Horizontal Reaction from Truss 1

[jimtheengineer10]

Attached is the truss diagram. The existing exterior wall is 2x6 studs @ 16" o.c. w/ plywood sheathing. What is the best way to handle the horizontal reaction? Thanks.

 

[KootK]

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.

 

[BAretired]

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

 

[FSS]

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.

 

[JAE]

Yes, there doesn't seem to be a connection across joint T.

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[KootK]

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.

 

[Triangled]

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..

 

[Signious]

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.

 

[BAretired]

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

BA

 

[jimtheengineer10]

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.

 

[BAretired]

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

BA

 

[jimtheengineer10]

Attached is a picture showing how the 2x8 floor joists are connected to the wall. Does this help or hurt at all?

 

[XR250]

@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.

 

[KootK]

@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.

 

[BAretired]

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

 

[Triangled]

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....

 

[medeek]

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

http://www.medeek.com

 

[KootK]

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.

 

[BAretired]

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

 

[XR250]

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 :>