Unbraced length for LTB - beam design 2

[ndrstruct]

Hi, i am currently designing a long lvl beam for one of my client. The beam is 24' long and is supporting roof trusses spaced at 24" o.c

The beam is 2-ply 20" deep lvl. The trusses are hanging at the top of beam as shown in the image.

1. I am wondering whether i can make use of the connected trusses for lateral bracing for the beam at top (compression side).
(so the unbraced length will be 24" at top).

2. Also i would like to know if i am connecting the trusses at the bottom (tension side) of the beam, and providing a 2x4 brace connecting top of beam back to the
top chord/web of the trusses ? will it be efficient?

3. I think in both the cases, the trusses need be checked for lateral loading from beam compression effect? is this required?

I would appreciate your responses.

 

[DaveAtkins]

1. Yes, the beam is braced against LTB by the trusses.

2. Yes, if the trusses connect near the bottom of the LVL, but you provide other bracing to the top of the LVL, the LVL is braced against LTB.

3. This is not normally checked. The load is typically small enough to be ignored.

DaveAtkins

 

[ndrstruct]

Thankyou for your response

 

[JAE]

DaveAtkins answers well here - but I might point out that I would NOT rely on the trusses - as sketched - to provide lateral resistance.

The trusses, most likely, need to be fully sheathed to provide lateral stability here...unless the trusses on the other end are secured to something rigid vs. a top of a framed wall or something.

 

[DaveAtkins]

Good catch, JAE,

I was assuming the trusses are fully sheathed.

DaveAtkins

 

[ndrstruct]

Any comments on this detail? blocking to be toe nailed to beam and face nailed to truss (top chord)

 

[XR250]

Should we open up the thread about loading the LVL below the neutral axis again?

 

[ndrstruct]

Hi, I have checked the hanger for loading below the neutral axis and is fine by simpson hanger selector

 

[lexpatrie]

Is there some established body of knowledge that when the LVL beam buckles in LTB that it always goes toward the brace so the brace always goes into compression, otherwise, toenails in tension? Sheathing in tension? Further, while this may deal with CL, as I recall the min of CL,Cv applies to LVL as well as glulam, so you may get a lower allowable stress anyway even if it's fully braced because it's so slender.

 

[ChorasDen]

I have checked the hanger for loading below the neutral axis and is fine by simpson hanger selector

I'm not sure you are fully grasping what XR250 is talking about. Please indicate where in the Simpson hanger selector that tension perp is addressed in their calculation for the carrying member.

 

[ndrstruct]

I don't either think it always goes in for compression and toe nailing has some withdrawal capacity. And yes the min. CL, Cv applies and is considered for design.

Yes, It has an option to check for loading perpendicular to grain and below neutral axis, since the members are week for tension perpendicular to grain. For hangers connected flush to bottom side of carrying member. They check this as well and allowable loads are reduced depending on the capacity of carrying member.

Thankyou

 

[ChorasDen]

It has an option to check for loading perpendicular to grain and below neutral axis, since the members are week for tension perpendicular to grain.

I don't see this option on the online tool, but I would be interested in exploring it more. Can you please post a picture showing where this option is located?

 

[ndrstruct]

In hanger selector, please go to flush bottom option. There you will see a option to turn on ANSI/TPI Evaluation which is technically used for the calculation of tension perpendicular to grain.

 

[ProgrammingPE]

The ANSI/TPI 1 (National Design Standard for Metal Plate Connected Wood Truss Construction) requirements for tension perpendicular to the grain (Section 7.5.3.2.1 in the 2014 edition) require connections to extend past the centerline of the supporting member by some distance relative to the magnitude of the reaction when:
[ol 1]
[li]P,perp > 800 lbs for Interior Connections (≥ 5d from end of the member)[/li]
[li]P,perp > 400 lbs for End Connections (< 5d from end of member)[/li]
[/ol]
If you set the "ANSI/TPI 1 Evaluation" input to On in the Simpson Hanger Selector Tool, it eliminates any hangers that do not extend past the supporting member's centerline if either of the two criteria listed above are applicable.

ndstruct's reaction must be less than the load limits since the connection shown is below the neutral axis.

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