Continuous wood built-up beam 1

[ajk1]

I am checking a 2-span 3-ply built-up wood beam in a newly constructed cottage. It is advantageous to take account of the continuity because one span is significantly less heavily loaded than the other span. It is over-spanned if checked as non-continuous. Can it be checked as continuous, if the ply members are jointed at the Code specified locations?

I did not design the beam; the issue arises because the contractor built it with fewer supporting piers than on the drawing, but he also increased the beam depth.

 

[manstrom]

Where are the code specified joints? What code specifies joint location?

If it a 3 ply beam and 2 of the plies are continuous, I would redistribute the moment based on this ratio.

Sounds like the contractor is going cowboy. If it needs a repair, repair it. Don't do him any favors.

 

[jayrod12]

Agree with manstrom

 

[ajk1]

"Cowboy" is an apt description, not only on this issue but others as well. Sound advice. Thanks. I believe the joint locations are specified in CSA O86, no? I have to go out now but will look it up when I get back.

Is it usual to design wood beams as continuous or does this come under the heading of "doing the contractor a favour"?

 

[CELinOttawa]

You can design as continuous, but even in timber this means continuous as best as I understand it... A beam with assembly joints is not a continuous beam unless these joints are designed for such a structure (ie: At or near points of inflection under all possible loading cases and designed to transferr the imposed loads).

No favours when plans are ignored without good cause. Also, if you play hero, the Contractor learns that Engineers may safely be ignored as well as YOU incurr all the liability.

 

[CANPRO]

ajk1, reference NBCC 2010 9.23.8.3. for splicing continuous beams.

Aside from Part 9, if it is already built you can analyze the beam in its existing condition to see if the splice locations are acceptable.

As for designing continuous wood beams...for me it depends on the contractor and how the size of the beam is affecting the rest of the design. I'm working on a large renovation job right now with continuous beams and I have no problem with it because I know the contractor does quality work and it will be built properly. I do some work with other contractors where if I show a continuous beam I make sure it works as a simple span because I know it won't be built properly. I usually only rely on the beam being continuous if I really need to in order to keep the beam depth reasonable.

I agree with others about not doing any favours. I don't look at it as doing a favour anyway...he deviated from the plan, if he wants to pay (or someone else pays) to check the existing condition then so be it. If it works, it works, if it doesn't...well then he'll have to fix it. I don't see this as you doing any favours, you're just analyzing the beam and reporting the results.

 

[SkiisAndBikes]

I have had success in taking field measurements of each splice location, then when analyzing as a continuous span in a program like WoodWorks, input each splice location as a point of interest. At each splice location you can then accurately determine shear and moment values and compare to the number of plies that you have at this specific location. One thing this method won't accurately calculate is any increases in deflection due to the splices.

It is often easier to add plies either side.

 

[ajk1]

ok, thanks for the all the comments. All sound very sensible. I have a feeling that even if the continuity is accounted for, it may perhaps still not work, but I will see.