Another Truss Reaction Question 4

[JStructsteel]

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)

 

[rb1957]

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 ?

 

[JStructsteel]

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.

 

[rb1957]

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 ?

 

[JStructsteel]

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.

 

[ChorasDen]

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.

 

[JStructsteel]

@ 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.

 

[JStructsteel]

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

 

[rb1957]

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 ?

 

[JStructsteel]

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

 

[ChorasDen]

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

 

[rb1957]

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 ?

 

[driftLimiter]

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?

 

[ChorasDen]

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.

 

[kissymoose]

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.

 

[JStructsteel]

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

 

[JStructsteel]

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.

 

[driftLimiter]

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.

 

[JStructsteel]

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.

 

[XR250]

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