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Brian
Brian, Architect
Category: Home Improvement
Satisfied Customers: 3123
Experience:  Licensed Architect- 17 years, L.E.E.D. AP
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We are building a balcony, whereon a built-up (3)2x12 beam

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We are building a balcony, whereon a built-up (3)2x12 beam will sit atop 6x6 posts and utimately carry the floor joists. We have consulted and obeyed code thus far, and utilized "over engineering" with regards XXXXX XXXXX & post and joist material selection etc..., However we are not familiar with proper "quarter point technique" and wish to ensure that our built up beam is properly constructed.

The overall beam length must be 48 foot 3 inches, it needs to be constructed of 3 plys of 10 foot long pressure treated spruce 2x12's. The (6)6x6 posts are arranged 9'3" O.C. The beam cantilevers 8 inches over the far edge of the two end posts.- We intend to use a nailing pattern of 4 spikes per side 16" O.C. So the question is where do we locate our joints when laminating (constructing) said beam?

Brian :

Hello and welcome to Just Answer.

The joints should be staggered within the 3 plys and the overall beam will be strongest if the joints are as close as possible to the 6x6 posts.

If you need further help or clarification on this answer, please do not hesitate to reply to this post.

Customer:

Thank-you for your response Brian, however in this case you are not telling me anything I do not know. I am afraid that question may be over-reaching as it probably requires the creation of small drawing that specifies the joint location in relation to the post locations, and follows certain rules regarding how many joints can be present within each "post-span". I realize that in our region- Alberta, Canada, our local building code, has an appendix to the floor systems section that has a sample drawing of what |I am referring to but I do not have access to any such examples at present(although I should have access to a code book tomorrow). Do you have any more specific knowledge of this type of beam construction?

Brian :

Yes, I will explain the quarter point construction and will verbally explain the joint location. If needed I could do a drawing, but I think I can explain it.....give me a second to type.

Customer:

Great!

Brian :

Joints are best made over bearing points, but joints located within the quarter points are relatively just as strong with a built-up beam. In regards XXXXX XXXXX "quarter point" you simply divide the span into four parts. In your case, the span is 9'-3"....so 1/4 of that is approximately 2'-4". 2'-4" is the quarter point of the beam. With a continuous beam, such as yours, joints need to be within 6 inches of the end quarter point of the beam span. This means that the joints need to be located 1'-10" or less from each 6x6 post. In this area, the bending forces and the shear forces are relatively low. There shouldn't be more than one joint at each quarter point. With your 3 ply construction in the beam, two must be continuous through the quarter point. All joints should be at least four feet apart.


 

Customer:

Does the "four feet apart" rule apply to joints that land directly over the post. I.E. One exterior joint on the post, one interior joint 1'10" beyond the post and second exterior joint no sooner then 1' 10" from the next post(which would require a 209" -2x12) ?

Brian :

Yes, the joints should be 4 feet apart regardless if a post occurs or not. This may make it hard to do with 10 foot lumber now that I think about it. Can you use longer lumber? If not, I may have to draw this to see if it will work.

Customer:

I do not think it will work with the 10 foot lumber, it may have if I had used one more pile and post thus reducing the spans, but.... So I either have to purchase additional lumber say 12, 16 and even 20 footers(which will not be readily available). or scrap this method and place all joints directly on each post essentially building 5 small 3 ply beams with the lumber we have?

Brian :

Well, the 4 foot joint separation is not critical....it's just ideal. As long as you keep the joints within the 1'-10" from posts you will definitely have a strong beam. Just try to space the joints as far as possible, from each other, with the lumber that you have.....at least stagger them and don't allow all three joints to align....even over a post it will be weak at that point.

Customer:

Well! you certainly answered my question, and gave me an education I will keep well into the future I will now pay you!

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