Construction Question- beams

[DougW]

In the cabin I'm building there will be an attic loft for 2/3rds and open to cathedral ceiling in other third. With the tie beams I was going with need a center span support to cut span to 11 feet to support weight of loft ( 10 dead , 30 live load) . The two columns will go right on top of piers supporting the structure.

The question is can I put in the columns and beam latter when I want to do the loft?

Would have to jack up the tie beams to fit in column and beam , but can't see any other problem other than that.

Also the way I designed the beam was , span 20 feet, load 10 +30 over half the building width 22/2 11' . The tie beams would sit on the beam which of course would be supported at either end by columns. Beam cam out to a 8x16 hem- fir no.2 . So if I could wait on doing it until need space it would save some cash.

[runethechamp]

Depends what controls your design.

A quick calc says that with double the span for your ties (which are effectively your floor joists in the attic right?), but a quarter of the distributed load, you're looking at 10 times as much deflection of your joists/ties. I have just assumed no tension in them, just the deadload and liveload for this calc.

The moments (and stresses) from the decreased load/extra span are the same in both cases (the weight and langth variation cancels out).

Soooo, if you are controlled by deflections, you might run into problems, if moments/stresses are controlling, you should be ok. Don't know what the building codes in Canada says (and I'm not even that familiar with it here in the US either, but I know structures pretty well).

[Duder]

Submit an RFI...
Just kidding, unless it's concrete I would listen to rune.

[DougW]

Depends what controls your design.

A quick calc says that with double the span for your ties (which are effectively your floor joists in the attic right?), true just on 5' centers but a quarter of the distributed load, you're looking at 10 times as much deflection of your joists/ties. I have just assumed no tension in them, just the deadload and liveload for this calc.

The moments (and stresses) from the decreased load/extra span are the same in both cases (the weight and langth variation cancels out).

Soooo, if you are controlled by deflections, you might run into problems, if moments/stresses are controlling, you should be ok. Don't know what the building codes in Canada says (and I'm not even that familiar with it here in the US either, but I know structures pretty well).

Thing is that will not have any load until I install loft flooring. So until I do that will just be rafters and ties ( big ceiling joists 6x10) If they were ties they won't need to be so big. Sized them so when I had a loft with a center beam could take weight of loft. Will put in beam and column before put in floor in the attic . Actually won't have attic just open cathedral ceiling .

To put another way , until I put in loft the ties will be ties. When I put in the loft the ties become attic floor joists ( with a centre support beam)

[runethechamp]

Are you saying the ties are 6x10s?? What spacing do you have them at? UBC seems to say that ceiling joists have a deflection limit of L/240, can't find anything about ties except that they should be sapce not more than 4 feet on center. If you give me the dimension of the ties (6x10?), sapcing and type of lumber you are using I can at least see if they are acceptable per UBC.

[Crass3000]

Tech Talk JONG

[DougW]

Are you saying the ties are 6x10s?? What spacing do you have them at? UBC seems to say that ceiling joists have a deflection limit of L/240, can't find anything about ties except that they should be sapce not more than 4 feet on center. If you give me the dimension of the ties (6x10?), sapcing and type of lumber you are using I can at least see if they are acceptable per UBC.

this what I put into the program , 2 spans each 11 feet, load dead 20 psf, live load 30 psf, deflection limit live l/360, total L/240,

result Hem -Fir No. 2 6X10
or Hem- Fir No. 1 6x8

[Gebster]

this what I put into the program , 2 spans each 11 feet, load dead 20 psf, live load 30 psf, deflection limit live l/360, total L/240,

result Hem -Fir No. 2 6X10
or Hem- Fir No. 1 6x8

OMG...quoting the UBC in TGR. Fells like I am back in timber design class

[runethechamp]

this what I put into the program , 2 spans each 11 feet, load dead 20 psf, live load 30 psf, deflection limit live l/360, total L/240,

result Hem -Fir No. 2 6X10
or Hem- Fir No. 1 6x8

What about spacing? What size are your roof rafters?

I can't really see how you need a 6x10 for this unless these are beams that ultimately will support floor joists going perpendicular to their direction, and I don't think that is the case since the floor joists typically would be aligned with your roof rafters. Sure you are using the right part of the program?

[DougW]

What about spacing? What size are your roof rafters?

I can't really see how you need a 6x10 for this unless these are beams that ultimately will support floor joists going perpendicular to their direction, and I don't think that is the case since the floor joists typically would be aligned with your roof rafters. Sure you are using the right part of the program?

spacing is 5' oc so load width entered is 5'

yes they are in line with rafters, not planning on conventional joists probably floor decking 2x 6 right on the beams so a 5' span minus width of beam. NOw that part haven't thought through. Right now just thinking of the structure that would support it. But the roof decking I'm puting on top of the rafters would be able to take the load.

Back to the orginal question , could I put the mid support beam in later??

[DougW]

missed ? - roof rafters - 12/12 slope - load 15 dead and 40 snow again 5' spacing so width of load 5'

came to 6x10 Hem-fir No. 2

[runethechamp]

My calc for a simply supported 6x10 with a span of 22 feet gives a deflection of pretty much a quarter of the allowable (L/240), just counting the self weight of the beam, so you should be fine according to that.

Edit: I guess this is a little bit ont he outside of what UBC calls "conventional light frame construction".

[DougW]

Rune, I see what you were checking. Wasn't even thinking of that.

I was thinking would it work from a construction point of view to install it later. would be like a reno in a way but preplanned.