE: Another Truss Reaction Question

Is this supposed to be a 263 pound drag truss? Never seen horizontal reactions called out by truss designer unless we call drag truss, even then I only see the loading side of it I suppose. Does it really make sense that a wind reaction would have different magnitudes depending on the direction for a truss? The 'windward' and 'leeward' are reversible for the analysis so wouldnt it make sense just to say +/- the maximum reaction in that case?

RE: Another Truss Reaction Question

If this is is wind controlled, could be due to the different boundary conditions of the truss that give the output a +251/-263 lateral load, depending on load direction. FEA truss software can be tough to decipher, but I particularly dislike that they almost never provide a reaction output based on load type. Reactions that include: dead, live, snow, and wind is infinitely more useful than a flat reaction value of 3000lbs, or whatever it may be on any particular analysis.

RE: Another Truss Reaction Question

Well there's no tail on the right side top chord, so maybe that little extra causes the 12# difference windward/leeward depending on direction.

RE: Another Truss Reaction Question

(OP)
@ rb1957 I am not sure where those numbers are shown....Are you taking the dimesions and multiplying by 2 to get the lateral load?

RE: Another Truss Reaction Question

(OP)
Looking more thru the truss submittal, they call out ridged surface at the bearing reactions. One of the Girder truss is in the order of 1K RL listed on the truss drawing. It falls on a beam, so i am figuring I should put 1K lateral load on the beam too. Not sure if that's normal, or most ignore it.

RE: Another Truss Reaction Question

Are you designing the building, reviewing the trusses, what is your role here? Normally it is the EOR who calls for a drag truss, and they would typically have a good reason for doing that. When I specify a drag truss there should also be details, or notes or something that indicates how it is connected to other elements. If I didn't care about how it was connected, then why would I call for a drag truss ?

I am thinking a drag truss is used to get loads from diaphragm to a vertical element. Or drag loads up into the diaphragm to pass them over to a vertical element.

I suppose it just depends how it is detailed and how the structural system is intended to work. Seems unlikely that one would want to use a beam in weak way bending to distribute lateral loads around but it does happen. Even less likely that vibration of the mass tributary to the middle of a beam needs to be restrained by a drag truss. The drag truss can serve many roles depending on how we spec it and detail it. For instance, you might not be dragging any load in the bottom chord at all. Or you might drag load from bottom chord to top.

RE: Another Truss Reaction Question

(OP)
Im designing the beams supporting the trusses. I have the truss submittal to get reactions, etc. All the trusses call out a 'RL' reaction. Its a residential structure. If go by the Alpine definitions above, the RL Is the wind load lateral load. Its a 11/12 pitch so there would be some horizontal load.

The Architect is the EOR if that helps any. A mess all around.

RE: Another Truss Reaction Question

I would assume RL is lateral but would be managed by the roof diaphragm so your beam only needs to resist up/down loads.

RE: Another Truss Reaction Question

I was trying to show that the assumed lateral loads seem proportional to the projected width of the roof.

I'd minimise the assumptions and ask the guy who did the work "what the eff do your numbers mean?"

another day in paradise, or is paradise one day closer ?

RE: Another Truss Reaction Question

RL probably means "Roof Live Load"

RE: Another Truss Reaction Question

Quote (TheRick109)

RL probably means "Roof Live Load"

Why would roof live load be both positive and negative?

"Reaction, Lateral" seems to make the most sens to me.

RE: Another Truss Reaction Question

Since we are all guessing I had nothing to lose LOL

RE: Another Truss Reaction Question

Guess I'm a bit late on this one.

As you already figured out, it's a horizontal reaction from a wind load case. I don't see any reason to think it's from a drag truss load.

FYI - A truss designer doesn't set up the wind load cases. We pick out the code (like IRC 2018) the MPH, and the exposure. The software sets up several wind load cases.

And I can tell you from experience they don't really effect the design of the trusses themselves very much. It might add CLBs in the webs of really long trusses. In very tall, steep pitched trusses it might possibly change a plate size. But not often.

Mainly it produced an uplift value on the truss drawings. We send a packet of truss drawings to jobsites with every load. Which the framer promptly throws in the trash.

In cases where tie-downs are installed, they're rarely installed correctly. So to me it's largely a waste of time to have wind loads. I realize things might be different in areas where they have building codes and inspections. But not where I work.

RE: Another Truss Reaction Question

Ron - here in (or rather on the edge of) hurricane country, we take those things a bit more seriously. If an inspector walks on a job site with trusses and the truss package isn't there, he'll shut them down until they get them with the approved permit set (and they'd better have a permit approval stamp on them, too). Permits aren't approved without finalized truss packages. Further south they're even more strict (and rightfully so).

The hurricane ties are another story. I agree they are often installed incorrectly - either the clip itself or the load path. Everyone loves to install them on the interior, but have no additional hardware. If the truss span is less than about 24' that might still be okay for some of the building, but not typically at the corners. And even if they put them on the outside, nobody really understands what is required to call the sheathing an uplift load path...especially if it's doing double duty as a shear/braced wall.

RE: Another Truss Reaction Question

Another thing I have noticed, and Ron correct me if I'm wrong, is that truss software doesn't typically account for actual wind zones rather the whole truss is designed for a zone, ie Zone 3 is typically a small L shape in the corner, but the whole truss may be designed for that, so IMO many uplift reactions are conservative.

RE: Another Truss Reaction Question

phamENG - I totally get that in hurricane country things are and should be different.

Although even there I wonder. I've read reports of inspections of houses after hurricanes. Some of the reports said the roof sheathing was only held on by 2 nails per truss. Shingles only had 2 staples in them. Plates only had bolts in them every 40' or so. (All of this is going from memory, so it may not be 100% accurate)

Designing the trusses for wind won't help if you can't keep the plywood on the roof.


Aesur -

I can't speak for all designers. Our software has an option to "design all trusses for end zones". I keep that checked all the time. And I teach newbies to do it that way.

Even at that - If our gravity loads are 50# (30 PSF live and 10 PSF dead) - Wind loads may only be a fraction of that. So if doesn't affect the design of the truss much.

RE: Another Truss Reaction Question

Quote (Redneck Ron)

Even at that - If our gravity loads are 50# (30 PSF live and 10 PSF dead) - Wind loads may only be a fraction of that. So if doesn't affect the design of the truss much.

...except for the 0.6DL + W load case

RE: Another Truss Reaction Question

I agree that wind rarely controls strength of members and usually has more to do with connections and load paths. Once you use that 1.6 duration factor, even really large wind loads begin to look a bit like regular live loads.

And yes, lots of buildings are poorly built. Most of those reports are likely about houses that got ripped apart. Those aren't the sort that I design. Robust load paths are the name of the game.

RE: Another Truss Reaction Question

The last mystery number, like 5"8 is 5 inches and 8 sixteenths.
The number is how many 1/16 of an inch. So 3/4 inch would be 12 for example.

That is how prefab wood truss manuf. do their shop dwgs.

Also, there should be no horizontal reaction. You need to call your truss shop and tell them to design the
truss with one end as a roller. They have basically modeled it incorrectly, which means that truss design is not
how reality is going to work.

We do this because it is near impossible to resist horizontal force from a truss with a tall spindle of a wall or column.
You cannot do it. The wall will bow out, the bottom chord will go into tension....

Prefab wood truss designers, despite good intent, are sort of a dangerous group. They know just enough to get themselves into
a real bind....like the guy who did your truss without rollers. They think they are engineers, but are not. I'm not saying
this to offend anyone. It is the truth.

We review their work closely for this purpose.

RE: Another Truss Reaction Question

Quote (bigmig)

Also, there should be no horizontal reaction. You need to call your truss shop and tell them to design the
truss with one end as a roller.

There has to be a horizontal reaction, otherwise the roof would slide off the walls whenever it was windy. If you have a pin and roller, the pin will pick up the entire horizontal reaction.

RE: Another Truss Reaction Question

Quote (CrabbyT)

If you have a pin and roller, the pin will pick up the entire horizontal reaction.

Actually, the roof diaphragm picks it up but how to model that? I suppose pretending you have a pin at the bearing is fine for most analyses until the software gets more sophisticated.

RE: Another Truss Reaction Question

XR - true, but it's nice to have it dumped in one spot for easy review. You can take that number, add the wall pressure to it, and verify the diaphragm loading from the original design.

RE: Another Truss Reaction Question

@CrabbyT I see your logic, but XR250 is correct. The roof acts as a diaphgram and transfers lateral load to the shearwalls. That is
why the roof does not slide off during a wind event. The shearwalls hold it in place so long as there is an adequate drag strut connection
between the roof and the wall.



RE: Another Truss Reaction Question

Quote (XR250)

Actually, the roof diaphragm picks it up

Since it's a flexible diaphragm, is it valid to assume that the wind would only get transferred to the (2) exterior side walls? Wouldn't the lateral force get transferred to the wall that the trusses are pinned to? My understanding of diaphragms wasn't that that directed the load to the outside of the building so much as they distributed lateral forces to the elements in proportion to the relative stiffness of those elements. I would contend that the side walls would have a lot more stiffness than the walls perpendicular to the force, but in my mind, it seems unconservative to neglect the lateral force acting on the perpendicular wall.

What I'm saying is, why not design the perpendicular wall to take up the lateral reaction of each truss, and why not design the side walls to half the total lateral reaction of the entire roof?

I just did a desktop experiment by making a house with paper walls and a paper roof. The roof has 3 "trusses" (they're actually just triangles) and a diaphragm. If I push on the center of the roof (directly on the center truss truss), the side walls take up enough force to cause the rear wall to buckle, but the front wall still deflects.

RE: Another Truss Reaction Question

CrabbyT - in a hyper realistic sense, sure, it'll transfer to elements based on their relative stiffness. But in light frame wood construction, who has time to design like that? It's a flexible diaphragm, for distribution to shear walls assume the diaphragm has zero stiffness and the load is distributed based on tributary area alone. Then look at diaphragm as a structural element with reactions at those locations to determine shear in the diaphragm. But the lateral load is taken by the diaphragm to the shear walls - often the exterior walls.

Your model is neat, but the scale and materials make it invalid. The paper lacks the ability to simulate wood framing, and the proportions of your little house don't match the reality of any building I've ever designed.

All of that said, a positive connection of the roof truss to the wall is important. It should be fixed to prevent lateral displacement at both ends. The 'roller' effect comes not from actually releasing the truss reaction, but from the lack of lateral resistance/stiffness in the wall that is pinned at the foundation and pinned at the truss. The reaction shown on that truss diaphragm is more about the load delivered to the LFRS and not the load delivered to the wall. The wall delivers load to the truss on its way to the diaphragm.

RE: Another Truss Reaction Question

Quote (phamENG)

The 'roller' effect comes not from actually releasing the truss reaction, but from the lack of lateral resistance/stiffness in the wall that is pinned at the foundation and pinned at the truss. The reaction shown on that truss diaphragm is more about the load delivered to the LFRS and not the load delivered to the wall. The wall delivers load to the truss on its way to the diaphragm.

This makes sense. It's like designing a 100' long steel box truss supported by towers. You put a 4-legged tower (a pin) at one end and a 2-legged tower (a roller) at the other.

In case someone Googles this thread in the future (and for the embiggenment of my own knowledge)... For the purposes of designing shear walls (let's imagine the 2 exterior side walls for simplicity), would the following equation give the total force on one of the shear walls?

Fshear = [RL * (ntrusses - 1)] / 2

where RL = the lateral truss reaction and ntrusses is the number of trusses? Subtract one truss because the end trusses only have half the tributary area of the other trusses.

RE: Another Truss Reaction Question

Only from the roof. You still have to add in the shear from the wall that's connected to the truss. Load path is wall->truss/blocking->diaphragm->shear wall->foundation.

And I'd probably go with Fshear(roof)=(RL/s)*L/2 where s is the truss spacing and L is the length of the roof. Should be the same result, but it fits my thought process a little better.

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