Uplift - Max To Ignore 2

[BSVBD]

What is the maximum uplift we can safely ignore?

On conventionally framed wood structures with pre-manufactured, wood, roof and floor trusses, the total uplift is provided on the truss profiles data provided with the truss submittals.

To specify hurricane ties, holdown anchors, etc, for every slightest evidence of uplift is unreasonable and costly. Yet, to ignore significant uplift reactions is unsafe.

The contractor / owner of a multi-family development company is pressuring me to provide an answer as to why i an specifying uplift hardware at location "A" but not location "B"?

What is a reasonable "threshold" or maximum uplift reaction to consider it negligible or consider that uplift hardware is not necessary?

Thank you all!

 

[jayrod12]

I guess that depends on the connection provided if there is no hold down. Toe nails, or screws. Common wire nails, spiral, etc. Then I would be looking at the withdrawal resistance of the typical installation.

For me, it's a low threshold. In my area, it's 3" common wire toe nails which don't have a whole lot of capacity.

 

[KootK]

What is the maximum uplift we can safely ignore?

0 lbs. Similar to what Jayrod mentioned, get your uplift from nails, hardware, or refined loads, but get it from somewhere.

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.

 

[Ron]

Agree with kootk and jayrod12.....any uplift needs to be mitigated.

I worked on a failure investigation many years ago that had a weld pop during uplift....the small resulting uplift displacement was enough to allow a dynamic impact that broke another weld on the way down and caused a joist girder to collapse....in the middle of a big box retailer while open! Fortunately no one was injured.

 

[BSVBD]

I agree, we need to have an understanding that all forces / reactions are accounted for.

But, do we really need to have a written analysis for every component?

When i consider at least (2) toe-nails per stud, the perimeter nails that fasten the sheathing to sill plate and the intermediate or field nails that fasten to at least (4) adjacent studs, we are already exceeding 1,000 LBS of shear resistance not considering a 1.6 duration factor.

As i reason this out, i'm coming to my own conclusion, that, I can at least account for many variables without having to document everything unless necessary and, with everything previously stated, i can avoid unnecessary hurricane ties and other hardware.

Thank you all for your input.

 

[KootK]

When i consider at least (2) toe-nails per stud, the perimeter nails that fasten the sheathing to sill plate and the intermediate or field nails that fasten to at least (4) adjacent studs, we are already exceeding 1,000 LBS of shear resistance not considering a 1.6 duration factor.

But the bottleneck is still the truss to sill connection, all by its lonesome, right?

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.

 

[JAE]

KootK beat me to the punch. Your sheathing nailing usually doesn't affect truss-to-sill connections unless the truss has a vertical end where some of the wall sheathing comes up the end of the truss.

Why can't you simply look at your two toenails (ugh...shudder) and see what capacity they offer. Then simply check on the truss reactions to see which ones need the additional ties.

Also - some jurisdictions require the ties no matte what - it is written in to their building code so check your applicable code language as well.



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[SlideRuleEra]

BSVBD - The "Wood Truss Council", "Truss Plate Institute", and "Structural Building Components Association" appear to agree with you:

Using Toe-Nailed Connections To Attach Trusses At Bearing Locations

Toe-Nailing For Uplift Reactions

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[JAE]

SlideRuleEra - nice.

However, toenails are tough to do correctly so those values are iffy in my book. (My own house has only toenails so I've always been real nervous in the spring with the storms we tend to have)



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[BAretired]

The other part of the connection which used to bother me was the stud to plate connection. These are usually not toe-nailed but nailed into the end grain of the stud, so they don't offer much resistance to uplift. We seem to be relying on the nailing of the sheathing to stud and plate, but there are cases where the sheathing is terminated half way up the top plate, so it is not engaging the upper ply where the trusses are attached.

BA

 

[DaveAtkins]

I would calculate the uplift at the end of a typical truss and use that value, not the value given on the truss manufacturer's shop drawings. They always seem to be unreasonably conservative.

That being said, I typically show hurricane ties, and I don't count on toenails.

DaveAtkins

 

[jayrod12]

SRE, thanks for that. it confirmed my suspicion, the typical connection around here is good for 60lbs (assuming installed correctly). That's 3 10dx3" common wire toe nails into spf lumber. It's as common as a chevy nova used to be.

 

[BSVBD]

As a general rule, I always specify a minimum Simpson H1 hurricane tie for ALL trusses. The H1 will typically satisfy my shear calcs also.

However, I need to clarify the original intent of my inquiry:

Specifically for wood girders, whether 1, 2, 3-ply or more, i usually specify the following:

"SUPPORTED GIRDER MUST BEAR DIRECTLY ON TOP OF A WOOD COLUMN (AS SPECIFIED IN SCHEDULE). THIS WOOD COLUMN MUST BE FULL HEIGHT FROM SPECIFIED GIRDER BEARING TO TOP OF STEEL BASE PLATE AT TOP OF CONCRETE BASE SUPPORT. STEEL BASE PLATE TO BE 1/4" THICK x SQUARE OR RECTANGULAR DIMENSION OF WOOD COLUMN."

This specification prevents the perpendicular crushing of top and sill plates that would occur under corresponding heavy reactions. In these heavy girder cases, i always specify a Simpson Holdown, or similar, at column base to foundation.

However on lighter girder reactions, as in the typical 1 & 2-ply girders, where the interruption of top and sill plates is not necessary, other than specifying a girder to top plate tie-down, my inquiry was intended to be for the column base to foundation. Please forgive me for not being clear on this.

In these lighter girder cases, when the uplift reaction is typically under 1,000 LBS, if the girder to top plates tie-down is anchored to ALL top plates, I consider that the 7/16" minimum wall sheathing to girder column (usually an equal quantity of studs as that of supported girder) and adjacent studs down to sill plate will often provide the 1,000 LB shear resistance i stated in my second post.

For the past several years, as i have been advised, for expediency, i have not considered specifying a column or stud to foundation connection for these "light" girder situations.

BA - I agree; when bearing walls are constructed, they are typically done so on a flat horizontal surface. Thus the end-grain is negligible for uplift resistance. Further, i agree with you that in many cases, especially in the lower profile residential and multi-family unit structures, the sheathing would be terminated half way up the top plate, so it is not engaging the upper ply where the trusses are attached. Regardless of what is shown on our drawings, I have not previously considered that. Thank you!

Thank you all!

 

[DaveAtkins]

I agree--for uplift, it is possible to justify "spreading" the uplift reaction through the wall, as long as the sheathing is adequately nailed to the post which has the uplift, and to adjacent studs (i.e., you must justify the load path).

DaveAtkins

 

[KootK]

you must justify the load path

Right. You must also take care not to double dip with regard to your capacities. There's a good chance that, when your truss is seeing its maximum uplift, there's a significant shear demand in the supporting walls. If sheathing joints land on the post, there will be shear demand on the fasteners coming from both sources.

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.

 

[manstrom]

A Simpson H2.5A costs about $0.50 and require 10 nails to install.

I require them on all roof trusses. Period.

Simpson also has a screw that installs through the top plate and into the truss. Costs more, easier install.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

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