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Horizontal Reaction from Truss

Horizontal Reaction from Truss

RE: Horizontal Reaction from Truss

You don't want to attempt to restrain the horizontal truss movement although, at wall corners, it's inevitable. I'd stiffen the truss to get the movement down to TPI recommended limits and then do what the rest of us do: stick your head in the sand and hope for the best. That's basically the crux of this industry article on the subject: Link. Loads lighter than anticipated, restraint from sheathing... yada yada.

If I remember correctly, the TPI limits are 0.75" under LL and 1.25" under total load. Your truss is goofy in the middle. It seems to be made in halves not connected at the bottom and has some extraneous horizontal webbing. Do you know why?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Reaction from Truss

The bottom chords are not connected by a truss plate which means that horizontal reactions are necessary. It would be much better to use a scissor truss and let the top of the walls move as dictated by the load and the truss geometry, as KootK suggested.

BA

RE: Horizontal Reaction from Truss

That's not a truss. That's two trusses leaning against each other. I would have the truss guys redo until they come up with something that makes sense.

RE: Horizontal Reaction from Truss

One truss run reflects movement and no reaction; the other restraint and no movement. Are those different options or different project conditions?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Reaction from Truss

Probably being manufactured in two pieces to render it shippable....
Didn't run/check the math, but looks like they supplied design forces for field connection at top and bottom chords,
Webs aligned to facilitate field connections..

RE: Horizontal Reaction from Truss

Going to mirror what others are saying and suggest you throw this back at the truss supplier.

It looks like it covers a fairly long roof (16 trusses total) so if they really want to have it constructed in halves, maybe suggest having a parallel chord girder truss run down the middle with skew hangers to hold the trusses.

RE: Horizontal Reaction from Truss

That's a good idea, Signious provided there can be a support at each end. The girder truss would be spanning 36'-0'.

BA

RE: Horizontal Reaction from Truss

(OP)
The girder truss option makes sense but there is a large fireplace directly in the middle so the column supporting the girder truss would fall right in line with fireplace. The owner wants to keep the peak the same as the other half of the building and wants to maximize headroom inside so that is how the truss guy came up with the truss he did.

RE: Horizontal Reaction from Truss

How do you propose to connect the two trusses at Joint T?

BA

RE: Horizontal Reaction from Truss

@Kootk;

Good article. I have questioned Simpson's engineers on their slotted clip Link
and asked how the wind load on the wall gets into the diaphragm. They could not give me a good answer. Clearly, they are more marketers than building designers.

RE: Horizontal Reaction from Truss

@Jim: neutral. If you beefed up your connections, you might be able to use the ceiling jousts as collar ties.

@XR250: frankly, I'm surprised that such a product even made it into production. Even if we didn't care about wall restraint, I'm sure that friction renders is ineffective.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Reaction from Truss

As I understand it, there are no ceiling joists. The bottom chord of the truss forms the ceiling line. Collar ties could be added but that would not maximize headroom.

There is no way the wall can be constructed to resist large horizontal forces. Perhaps the best solution is to reduce the slope of the bottom chord and tie the trusses together at Joint 'T'. The geometry of the trusses could be adjusted to render the horizontal deflection acceptable.

BA

RE: Horizontal Reaction from Truss

Agree on Simpson clip, I don't think it works.

Wondering on truss if, instead of reducing slope on bottom chord, the slope on the top chord was adjusted towards parallel to bottom chord, retaining ridge height at exterior and interior, kinda like two parallel chord trusses, creating a larger moment of Inertia and reducing deflection..... messes with architectural lines though...

photo is confusing to me as it looks like a ceiling at the roof line, but then, why would this discussion be occurring... if this is a second story addition, then the exterior walls at the new trusses are not existing....

RE: Horizontal Reaction from Truss

Notice the plate at joint H, is this plate to be installed after installation of both halves of the "scissor truss"?

This is the weirdest scissor truss I've ever seen. As mentioned above there is not connection at joint T, the truss will split apart at this joint.

A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com

RE: Horizontal Reaction from Truss

My guess is that the odd horizontal webbing members are intended to receive some manner of field applied tension strap. Naturally, slip in that connection would exacerbate the calculated horizontal deflections.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Horizontal Reaction from Truss

I think the plate at joint H is actually two plates, 6x8 MT 18HS so the two half trusses are separate.

The horizontal forces at top and bottom are 5240# and 5948# respectively which are to be resisted by a field connection of some sort. The detail of the connection is not provided.

BA

RE: Horizontal Reaction from Truss

funny, don't the truss drawings typically call out a special connection is req'd to resist "X" amount of load?
Otherwise, it might just get toenailed together :>

RE: Horizontal Reaction from Truss

Looks like it's 20 feet tall and 48 feet wide to me, hard to ship in one piece in the areas I'm familiar with

RE: Horizontal Reaction from Truss

Note 11. covers the responsibility for connection design:

11. All additional member connections shall be provided by others for forces as indicated.

That would include the horizontal force at Joints H and T and the reactions at Joints B and N.

BA

RE: Horizontal Reaction from Truss

This truss doesnt look right. Members too skinny for dimensional stability? Multiple ply trusses maybe? Increase to LVL chord members maybe to get some EI out of it... collar ties possible to prevent this thing from doing the splits? If not... is it possible to rely on the wall top plates to span horizontally as beams or bedf them so they can? ... would need some serious restraint of top plates if they need to span like that...or install an external buttresses to keep walls from rotating out?

Either way the truss seems spindly as they have it shown...

RE: Horizontal Reaction from Truss

They are single ply trusses spaced at 2'-0" centers. The 2x6 top chords are considered continuously braced by roof deck. Apart from the rather unusual condition that the field connection at joints H and T are by others, the trusses appear to be typical of practice in North America.

The doubled top plates cannot span horizontally as beams over a 38' span. External buttresses are theoretically possible but probably not acceptable by the owner and likely not economical.

The Engineer of Record has to come up with a clever solution to the connection problem and perhaps modify the truss geometry in order to reduce horizontal deflection at the supports.

BA

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