Loft Joist Methods...

[ajbremer]

Thursday Morning - February 9th, 2012 Oklahoma

Ok, on John's 20x30 1 1/2 plans - the loft joists, what to use?

I have seen many different methods here, parallel to long walls, parallel to gable, (seems to be
the norm.), double 2x12's on the 20' width, i joist on the 20' width, etc.

My first thought is to use I-Joist parallel to the gable walls every 16". This seems like it
would give more support to those long walls that hold the roof up. I called a lumber place
and told him how some people use doubled 2x12's. He said two 2x12's would cost $48.00
but one 'stronger' i-joist cost $38.00.

I'm going to have a loft at each end of the 30' length. One will be around 10' and the
other will be about 8'. There will be a catwalk between the two lofts. There really shouldn't
be that many variables in this choice. I'll also be supporting the loft with sistered studs inside
the wall cavities.

My thought is that I should go with i-joist every 16" on the 20' width and connect the catwalk
by way another couple I-Joists (or doubled 2x12's). The catwalk joists will be around 12' long.

Any help or comments on this would be greatly appreciated. This is the next step in my build.

[Don_P]

First look down, bear on the girders below. Then look up, tie the roof together as best you can. The joist should be running parallel to the gable wall, bearing on the girders and tieing the rafters. Sheathing should be on the walls before you add top weight, a few nails in some braces is risky with weight swinging around up high.

I joists will have a harder time developing a good tension connection to the studs if you are restraing thrust from the roof. Why are you comparing a single I joist to a double 2x12? #2 SYP 2x12's @ 16" oc will work.

The joists supporting the catwalk are taking a serious point load at midspan, they need to be LVLs and some cyphering needs to go into that.

There is no tie in the center of the building, technically it needs to go to ridgebeam construction there. Run the handrail posts for the catwalk from the LVL joists all the way up to doubled rafters that in turn support an LVL ridge over the bridge (kinda has a ring to it  ). The connections need design. The LVL's just took more load. Trace and reinforce the supports under those LVL's down to ground, directly over piers if at all possible, (We stepped over the engineer line at the catwalk and have been traipsing down an ever narrowing road for two paragraphs now, just sayin)

[ajbremer]

Friday Night - February 10th, 2012 Oklahoma

Wow Don_P,

You have covered a lot of information right there! Thank you for letting me know about getting
my sheathing on the walls before I start to add weight near the top, I didn't think about that.
I also understand what your trying to say about the 'tension connection' of the I Joist to the studs.
Because of their shape, they wouldn't have the surface area that let's say a 2x12 would have against
the studs. Maybe I could fill in their cavity with a board and then nail it to the stud?

I was comparing the I-Joist to doubled 2x12's because I thought that some people may have used
doubled 2x12's for loft joists. I guess I thought that I had read, heard, or seen where some may
have said that a single 2x12 spanning 20' wouldn't be strong enough or as strong as one of
these I-Joist spanning the 20 feet.

Thank you also for pointing out the point load at mid-span of the catwalk. You mentioned that the
catwalk joist should be LVLs, couldn't they be those I-Joist? The main loft end has an interior wall
at the end and under it is the middle girder. The other loft end is where I could put a post or
a nice looking shellacked tree post. Two 10 or 12 foot long I-Joist would be plenty strong enough
between the two lofts in this scenario wouldn't it?

Another way to support a catwalk is found in these next pictures that I took while visiting my parents
in Florida during Christmas of 2011. They live in Zolfo Springs and we took a 45 minute ride to
Punta Gorda, Florida at a place called Fisherman's Village. They had a lot of stores on a huge pier
and I just happen to notice a catwalk above us as we were walking. Are these catwalk supports
something like you were describing? This is what I would love to do! Here's the pics:

[Don_P]

The main loft end has an interior wall
at the end and under it is the middle girder. The other loft end is where I could put a post or
a nice looking shellacked tree post. Two 10 or 12 foot long I-Joist would be plenty strong enough
between the two lofts in this scenario wouldn't it?

Basically this will work, but some details... a tji cannot hang from another tji, they are designed for uniform top  this would put a side hung point load on them. The catwalk joists could bear on the wall at that end. The other end I think I'd use an lvl, They are made to have hangers and I joists dangling from them,  it will be considerably smaller if there is a post in the middle.

You can fill the tji sides flush at the stud end, I'd use chunks of ply on each side, C clamp them and use a healthy number of nails.

If you move the the floor to roof weathered vertical posts to the corners of the white railings at the ends of the catwalks, that would be what I'm describing. I'm trying to get some structural ridge supports. you had mentioned the possibility of a dormer, it'll have issues with tieing, there is an issue with thrust over the catwalk section. A structural ridge makes that all go away.

[MountainDon]

Or using those nice engineered roof trusses that we saw drawings for a few months back. The truss engineer should be able to work out a dormer design I would think. Then yiu would eliminate and wall lateral loading.

[rick91351]

Or using those nice engineered roof trusses that we saw drawings for a few months back. The truss engineer should be able to work out a dormer design I would think. Then yiu would eliminate and wall lateral loading.

The truss guys here are very good or their software is very good at such.  So long as you are buying from them 'no extra charge'.  Gee I wonder if it is built in the the cost?     Grin.....   Actually they do a great job here and would think it much the same there. 

[Don_P]

They can also design the floors and catwalk, most of those plants sell and support a full line of engineered wood products, trusses, TJI's, LVL's,  Gluluam, etc.

The caveat with their engineering is that their design usually only covers their members, you are responsible for supporting and bracing it.