Sill Plate Crushing at Girders

[BSVBD]

I've read at least (2) past posts regarding "Crushing of Plywood" and "Wood Bearing Plate at Trusses".

Now what about sill plate crushing at heavy girder reactions?

After reading the prior threads, i can be comfortable with a slight over-stress of perpendicular compression. But when i have a 16K reaction bearing on a 3.5x3.5 parallam, the compression perp exceeds 1,000 psi! I don't want to ignore that!

With one commercial general contractor, for over a decade, i've been specifying that the column shall interrupt both the top plates and sill plate and bear on a steel plate on concrete foundation wall. I've had no problem with that contractor. Did they actually comply in the field? As reputable as they are, i trust they do.

A different contractor, for a multi-family developer, is questioning this specification. After meeting and hearing my defense, they do understand and will comply for at least one particular heavy reaction location. Desiring to please the client, i am considering to raise my threshold of compression perp, but i must stay within reason.

After reading the aforementioned posts, i can conclude that crushing, WITHIN REASON, is not a major issue. But, where is that threshold? Do i accept nothing higher than 425 for SPF and likewise 625 for DFL or how much can i be comfortable exceeding this?

Related to this topic, in a prior post, i inquired, without a reply:

If the wood actually DID begin to crush, wouldn't the allowable compression perpendicular actually increase as the wood is crushing, and thus, soon terminate the crushing? Any opinions?

Another consideration is that if crushing DID occur, if brittle finishes, such as gypsum, are on that wall, cracking of that finish will also occur.

Another other thoughts?

Thank you!

 

[TehMightyEngineer]

One thing to note is that the stress allowed for compression perp. to the wood grain is the stress at which you will have 0.04 inches of "crushing" deformation. Thus, it's rarely a safety or structural issue and mostly a service issue as you noted. These crushing failures can cause cracking or visible effects in drywall or other sensitive finishes, could cause things like windows to stick, and could cause a cumulative effect if you had multiple stories which had similar crushing defects. Combine this with shrinkage and it can be a concern.

In practice I've often ignored perp. compression if I didn't have a serviceability or performance issue. Metal bearing plates can also get you out of these failure modes pretty easily as well.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[structSU10]

I'm not sure if you are aware, but the bearing perp to grain values in the NDS are correlated to a specific amount of crushing ( per NDS 2005 4.2.6, its based on 0.04" of deformation when loaded by a steel plate). they also give an adjustment value if you want less crushing. The commentary does state 'it does not lead to structural failure'. It discusses ASTM D2555 from which they derived their values - which may be a place to look and understand some of the more 'ultimate' values perhaps...

This doesn't really answer your question, but gives some places to look.

 

[mike20793]

Nicely stated TME. For very heavy reactions, I typically use the steel bearing plates on top of my sill plate or make sure I have enough bearing area through the size of the member. There have been a few occasions where I've used engineered lumber sill plates. Sometimes column size can be governed by the crushing perp to grain, unless you don't mind interrupting the sill plate. Sometimes, I'll use a 6x8 post over a 3 1/2 x 3 1/2 PSL, but it depends on the project. I don't ever ignore it and it can be serious problems in boundary members of shear walls in 4 or 5 story wood buildings. That being said, I've only seen serviceability issues caused by crushing of the sill plates in person a handful of times.

 

[msquared48]

Anything over 500 psi, I commonly use metal bearing plates.

As noted, too, the plate will crush to a certain extent to achieve the load seen, similar to what soil does in bearing. The larger and thicker the plate, the less crushing.

Mike McCann, PE, SE (WA)

 

[EngineeringEric]

Anybody use the Simpson Truss Bearing Enhancer clips? TBE4/6?

I usually limit my stresses at 400 for SPF plates, if over i switch to SYP plates. And i actually have never been over 600 knowingly...

 

[BSVBD]

I have specified Simpson TBE's. Very useful depending on the application.

Something i forgot:

I would not specify SPF or DFL for sill plates. Since sill plates (typically) bear on concrete, unless we provide a separation barrier, i should only be specifying preservative treated SYP sill plates. Apparently, for the heavy girder situations, that is why i've specified the steel base plates. Through my research, SYP is the only species that is preservative treated in this area.

Thank you all!

 

[jayrod12]

We use SPF plates on concrete all the time. There are a few rules that have to be followed but it is acceptable by our building codes.

 

[Michel60]

When needing to use a steel bearing plate under a post what does that detail look like? Is the plate loose or fastened to the sill plate? What about the post, how is it fastened to or through the bearing plate?

 

[allgoodnamestaken]

If you just want the non-design wind velocity pressures, those are in the National Building Code of Canada. But the process of taking those values and creating design wind load values is dependent on a lot of other factors that are site and design specific.

I don't know anything about the legality of taking the wind tables from the NBCC and creating an interactive map out of them. You'd have to talk to an intellectual rights lawyer about that.

 

[medeek]

I recently has a situation where I had spec'ed out a Simpson Strong-Wall SB and the contractor framed a sill plate between it and the concrete stemwall. I had calculated about 13,000 lbs of load at (tension/compression) in these shear panels so I let him know that was not going to work and he corrected it. The installation instructions from Simpson were also pretty direct on mounting the panel directly on the concrete:

http://www.strongtie.com/ftp/catalogs/C-L-SWSB13/C-L-SWSB13-p18-22.pdf

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

http://www.medeek.com

 

[BSVBD]

Michel60,

The steel plate can be loose, otherwise fasten it to concrete with tapcon screws or other. I have previously specified the following:

A STAINLESS OR GALVANIZED 1/2" x 8" THREADED ROD WITH 4" PROJECTION MUST BE CAST IN PLACE INTO CONCRETE FOOTING CENTERED ON WOOD COLUMN AND PROJECT 4" UP THROUGH BASE PLATE AND BOTTOM OF WOOD COLUMN.

You could also specify to have the threaded rod installed, with epoxy, after the concrete pour.

To anchor the wood column to prevent uplift, specify any of the Simpson Holdowns or Tension-Ties. The accompanying anchor, required to fasten the Holdown or Tension-Tie to concrete, would be installed above the adjacent wood sill plate (which is interrupted by the steel bearing plate), penetrate and go through the sill plate and anchor to the concrete.

 

[manstrom]

I try to adhere to the fc perp in the NDS. A failure wouldn't be catastrophic, though. Just a serviceability thing. If a reaction gets huge, I try to use a Simpson cap or bearing blocks on each side of the beam. Basically, widen the beam to get more area.

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.

-R. Buckminster Fuller