Built Up Beam

[00Z]

I have a built up beam made out of a 2x10 sandwiched between to sheet of 3/4" plywood, and I have a couple of questions.

First I don't have the NDS supplement to get the fb and fv values for plywood but I think that they should be higher than DF#2 so I was going to just assume the DF values. Anyone opinion on this assumption?

Second when calculating out my shear flow to determine nail spacing I'm getting really high values. I'm using the equation f=VQ/I. In determining I=bh3/12 I am using a h=4.625" instead of 9.25" is that incorrect.

The plywood is stitch nailed with 4 rows of nails. So I have 2 nails above the neutral axis. In determining Q=yA, I am using y = 4.625"/2 (distance from N.A. to center of two nails) and A = 1.5" x 9.25"/2. Do I have to calculated out for each row of nails or can I do it as a group.

I know this is a lot to ask, especially for a first post but would appreciate any input. I have been a long time lurker and have found the site to be very informative.

Thanks.

 

[Lion06]

There is no shear flow between the plywood "plates" and the 2x. The nails help the two pieces deflect together. Shear flow happens when you're using a built-up section - two or more members side-by-side with varying stiffnesses isn't a built-up member. They are just sharing load based on stiffness. The fastening pattern just ensures that they will deflect together (mostly because all pieces may not be in direct contact with the plywood subfloor loading them).

 

[Lion06]

If you want to design the fasteners, assume all the load gets into the 2x and design the fasteners to get the load that the plywood plates will take from the 2x into the plywood.

 

[00Z]

Thanks for the quick response!

 

[Lion06]

One last comment I would make is your support conditions could require you to design the fasteners at the ends to get the shear from the plywood back into the 2x.

 

[apsix]

With the ply, for about half (+/- 1) of the plys the grain is perpendicular to the stress and therefore won't contribute to strength.

 

[RHTPE]

Generally you would be correct in your presumption that fb & fv of a Structural plywood is greater than that of DF #2 (DF #2 base values: Fb = 850 psi, Fv = 180 psi). I would NOT allow the use of a 3/4" CDX plywood.

No doubt your 2x10's span is greater than an 8' sheet of plywood so you will have to contend with the discontinuity of the plywood. I would use the plywood plates to increase bending strength only - shear would have to be carried by the 2x10.

apsix is correct - only 3 out of 5 (or 4 out of 7) plies will have grain running parallel with your 2x10, so only about 57-60% of the plywood will contribute.


Ralph
Structures Consulting
Northeast USA

 

[hokie66]

Using plywood this way, what happens at the end of the sheet? You have a discontinuity there. Better to plant an LVL on the side? That way, the grain all benefits, and lengths are not an issue.

 

[Lion06]

Guys, even when plywood is used as intended, not all of the plys are working for you in bending - some plies have tension across the grain, that's just what you get when you have the plies orthogonal to each other.

The point about the discontinuity in the plywood is something that I didn't think about.

Why not just use two 2x's?

 

[00Z]

RHTPE, It is structural plywood not OSB and the plywood does not span the whole way without a break, the breaks are staggered on either side.

hokie 66, I agree that an LVL or just a 4x beam would certainly be a better option but I am being called in after the fact. This beam is something the contractor has already put together. I'm sorry I ever got involved.

Thank you all for your input.

 

[00Z]

I am probably setting myself up for ridicule but....

The beam is actually a 4x6 on top of a 4x10 with 3/4" plywood on each side to make a 3" x 14.75" beam. I have attached a detail (keep in mind this is a work in progress...). The 4x10 on the bottom breaks above the post on the right the the the 4x6 breaks above the post on the left.

This is the existing condition not something I designed. So far my only contribution is to add the posts to the detail, the rest is the work of the contractor. This beam is sitting on top of the bottom chord of the stick framed trusses and is spanning 24'. The beam was put in to replace a bearing wall that was located about a foot away from the beams new location! It is supporting a tributary width of 15', (no snow loads). Apparently everyone was surprised when the inspector wanted an engineer to sign off on it.....I know I should have run away at first sight.

I may still tell the owner that there is just no way for me to make this work and that he will have to just add a new beam below the trusses where the bearing wall was located.

 

[TXStructural]

Do not let the contractor pressure you on the right fix. That happens all the time, but I have seen bad plywood repairs, which did not the problem.

If you are looking for increased shear at ends, you can reinforce the ends as you describe. Nails will not work well as they will tend to withdraw as the plywood tries to rotate away from the lumber. Thru bolts and lags are better than nails in these situations. If you use an adhesive, there are very few rated for structural use, and the last time I spec'd one (3M), it was $10-15 a tube. Look for an ICC-ES on it.

If the deficiency is bending capacity, you need to have a better fix. LVL is often the best fix to get more capacity out of the same depth. Be sure the top of the member is secured against rotation.

 

[RHTPE]

Am I correct in assuming that the trusses are supported by this built-up beam? Essentially suspending the bottom chord from the built-up beam?

Don't like it one bit.

Ralph
Structures Consulting
Northeast USA

 

[00Z]

That is what the contractor is trying to do by clipping the bottom truss chord to the bottom of the beam. I still have to evaluate moving the bearing point of the truss. The interior bearing wall that has been removed that the beam is suppose to replace was offset about 1 foot from the beams current location....one problem at a time. I was just trying to figure out if I could even make that hokey beam work before I got into the bearing point of the trusses.

 

[TXStructural]

My last post crossed yours, so I recant.

Please look at this from start to finish of the load path.
A few items seen in quick look at only the beam-truss area, without evaluating the nailing or wood capacities:
1) The trusses have bending in the bottom chord
2) The new beam is trying to roll over supports
3) The slender posts have very limited capacity

The new support beam is obviously no longer at a panel point of the trusses. The bottom chord is in bending and stress reversal on the trusses could be an issue if they were not designed to bear where they bear. Install lumber from one or more upper panel points to move load onto the beam, or take a similar approach to remove bending of the bottom chord. Thru-bolt the new members. Evaluate the chord to web member connections for capacity using NDS.

You need kickers/braces at the top of the beam above all supports to restrain lateral movement of the beam. It should be OK to brace this to the bottom chord if the trusses are restrained.

The plywood is acting to resist shear between the upper and lower parts of your beam (now I understand your original statement). Design the nailing for that shear, or install thru-bolts or lags with plate (or heavy) washers to improve capacity if you don't have enough with the nails. The plywood value you would use is "through-the-thickness shear".

 

[BAretired]

Second when calculating out my shear flow to determine nail spacing I'm getting really high values. I'm using the equation f=VQ/I. In determining I=bh3/12 I am using a h=4.625" instead of 9.25" is that incorrect.

Actually, h is neither 4.625" nor 9.25" It is the depth of the beam which is 9.25 + 5.5 = 14.75". I = bh^3/12 = 802 in^4.

The plywood is stitch nailed with 4 rows of nails. So I have 2 nails above the neutral axis. In determining Q=yA, I am using y = 4.625"/2 (distance from N.A. to center of two nails) and A = 1.5" x 9.25"/2. Do I have to calculated out for each row of nails or can I do it as a group.

You are interested in the horizontal shear between the 2-2x6 and the 2-2x10, A = 2*1.5*5.5 = 16.5 in^2 and y = 14.75/2 - 5.5/2 = 4.625" (distance from N.A. to center of 2-2x6). So Q = Ay = 76.3 in^3 and VQ/I = V*76.3/802 = 0.095V #/" or 1.14V #/'. This is the force per unit length which must be transferred between the 2-2x10 and the 2-2x6 in order to make them act as a composite unit. For a uniform load, V varies from a maximum at the support to zero at midspan, so you can calculate the nail spacing throughout the span.

BA

 

[dhengr]

OOZ:
You say... “I was just trying to figure out if I could even make that {hokey beam} work before...” @ 21NOV10, 22:19.
You mean Hokie66 has a beam named after him? No wonder you got problems.

I’m repeating and thinking some of what TXStructural just said, maybe in slightly different words. Are these manufactured metal plate trusses? Do they span from outside wall to outside wall, or did they really bear on the removed wall? Were they really designed to bear on that wall, or is it just a wall under the trusses? Are the trusses homemade stick trusses and intended to bear on the removed wall? You (the contractor) can not just move that bearing point a foot to one side for convenience, the bottom chord of the truss will not take that (TXS’s secondary bending in the bot. chord). Is that new bm. made with 2x6 atop 2x10 + 2 -3/4" plywd. side plates, or is it 4x6 atop 4x10, etc.?

Show us an elevation sketch of the truss, with all truss members, and joint connections, in relation to old brg. wall and new beam locations. I think this contractor’s bm. is an abortion, and your posts and cribbing or whatever under the first fl., to fix it, are difficult, complicated, and expensive to do. You might be better off to remove that bm., save the 4x4 posts and cribbing, etc., and design a real beam with LVL’s spanning 24'+. If the diag. webs of the truss are located properly (lucky configured for you) I would put the new LVL beam 12 or 18" above the bot. chord to miss the diags., and back in line with the old truss brg. and supposed brg. wall, where you can add more studs for brg. at the ends, left and right. Then I would block btwn. the bot of the LVL bm. and the top of the bot. chord and strap the two together in tension. You will have to increase column structure at each end brg. down to the found. Lets see that truss elevation. I think the contractor shot a bull, and your doing a half assed fix job to save him a buck is a mistake on your part. By doing it right you are trying to keep him out of future trouble.

 

[00Z]

dhengr, I think your right about doing a half assed fix job. I have decided to tell the owner and contractor that they just have to do it right. I appreciate all the input.

 

[msquared48]

This is really rinky dink here... Not even half assed!

Did the contractor even get a permit?

The pier blocks - are they temporary shoring for the "pseudo elastic, mostly plastic beam" or permanent?

Glad you are going to have him do it right.

Sorry that I missed all the fun here, but I had to throw in my two cents ... after the fact.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask