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]

The Canadian Wood Council has responded to me and they say that it is standard practice to consider all 4 plies equally effective, provided OBC Clause 9.23.8.3 is complied with, inlcuding the somewhat more rigorous nailing.

I see OBC Clause 9.23.8.3 is similar to the Alberta Code Clause that BARetired referred me to earlier. So looks like BARetired (and perhaps others) was right!

(I remain a bit dubious of the prudence of this practice though, partly because the Canadian Wood Design Manual says that "Built-up beams are often used where the loading is light and dimension lumber is used for secondary framing, thus simplifying the type of materials required for the structure").

 

[BAretired]

But even if you consider all four plies effective, the beam is still overstressed, so remedial measures are required. If you can combine the remedial measures for the beam with measures required to provide overall lateral stability, that would kill two birds with one stone.

BA

 

[ajk1]

Yes BARetired, precisely! That is what I would like to do if possible. So next step is for me to calculate the wind load.

 

[dhengr]

Much of the bulk material could be taken out to the site over the ice, during the winter, thus less need for boat traffic. I like the idea of the continuous strip footing around the exterior, with some conc. blk. stem wall up out of the ground, and then a stud wall up under the beams. I would set this wall so that the ext. sheathing went up onto the ext. face of the beams, and rim joists at the main fl. diaphragm. I would also work on details which eliminated the existing blk. piers and wood posts under the exterior beams. The beams will act as the cont. top pl. type tie around the bldg. if tied together at the corners. I would use a single top pl. on the stud wall, under the beams. Lay the studs out so portions of the wall can be built to within a few feet of each pier. Then remove the posts, piers and their footings, and infill the cont. strip ftg., stem wall, and stud wall to complete the exterior basement walls. You may still need to do some remedial work on interior beams and piers, you haven’t revealed anything about them. This gets you up to the main fl. diaphragm with a meaningful lateral support system. And, it still sounds like there are plenty of problems above that level.

 

[ajk1]

Thanks dhengr. The height of an un-reinforced block wall, with no lateral support along its top, is limited to 3 times its thickness. So 8" block can go up only 24" unless there is lateral support at its top. Also, frost protection there is about 5'-6 or more, according to the frost depth maps, so would involve some excavation where it is not on rock. But may have to do that if can't think of anything else.

Most of the cottage was built in the summer of 2013, so they seem to be able to get materials to it by boat.

 

[Canuck67]

I just took a quick look at the WoodWorks output provided, but admittedly have skipped many of the posts in between so apologies if I repeat something already commented.

It appears that the 2x12's are pressure treated, based upon the Kt values shown. As per the CWC Wood Design Manual, this strength reduction is typically not required for most types of current preservative treatments which have little to no affect on strength, unless the 2x12's were incised. Were the 2x12's incised? You could pick up 25% capacity if not incised.

 

[ajk1]

Yes they were incised. A eek ago we had the wood inspected for the incision marks and they were definitely there. My understanding is that virtually all pressure treated wood with preservative is incised in Ontario.

 

[ajk1]

I was puzzled as to why I could not get the same answers as in th Span Book. Below is the response that I got from the Canadian Wood Council. Any comment? The whole thing seems very confusing to me. In checking an exisiting built up beam, should I use the additional 1.2 multiplier (which is in addition to the 1.1 multiplier) for resisting moment?


1) The Wood Design Manual (WDM) follows the CSA O86-09, and uses the system factor for built-up beams of 1.1 (Table 5.4.4, Case 1), while the Span Book follows Part 9 of the NBC, which allows for an increased system factor of 1.2 as well as an additional construction factor of 1.1 (See page 28 of the Span Book). The increased system factor and additional construction factor are not mentioned in Part 9, but is allowed based on good practice.

Example: Span governed by bending, width = 3.6 m, 3 ply 38x286, S-P-F No. 1 & 2, two floors in a house, the Span Book says that the span is 2.79 m

Factored wf = 3.6 x (2x0.5x1.25 + 3.3x1.5) = 22.3 kN/m
Mr = 18.2 kNm x 1.1 (construction factor) = 21.78 kNm
wL2 / 8 = 22.3 x 2.792 / 8 = 21.7 kN.m, therefore Mr = Mf

 

[jayrod12]

I don't understand how the O86 would only allow a 10% increase but the NBC allows a 32% increase but doesn't directly reference it anywhere.

And miraculously your example fails as per the CSA O86 by 21% (according to woodworks)

 

[ajk1]

Yes thank you Jayrod, I have precisely the same discomfort as you.

I think I will draft a letter to NBC, Ontario Building Code (OBC) and CSA and the Wood Council about this. Seems like lack of coordination between the groups, as well as lack of clarity.

Probably too late to do anything about it now for the upcoming 2015 editions, except perhaps the Wood Design Manual and Span Book and the OBC. Maybe some others should also send letters if they are in agreement that there is a problem

I wonder if the U.S. has something similar to the NBC Part 9 for small buildings and to the Span Book and what they say. Would be interesting.

 

[BAretired]

Part 9 has always been less restrictive than Part 4 of the NBC. Last time I wrote to NBC/CSA pointing out contradictions, I was told that Part 9 is based on traditional usage and if I was aware of a specific situation where the use of Part 9 resulted in failure or unacceptable performance, to let them know. I had no examples to offer so I let the issue die.

Good luck with your letter.

BA

 

[CELinOttawa]

When I interned with a very old and well established Structural Engineering firm the differences between Part 9 and Part 4 were one of my first lessons.

If you think the spans look dodgy, try doing any of the most basic vibration checks for the tables in Part 9.

If you think that's bad enough, try to make the Part 9 handrails work.

If you don't like any of that, then don't look at the foundation provisions of Part 9. They're practically the root-mean-square of what you'd really need.

 

[jayrod12]

So because contractors have been using undersized beams for the last bunch of years (traditional usage) they are allowed to deviate from the design standards? That is a cop-out answer if you ask me.

Remember that for span tables to be used the member must meet the specific criteria laid out in part 9 including uniform loading. Once the loading is non-uniform or a point load is to be supported by the span (which is becoming much more common as the desire for open concept floor areas grows) then you cannot use the span tables and it kicks you back to using the O86.

 

[CELinOttawa]

I agree Jayrod, wholeheartedly, but that it not what really happens... Even worse you should take a look at some of the TACBOC details of what is permitted without an Engineer's involvement in the Toronto area. Shocking.

 

[BAretired]

TACBOC = Toronto Area Chief Building Officials Committee

Does Toronto have building standards which differ from the Ontario Building Code?



BA

 

[ajk1]

I am not quite so much against the leniencies of Part 9, as I am against the lack of clarity and explanation, and lack of some commentary to bring these differences to the user's attention. This would apply particularly to the Wood Design Manual...and to Woodworks program...or are they intended to be used only with Part 4?

Jayrod12 makes some particularly good points; along the same lines, I have a perimeter built-up beam, but I suppose I can use neither the Tables in the Span Book because they do not include perimeter built up beams, nor can I calculate it using the 1.2 x 1.1 factor for bending in the Span Book ...or is it ok to calculate it using these factors even though there are no tables for perimeter built-up beams?

As for clarity, the NBC and provincial codes should be make it clear that when a multi-span beam has butt joint in the span for some of the plies, that its strength can be calculated based on all plies being equally loaded (if loaded from the top) and based on moments derived from full continuity of the member, if the Code requirements for splice locations , nailing and the like are followed and the strength is checked at the splice point based on only the non-butted plies. Currently it leaves it to your imagination as to whether all plies are equally effective, etc.

 

[CELinOttawa]

They used to, but have (officially) adopted the OBC because of a provincial law. I know a few of the Building Officials around here (Ottawa) and one is a close personal friend from way back. He's flat out told me that Toronto has a reputation for behaving as if they still had their own code, frequently permit works based on the standards they applied before the OBC became the whole-province mandatory code, and permitted construction based on "custom and practice" reflected in the TACBOC standard details books that Ottawa wouldn't even touch with a ten foot pole.

Some of this may very well be "little City" people being resentful of "big City" status within committees, etc, but I trust the source of this info implicitly. That and I've seen some real disasters (no pun intended) with permits and without a prayer in the Toronto area.

 

[ajk1]

I live in Toronto and am not aware of what you say. I would be very surprised if that were true, at least in terms of structure. I have no knowledge in non-structural matters. Anyway, the structure to which my questions are addressed is near Bracebridge, nowhere near Toronto.

My experience is that as far as structure is concerned, building departments are in general not very proactive. They are most concerned about collecting the permit fee, and that the submitted drawings not be in colour and the like.

By the way, a house under renovation in Toronto, about 2 kilometres from where I live, collapsed yesterday, reportedly while the foundations were being underpinned,and one worker was killed. This is the 2nd time that I recall a house collapsing while being underpinned in Toronto in the last two years. You would think that the building department would be more vigilant on house underpinning, but I wonder if they had a building permit...

 

[CELinOttawa]

Look up the TACBOC standard details, then tell me how surprised you are.