I'm building a modified version of the Builder's Cottage from the Enchilada plans set with the extended side walls.  The primary modifications are the cabin will only be 14x16 and the extended side walls will be 10' instead of 12'.

My question concerns the foundation beams.  I'll be using 4 poured piers on each side for a pier spacing of roughly 4'-10".  The plans call for a built up beam of 3 plywood sandwiched 2x12's for a 8' pier spacing.  What comparable size beam would I need with the 4'-10" spacing?

Secondly, would it be a bad idea to use box beams for the foundation rather than the built up 2x beams?  I've read the APA document concerning box beams posted elsewhere and as far as span ratings, they should be stronger than a similar built up beam.  I always see them referenced for use as roof beams or headers though so I wonder if compressive strength would be an issue for a foundation beam.  If possible, I'd prefer to use a box beam because I can build them straighter and transport them easier. 

Thanks.

I'm not sure about the box beams.   However the easy way to build and handle the built up beams is to build them in place on the posts where they will be used.

http://countryplans.com/smf/index.php?topic=2335.msg60954#msg60954

scroll down to the second picture.

Thanks for the response.  If I use built-up beams, I probably will build them "on-site" rather than "in-place" because I was planning on pouring the piers with the beam temporarily in place and post anchors attached.  That way I don't have to perfectly level and align the pier tubes and footings before putting the beam in place, just level the beam itself.  I'm building on solid rock so the piers aren't buried, just a 16x16x5 base with 8" sonotube uprights and I'll be able to move them around a bit and cut the sonotubes to the appropriate height based on the beam itself.  So my main concern is building a straight beam.  The box beam approach interested me because I know I could build it straight in my shop at home.

But maybe I'm going about it wrong and should just try to pour the piers with embedded post anchors straight and level and build the beam in-place like in the pictures you mentioned.

You can use a 2x4 or two to hold the brackets in the concrete. Mount then with screws and remove after the concrete sets.  The weight of those beams add up quickly


something along these lines....



Thanks to PEG for the image.

I did the same... I built them in place.  

Resizing the beam will need engineering advice to be certain.  

I built my beams, used 2X10's on piers that were spaced approximately 6'.  My cabin is 14X20, 8' walls, significant potential for snow load.  I used no engineering to size these... I just swag'd it and am hoping that it will be fine.  Based on other long term structures already on the property, I am overbuilt.  

I don't think box beams would have been a good choice for me... the beams would have been in a high humidity environment, and I would be worried about delamination.  My pix with my beams start here : http://countryplans.com/smf/index.php?topic=4782.msg94308#msg94308

Quote from: MountainDon on May 17, 2010, 12:02:04 PM
You can use a 2x4 or two to hold the brackets in the concrete. Mount then with screws and remove after the concrete sets.  The weight of those beams add up quickly


something along these lines....

That was a good idea.  Keep in mind that the bases of the simpson clips are frequently non-planar, so be careful when setting the clips in the concrete base prior to building the beam.  Low spots can be shimmed, high spots necessitate some wood removal. Yuck. 

I agree, using the 2x4's as a temp beam is a good idea.  I'm convinced, I'll pour first and build the actual beams in place.  Building in-place makes the beam sizing less of an issue as far as transport so I'll probably mimic NM_Shooter and use 2x10's.

I agree with the suggestions here. I suspect that using built-up 2x10 beams will be more than enough for the spans suggested. Also the use of a couple of stiff 2xs for bracket holders is a better suggestion that prebuilding the beams.