crushing of wood fibers

[rktect 1]

I am trying to have a bit of better understanding as to the crushing of wood fibers.

The specific problem is that we had a resident build a room addition over an existing wood deck. The deck beam at the outside perimeter is a split beam of 2-2x12. So a single 2x12 is on opposite sides of a 4x4 column which is then bolted with either two 3/8" or 1/2" through bolts ( to be field verified). There are four columns and I have guestimated that the loading at the two outside columns is about 3200 pounds and the two inside columns is about 5600 pounds. So each bolt carries between 1600 and 2800 pounds. How does this effect the beam? How does this effect the column? Are the bolts an issue for shear?

I had read that most, depending on species, wood fibers crush at 625 psi. (edited psf to psi)

 

[Yankee]

If I understand your question, that type of connection is precisely disallowed. the 2x12's must be notched into the posts to sit (bear) on a shoulder AND be bolted.

 

[rktect 1]

If I understand your question, that type of connection is precisely disallowed. the 2x12's must be notched into the posts to sit (bear) on a shoulder AND be bolted.

That is correct as to the requirement. And I will have them correct this. What I have out there is not what should be there and I want to understand what is there in order to formulate my review comments. I can easily say that the requirement is "x" but I do want to understand the why.

 

[GHRoberts]

All good questions.

625psi (square inches not square feet) seems more reasonable but ...

I believe you want chapter 8 of the NDS for Wood Construction. Bolted connections. Dowel bearing strength. Multiple Shear. There are numbers for all species of wood. I will just give first glance estimates.

If I get the directions right, the posts should support 2000 psi of bearing (more or less). The girders should support at least twice that.

The 625psi (psf in your post) appears to be the design shear strength based on 1/2" bolts. Two 1/2" bolts in double shear gives you 4*625 pounds of support - 2500 pounds.

---

Having taken so long to make this post I suppose we will start on a disallowed by prescription versus engineered discussion.

I don't like the design, but I do think engineering shows it is a code compliant design. At least the same engineering that is allowed elsewhere shows the design supports the load.

 

[Yankee]

Sorry, I should have known : )

 

[rktect 1]

The 625psi (psf in your post) appears to be the design shear strength based on 1/2" bolts. Two 1/2" bolts in double shear gives you 4*625 pounds of support - 2500 pounds.

So, basically, they have exceeded the shear strength of the wood when over 2500 pounds? The fibers would start to crush?

I think what I am trying to avoid with this is that if, as you say, they have 2500 pounds for two 1/2" bolts in double shear, if they add one more bolt they would then have 3750 pounds and basically have an engineered solution instead of the required 1.5" of actual bearing. OR does that actually sound acceptable?

 

[bgingras]

"if they add one more bolt they would then have 3750 pounds and basically have an engineered solution"

Only if an engineer designs the connection and stamps it. No stamp then they must go with prescriptive per code, which I don't have in front of me, but would require at least 1 1/2" of bearing and a bolt doesn't provide that, right? Sun room additions on existing decks are so much fun. I failed one on a final that was engineered, except for the connection to the house which had a point load from a bearing ridge posted on a 2' cantilever that was dead center of a 6' sliding door. They couldn't understand why I wanted an engineer to spec the connection and were quite unhappy when they had to install a (3)x 2x10 through half the house to carry the post AND install an LVL over the sliding door. Sorry, ran off topic a bit.

 

[peach]

one more bolt.. the ledger could split.. what a terrific solution!

My point of course is.. relying on a few bolts to hold a deck to the wall isn't such a great idea.. converting it to something else maybe even less of a great idea.

 

[GHRoberts]

rkteck 1 ---

As both Yankee and I alluded to the issue is not just the engineering.

You need to make a decision as to if the prescriptive prohibition should be followed. That is a tough call.

 

[rktect 1]

Just got back from a field visit.

Turns out that the original deck was constructed with the split beam on 4 columns as indicated above. The new drawings and engineered report shows this csame onstruction and provides calcs for the "to be added" lateral bracing at the columns, the existing supposed roof rafters and the existing supposed floor joists. Unfortunatly, I don't think the engineer nor the architect have seen photos nor been to the site. When they built the addition over the deck, they removed the 4 cexisting columns and piers and replaced them with 5 4x6 columns on new 12x12 concrete piers. The beam is still split but both 2x12's rest directly on the wood column as the columns are notched. The beam is then nailed to the column with about 6 nails on each side.

The engineers report gives design criteria on the first page for live and dead loads, then in the calcs on the next few pages changes this design criteria. No big deal for the structure as the 2x8 roof rafters and 2x10 floor joists are fine per the prescriptive tables in the IRC. I think it may affect the lateral bracing though as there are 5 columns and not 4. Still have to check the beam size and span but it looks ok so far being a doublt 2x12.

My review comment will be far diffferent then I had originally thought.

 

[vegas paul]

New construction, 09 code, built without permit... hmm... Make him sprinkler the addition!

 

[peach]

VP.. depends, I guess on whether or not the existing building provisions are adopted.

 

[Phil]

Once you leave the presciptive codes and pre-approved connectors behind, wood connections require engineering. The bolted connection described in the original post has several limit states that need to be checked besides simple crushing of wood fibers. For instance, the bolt can flex or bend while some of the wood fibers are crushed. This connection with two 1/2" bolts might have a 1300# capacity total. Even with the beams bearing on the post carrying a 5600# load (5600/10.5sqin=533 psi), the beams could be exceeding there allowable compression stress perpendicular to grain. It depends on the wood species.

If you want a quick reality check for a bolted or nailed connection's capacity, the American Wood Council has a calculator on there web site (http://www.awc.org/calculators/index.html ). But is don't think it accounts for everything. It is best to hire an engineer. There can be capacity reductions for group action or geometry or . . .

 

[JBI]

The prescriptive provisions of the Res Code do not negate Engineered design based on sound engineering principles. As long as the Engineer/Architect provides the calculations to verify compliance with the performance requirements of the Code.

 

[Darren Emery]

Dealing with this issue today - can anyone point me to where in the IRC the bolted connection is prohibited?

 

[steveray]

R502.6 Bearing. The ends of each joist, beam or girder shall have not less than 1.5 inches (38 mm) of bearing on wood or metal and not less than 3 inches (76 mm) on masonry or concrete except where supported on a 1-inch-by-4-inch (25.4 mm by 102 mm) ribbon strip and nailed to the adjacent stud or by the use of approved joist hangers.

Good place to start Darren.....Not Prohibited, just not prescriptive...

 

[steveray]

I am not an engineer, but I did sleep in my bed last night.....To some extent you would be looking for compression (crushing) values for SYP (most likely) parallel and perpendicular to the grain...I know I had a builder a while back posting down 24,000+ pounds and I did the calcs and the PT sill plate failed in compression....Things we shouldn't have to check.....

 

[JCraver]

R502.6 Bearing. The ends of each joist, beam or girder shall have not less than 1.5 inches (38 mm) of bearing on wood or metal and not less than 3 inches (76 mm) on masonry or concrete except where supported on a 1-inch-by-4-inch (25.4 mm by 102 mm) ribbon strip and nailed to the adjacent stud or by the use of approved joist hangers. Good place to start Darren.....Not Prohibited, just not prescriptive...

I had that section as my first thought also, but wouldn't the word "ends" in that sentence infer that this section is only talking about joists running perpendicular to the support (posts and beam, or ledger, whatever)? And if using that section as your reference, wouldn't you also have to accept a double rim where only the inner 2x rests on the notched post (so the outer 2x would not be bearing on anything, simply screwed/bolted to the inner board)?

DCA 6 forbids the installation with just bolts, but neither that code itself nor its commentary references an IRC section that does so (that I could find).

Tricky deal. I just hand my deck guys a copy of DCA 6 and tell 'em if it's not built this way, I'm gonna' fail it.

 

[Darren Emery]

And if using that section as your reference, wouldn't you also have to accept a double rim where only the inner 2x rests on the notched post (so the outer 2x would not be bearing on anything, simply screwed/bolted to the inner board)?.

Also dealing with this exact situation today. I flagged the condition, as I feel that if the outer 2x is not bearing, then only 1/2 of the beam is bearing, and you cannot give full span credit to this improperly supported beam.

 

[steveray]

.......And if using that section as your reference, wouldn't you also have to accept a double rim where only the inner 2x rests on the notched post....... .

1.5" bearing depth, full width of member(s)....If that is what you were referring to.....Correct Darren...

 

[Mark K]

Proscriptive provisions only work in limited situations but individuals who are not engineers attempt to apply them in creative ways. It is no wonder you are having problems.

The problem is that an engineer should have been involved with the original design and was not. The solution is to require the building owner have an engineer look at it and make recommendations.

 

[mark handler]

Some licensed engineers don't think about the crushing of plates

Had one that wanted the sill plate run through the column base supporting two stories and the roof

 

[Paul Sweet]

I believe that crushing perpendicular to grain is only in the 300 - 500 PSI range, depending on wood species, and is further reduced for wet use, treated lumber, etc. The sill plate or girder supporting a column will usually fail in crushing before the column fails in compression. Bolt tables give different allowable values for loading parallel and perpendicular to the grain, as well as for different species.