Wood Truss to Concrete Wall - Out-of Plane Connection

[dhoward26]

I'm having a debate with another structural friend over connections of wood trusses to concrete walls. I learned one way and he learned another and I'm not sure at this point which is the "correct" way. I attached a detail of the connection.

For out-of-plane wind and seismic forces when connecting a wood truss to a concrete wall, is it acceptable use Simpson A-35's (or something similar) or do you have to have direct connection to the anchor bolts in the top of the wall with a steel angle of some sort? I'm aware of the code requirements about not placing wood in cross-grain bending, etc. The way I was taught through my EIT years and first couple of PE years was to use Simpson A35's, but I'm questioning the validity of this now after doing more research.

 

[dhengr]

dhoward26:
You note a “dbl top pl.,” but show a single top pl., more like a sill pl. on a found. wall, and the single pl. should be o.k. if it checks structurally. The double top pl. on a wood stud wall has a slightly different set of loadings and reasons behind it. That sill pl. (or a dbl. pl. must span from A.B. to A.B. as it transmits either lateral loads (wind or EQ, lbs./l.ft. of wall) up into the trusses at some discrete spacing and into the roof diaphragm; or as it transmits uplift (lbs./l.ft. of roof plane), again at discrete truss spacings, down into the wall. Note that Simpson’s A34 or A35 must be applied to both sides of the trusses if their F2 forces can act in either direction perpendicular to the truss plane; i.e. they are no good when you pull them off the member they are nailed to, but quite capable when your load pushes against them. When one leg is bent their A35 can be made to work in three directions, their loadings A1, C1 & E, their connection type [2]. The A35 connector your arrow points to will be effective in transmitting loads perpendicular to the plane of the wall; but in only one direction in the plane of the wall unless it is applied to both sides of the truss. It would not be very effective (no listed cap’y.) against uplift, because you are pulling the nails out of the sill pl. The A35 you show standing upright on the right side of the sill pl. might work in three directions, but you have it turned the wrong way. The leg attached to the side of the truss should be on the left side, so its bottom portion can be bent and attached to the top of the sill pl.

I don’t see a ‘cross-grain bending’ problem, whatever that is. You can get cross-grain tension too near the loaded edge btwn. the A.B’s. and the sill pl. if they are not careful about centering the A.B’s. on the sill pl. location. The sill pl. and A.B’s. must be checked for this concentrated loading at 4 or 6' o/c, and the real code criteria is bolt bearing on the wood and edge distance in the tension direction. I’m sure this same tension perpendicular to the grain/edge distance issue dictates Simpson’s C1 cap’y. values in that direction.

 

[dhoward26]

Thanks for the reply. I have the vertical Simp. connector for uplift only. This is just an illustrative detail for purposes of discussion about perpendicular wall forces.

My argument/concern is that the anchor bolt in effect splits the sil plate in 2. You are then relying on the Simp A35 to hold the plate together as it seams there would be a tendency for the plate to split in half. Maybe I'm over analyzing how I was taught now as it seams to be standard of practice to use A35's for the force transfer perpendicular to the wall...at least in residential and light commercial construction.

 

[msquared48]

I commonly use Simpson hurricane clips from the truss to the top plate to resist the uplift forces. Seen this done all the time regardless of the cross-grain bending issue, but I do see your point. To get around that issue you would have to use an embedded strap every 4 feet or so with the strap wrapped over the top of the truss and nailed off, taking the plate out of the equation for those forces.

Mike McCann
MMC Engineering

 

[dhengr]

dhoward26:
Your concern is legit. and that splitting can happen, if the lateral loads are large enough. That’s why you pay attention to edge and end distances from the bolt hole, and why you care that they place the A.B’s. properly. That may also be why you would use a larger (wider) sill pl., to improve the edge dist. and to allow some tolerance for the A.B. placement. Take a look at any good wood design text book for bolt loading and allowable bolt loads. Take a look at the NDS for these same topics. The A35's will tend to reduce the splitting of the sill pl., but they may not always be near enough to the A.B. to be effective in doing this. If the lateral loads are large enough, you may need to decrease A.B. spacing, increase the A.B. size or look for another means of transferring those loads in a less concentrated means. MikeMc suggests one means. Note that one of the advantages of the A35 over the A34 is that it can be made to counteract loadings in three directions with one connector, if those loads fall within its cap’y. The cost of an additional connector and the installation time are significant, if you can avoid the second connector.

 

[RFreund]

I'm slightly confused, is the issue:

1. Uplift from the truss causes 'cross grain bending' due to the anchor bolt/hurricane clip couple.

2. The horizontal (out-of-plane) force being perpendicular to the grain of the sill/top plate.

I've attached a sketch but I may still be missing the boat here.

EIT
www.HowToEngineer.com

 

[dhoward26]

RFreund: the concern is both uplift and out of plane loading.

 

[XR250]

There is also in-plane loading from the roof diaphragm shear. Might wanna add some blocking.

 

[ron9876]

I must be missing something. Why not use a typical embedded Simpson truss connector?

 

[dhoward26]

ron9876: Are you talking about the Simp. TC connectors or mudsils anchors?

 

[ron9876]

I mean truss anchors like Simpson META.

 

[msquared48]

Or a Simpson MAB15 at 24" on center to stabilize the plate from bending due to the uplift loads. See page 31 of the new Simpson catalog (2013 - 2014). Shows 900 to 1000# in uplift capacity.

Mike McCann
MMC Engineering

 

[dhoward26]

Not sure about the META. Seams as though the contractor would have to be pretty precise on the truss layout. It doesn't leave much room for error.

 

[woodman88]

"I'm aware of the code requirements about not placing wood in cross-grain bending, etc."
Where is this?
I am not aware of it and have done a fair amount of designs in wood with this occurring. You should always be careful in designing such details, but they can be done. (If not, then please show me where in the code this is).
For the truss to plate connection how much force do you have? I usually do not have a problem with the trusses for my structures with just the Simpson H type anchors.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[dhoward26]

woodman88:

ASCE7-10 - 12.11.2.2.3 & 12.14.7.5.2 Wood Diaphragms In wood diaphragms, the continuous ties shall be in addition to the diaphragm sheathing. Anchorage shall not be accomplished by use of toenails or nails subject to withdrawal nor shall wood ledgers or framing be used in cross-grain bending or cross-grain tension. The diaphragm sheathing shall not be considered effective as providing the ties or struts required by this section.

I believe it was in past IBC provisions as well as some "vague" information in the NDS.

 

[dhoward26]

The previous section to the above is:

ASCE7-10 - 12.11.2.2.1 Transfer of Anchorage Forces into Diaphragm Diaphragms shall be provided with continuous ties or struts between diaphragm chords to distribute these anchorage forces into the diaphragms. Diaphragm connections shall be positive, mechanical, or welded. Added chords are permitted to be used to form subdiaphragms to transmit the anchorage forces to the main continuous cross-ties. The maximum length-to-width ratio of the structural subdiaphragm shall be 2.5 to 1. Connections and anchorages capable of resisting the prescribed forces shall be provided between the diaphragm and the attached components. Connections shall extend into the diaphragm a suff cient distance to develop the force transferred into the diaphragm.

 

[dhengr]

Woodman88:
I questioned the meaning of ‘cross-grain bending’ too, in my first post, 3JUN13 @ 15:08. I think I know what they must mean in this context. Although, I don’t have the ASCE7-10 code, maybe you can see the context, and some more background better than I can.

We both know that if the bolt bearing load is applied toward a sill edge, with too little edge distance, you can get splitting along the grain, and I think they call that tension across the grain. And, that’s one of the bolt location and loading considerations important in this sill pl. detail; it’s akin to a bolt which is too near the top edge of a ledger beam on a deck, which will split the ledger, along the grain, when loaded. The ‘cross-grain bending’ is a misnomer, but I suspect that what they are talking about is bending or loading which causes bending stresses which will cause splitting along the grain. And, that could happen when you bolt the sill plate down, and then apply the uplift clips to one edge of the sill plate, putting the bottom face/side of the sill pl. in tension, in bending. One of the fixes for that is to use 3x3 washer plates (or larger) at the A.B’s. Both situations cause tensile forces/stresses across the grain, tension across the grain, or splitting along the grain.

dhoward26:
These are both important considerations for your detail, and must be accounted for, and the problem is made worse by the fact that you are concentrating the loads at the A.B’s every 4 or 6' o/c, but I don’t know the magnitude of your loads. In wood design, I think it’s fair to say, that more smaller connectors are usually easier to rationalize. You do need a reasonable and fairly direct load path, which you can rationalize/prove, for all of the loads. I think that’s what they mean when they say direct load path, continuous ties or struts, etc. At some larger loads you might need a direct tie or strap to each truss. But, I do agree that most contractors will resist using the META anchors because they are difficult to locate accurately at the time they are installed. Obviously, toenails and nails in withdrawal are not considered effective. And, continuous load path just means well defined and reasonably able to be calculated and proven.

 

[dhoward26]

dhengr: Thank you for the post. That was a good summary and exactly the points I'm looking at. The loads aren't huge by any means. Standard residential (or light commercial) but concrete walls with a foot print around 60x40ish. Loads from the wall due to wind or seismic in the 200-300 lb range.

I have used steel angles hooked to the anchor bolts in the past so I'm really starting to think that doing that is the best method of attachment. But then I'm still stuck with expecting the contractor to place the anchor bolts correctly.

I think the uplift is easy enough to solve with a Simpson MSTM...but that doesn't take care of the force perpendicular and the bolts creating cross-grain bending. I'm not sure if you could rely on something like A35's from the truss to the sill plate to hold the sill plate together so that no cracks occur between the bolts when loaded.

 

[ron9876]

I don't think I have ever designed a "normal" wood truss to masonry/concrete connection that wasn't an embedded strap. Contractors don't seem to have a problem. Must be a difference in markets.

 

[RFreund]

Just for some quick clarification (mostly for my own clarification) -

...but that doesn't take care of the force perpendicular and the bolts creating cross-grain bending.

I believe the force perpendicular to the bolts (out-of-plane load) puts the sill in tension perpendicular to the grain.
The uplift and resulting T/C couple with the bolt (sort of a prying action) would put the sill in cross-grain bending depending on the connection (if the hurricane clips are used the force is on one side, on the narrow face which would seem to create the T/C couple I'm referring to; see my sketch above).

Let me know if I have this wrong.

EIT

http://www.HowToEngineer.com