Built-Up Sawn Lumber 2-Span Continuous Beam

[ajk1]

Background:

I am checking a continuous 2-span built-up sawn lumber beam of a cottage that was constructed within the last year (I had nothing to do with its design or construction). The beam that I am checking is the main floor perimeter beam supporting the main floor, and the wood stud wall above which in turn supoorts a loft and the roof.

Preliminary checking indicates that this beam is over-spanned. The spacing of the supports (number of supports were reduced), and the size of the beam were both increased by the contractor, from the original system designed cottage, but no engineer sized the new setup.

Although certain simpifying shortcuts could be made when designing such a beam, I cannot take such shortcuts when checking because I may put the owner to needless expense strengthening the beam, where perhaps no strengthening is required if more accurate checking methodology is used.

Given:

The 2 spans are about 7 feet and 11 feet, and 2 of the plies of the 4 ply beam are butted at about the 2 foot locatiion from the centre support in one span, and about 3 foot from the centre support in the other span.

Questions:

1. Does Woodworks software account for the location of the joints?

2. When checking manually, how should the butted plies be dealt with? For example, is there a "development length" over which the ply picks up its share of load from the adjacent plies of the 4 ply beam? Are the nails generally adequate to make that tansfer of load?

3. Are "clear" spans rather than centre-to-centre spans entered into Woodworks?

4. Is there any worked example of design or checking of a built-up continuous wood beam with some of the plies butted within the spans?

 

[ajk1]

Yes an extra support is precisely what I have been considering. It should not cost that much, but the cottage is reachable only by boat, so it is a bit of a bother to bring all the materials (for concrete footing, block piers, etc.) but still the most cost effective solution. I just wanted to be sure that I was not doing something unnecessary.

I note that the Span Book permits a live load reduction factor of 0.8 for "lintels" supporting 2 floors and a roof. Why does it not say anything about this for a beam supporting 2 floors and a roof? Although this will not get it out of the over-stress condition, I was just wondering why this is limited to lintels.

 

[ajk1]

Attached is the output from the Woodworks file.

 

[KootK]

Hold the phone... I see cantilevers that have mucked up the whole "insignificant moments at the quarter points" business.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

Seems to me the cantilever on the right can do nothing but help the situation for the 11' span. The cantilever on the left is of little consequence.

BA

 

[KootK]

For a gazillion posts here, we've been hanging our hats on the butt joint being fine because it's a point of inflection. Now, as a result of the left cantilever, there is no point of inflection. Rather, the butt joint see a max moment upwards of 2/3 of the maximum negative moment at the support. In the context of this discussion, that seems pretty consequential to me.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[dhengr]

Ajk1:
When you get done with this beam problem, take a look at the lateral loads on a whole bldg., going through the first fl. diaphragm, then through two or three standing beam lines sitting on top of some posts sticking up out of the ground. How many posts, 6, 9, 12, and how deep and on what footings. What are these posts and the post/beam connections good for laterally? A 6 or 8" post cantilevering a couple feet above grade and buried a few feet in a loose filled hole is not worth much laterally.

 

[ajk1]

yes dhengr, that is very perceptive and the next item on my "to do" lists of things to check. The lateral load issue is more severe than you suspect, because the whole cottage is several feet (7?) up in the air! I was going to put the lateral load issue as a new string, but you beat me to it.

And some of the posts are eccentric on the piers, the piers have no mortar in the joints, the footings may be undersized, and on and on it goes with odd-ball things.

The loads on the cantilevers are quite small, so I don't see that they change the basis of the fundamental question. If the cantilevers were not there, the question would be the same, namely can we assume all 4 plies resist the load equally, as implied by the Alberta Building Code?

When I get into the office Tuesday, I will see if the 2010 NBC Part 9 is like the Alberta Building Code. I will also have to have another look at CSA O86. I should also check ASCE although our copy of it is quite old.

 

[BAretired]

I didn't notice the reference to Appendix A when I posted before. See the attached link.

BA

 

[ajk1]

ok

 

[KootK]

A lot of cottages in my area are built on hilly sites and have downhill piles/piers that project fairly far above grade. It's quite common to see the piers cross-braced from grade to the tops of the piers.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ajk1]

How do they cross brace concrete masonry piers?

 

[ajk1]

How do thye cross brace masonry piers?

 

[BAretired]

You could brace the beam as shown (see link). That would provide lateral resistance and brace the long span in the middle.

BA

 

[ajk1]

The footings are below grade and at significantly different elevations. The wood would be in contact with the soil. Connection of the wood to the concrete masonry pier may be problematic. I have done that sort of supplementary steel support, to concrete beams in parking structures, but I am not so sure about it for this cottage; I will give it further thought. Thanks.

I wonder about placing carbon fibre reinforcement each side of the piers to give it some tensile strength.

 

[RHTPE]

Missing from this discussion is how the floor joists connect to the built-up beam.

If they bear on top of the beam, then (for the most part) all plies should have the same deflection profile.

However, if the floor joists connect to one side of the beam, then the distribution of the load to each of the 4 plies becomes rather gnarly, unless the joist hangers are through-bolted to the built-up beam.

KootK's point #6 is true in either case.

Obviously, the connections from ply-to-ply will influence the load-sharing capability of the 4 plies.

If I were to try to do a comprehensive analysis of the condition, I would first examine a 2-span, 2-ply condition cantilevered over the center post, presuming that where the 2-ply beams overlap the load is shared equally. If that proves to be satisfactory I would stop there.

Trying to take advantage of the (2) 5' long plies on the left side and the (2) 8' long plies on the right side really seems like a futile effort. They offer little additional shear & moment capacity unless one gets into a rather complicated analysis of how the plies are fastened together.


Ralph
Structures Consulting
Northeast USA

 

[KootK]

@AJK: they don't brace masonry piers. For some reason, I was thinking steel supports. My bad; false hope.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[BAretired]

PWF material is frequently placed below grade in preserved wood foundations. A knee brace at each end of the 11' span down to the footing would reduce the beam span and provide some lateral resistance. The connection could be to the concrete footing, not the masonry pier.

BA

 

[ajk1]

Yes I suppose although we always avoided wood below grade in our projects, even though it is permitted with the right treatment. It is something to keep in mind.

 

[BAretired]

After viewing the photo in thread507-370835, there seem to be more serious concerns with respect to this cottage than the strength of the wood beam.

From my own experience with contractors who produce work of this caliber, I believe that you will have a great deal of difficulty in getting him to comply with measures which you deem necessary for a safe structure. Nobody could blame you for opting out now. If you decide to continue, be prepared to be frustrated throughout the remedial work. Good luck.

BA

 

[ajk1]

The owner is withholding a large chunk of money so the contractor appears at the moment to be cooperating.
I have told the owner that we don't want to do it.
The contractor says he has built cottages this way for 30 years, including elevated cottage and they are standing up.

To try to resolve the effect of the butt splice of 2 of the 4 plies, I ran the built-up 4 ply beam on Woodworks with the 2 shorter discontinuous plies being supported by the 2 adjacent continuous plies. i.e. at their end at 0.28 of the span from the support, they apply a load on the 2 adjacent continuous plies. The results were as follows:

For the original run of 4 un-spliced plies (i.e. each taking 25% of the load and fully continuous:

V_f / V_r = 1.21; M_f / M_r = 1.35

With the shorter plies dumping their load on to the adjacent continuous plies:

V_f / V_r = 2.06; M_f / M_r = 1.86

From which I conclude that for moment should consider about 70% of the Mr of all 4 polies, and for shear should consider about 60% of the Vr of all 4 plies. This was for the butt point at 0.28 of the span.

I know this is very rough, but better than nothing. For new construction I would consider only 50% effective, but for checking an already constructed building, the above percentages may perhaps be acceptable, until someone shows otherwise. I have sent an email to the Canadian Wood Council to ask them this question about how to deal with splices.