Comments on: Collar Ties and Ridge Beams

By: Tim Garrison

Tim Garrison — Tue, 08 Nov 2011 15:47:07 +0000

Hi Guts,
Welcome to the site, good to have you.

I couldn’t agree more that most guys in the field are sorely lacking in basic structural concepts. This site, books, and software is devoted to them.

I find that lack of knowledge in this area generally costs guys $ and jobs. Most builders are aware that they don’t really understand the concepts and so to compensate overbuild. In these tight times, who can afford that?

I was at a big sawmill yesterday helping the plant foreman with a steel cover for a new boiler. They welded roof trusses together with angles and flat bar! The county got wind and red tagged them. Now I have to somehow salvage this mess. I’ll do it, but it’s going to cost double what it would have if they’d have come to me before cranking up the welders. I see this kind of inefficiency every single day. It just tears me up.

By: Tim Garrison

Tim Garrison — Tue, 08 Nov 2011 15:39:48 +0000

Hi Snowman,

I thought the ridge board was under the top chords, but now I understand that it is truly a ridge board and the top chords butt up to it. Can’t get a gusset on that. Here’s a case where a collar tie can actually be useful. Can you use a 2×4 collar tie snug up against the bottom of the ridge board? This can be nailed to the top chords with 16s or SDS screws. This becomes your gusset.

Regarding my software’s ability to analyze trusses, no, it doesn’t do that. ProBeam designs any single beam, joist, or rafter. Truss software is massive, beyond what ConstructionCalc is about. In fact when I have a truss to analyze, I farm that task out to the local truss company who can bang that analysis out in less than an hour. Without the software it takes 4+ times longer.

By: GUTS

GUTS — Tue, 08 Nov 2011 14:08:03 +0000

TIM , you are good.

I have been reading a lot on this site. All of you have a lot to bring to this forum and most important is the help you give to all that want to receive it .
Here in northeast lots of roof structural failures have occurred over the last few years. Some from “contractors want to be” that refuse to get an education of structural engineering and or willing to read or apply any of it . I think some of this is disbelief in how things are in the real world .
The spans in clear openings beams for weight carrying roof design is in shambles. Some buildings people buy a mess or something a little off the edge.
I have noticed some of you are jointing/connecting lumber for structure such as roof design/repair/strengthening . Gluing with pl and gusseting using steel 12 gauge and thicker with thru bolts ,nuts washers is best . This is good for multiple pieces of 2X and greater thickness wood. For 2X and less ;Next to that is the glue and right kind of plywood with deck screws with washers to get more squish . Epoxies and resorcinol have their uses . Epoxy will crack and not bond well to some wood at all conditions that a “wood” may in it’s life time in a structure. Yes there are 3000 to 5000 pound epoxies out there but that is for the epoxies only not the bond or lack there of to wood that is flexing and or changing size from natural environment conditions. You all know that water soaked MDF is like oat-meal. So like the glue is the material good for the intended purpose. May I suggest that you build a test connection then do destructive testing. Like the only sure way to keep roof gutters from freezing is move them close to earths equator. No , heating then with free energy will not cure all. The heating system may fail. Test and prove your designs.
GUTS

By: snowman

snowman — Mon, 07 Nov 2011 20:29:56 +0000

Tim,
Thanks for responding to my truss question.

->You said:The gusset at the ridge has almost no stress in it. A moderate
notch there isn’t a problem. If the notch gets too big, say more than 25% of
the top chord depth, use thicker plywood gussett to compensate.

Unfortunately the ridge board is the same depth as the top cord; both are
2×6. So the notch would be ~100% of the top cord depth. My only hope is
there is no significant stress (assuming equal roof loading) across that
joint. I’m hoping the actual path of the tension force from the web is only
passed to the top cord *above* the web, rather than across to the top cord
on the other side. Do you see it that way too?

Can your structural software calculate Pratt truss stresses? If so, what is
the magnatude of the stress at the ridge gusset and the heels when the roof
is fully loaded with 50psi live + 10psi dead. With that information I can
properly scale the increase I may need in gusset surface contact area, so
the design integrity is maintained. With cured epoxy shear strength at
over 3000psi and the Loctite PL premium at about 500psi, I better check the
numbers.
Thanks, snowman

By: Tim Garrison

Tim Garrison — Mon, 07 Nov 2011 18:39:11 +0000

Hi Snowman,

The gusset at the ridge has almost no stress in it. A moderate notch there isn’t a problem. If the notch gets too big, say more than 25% of the top chord depth, use thicker plywood gussett to compensate.

Regarding that old school glue / epoxy, yeah it’s a mess and I don’t recommend it. Your Loctite is good. Here’s my standard spec on wood glue:

Glue. Where specified wood glue shall be commercial grade with minimum shear strength of 450 psi at 28 days. Use Liquid Nails LN-940 or equivalent. Prep and apply per manufacturer’s recommendations.

By: snowman

snowman — Fri, 04 Nov 2011 20:33:51 +0000

Tim,
I’ve been considering the truss retrofit discussed above and I’ve become concerned about the gusset at the ridge (Gusset “B” 10″H x 12″W in the link above). Because there is a ridge beam, I will need to notch the ridge gusset to fit around the 2×6 ridge board. I’m assuming this will compromise the truss design. If this does compromise the design, do you see a way around this? My impression is the king post is not under much stress but the webs are under major tension.
The truss was first designed in the 60′s (with revisions thru ’89) and they recommended using Casein, Resorcinol or Epoxy resin as the glue. While I’m willing to use epoxy, it is messy and will require mixing many batches. In view of this, I’m considering using a more modern adhesive such as Loctite PL Premium or PL and I’m wondering if it will be strong enough. I don’t have the calculated stress values. All I have to go on is the recommended gusset size for each joint (in the link), the nailing pattern and the stress assumptions for the plywood (from the MWPS9 publication).
Plywood stress assumption: 250 psi shear and 53 psi for rolling shear.
Nailing pattern: 5d or 6d box galvanized; 4″ spacing along grain, 2″ spacing across grain.
The nails are also used to hold the joint tight while the glue cures.
Thanks for any advice you can provide. – snowman

By: Tim Garrison

Tim Garrison — Wed, 26 Oct 2011 21:36:19 +0000

Hi Snowman,

I would go the truss route. The challenge will be the heel connections because the rafter and ceiling joist are side by side. A way around that is to sister a 2x, say 2′ long to either the rafter or joist to flush out a single face upon which a plywood gusset would be attached. I’d glue the 2x in place and glue (Liquid Nails or similar) the gusset as well, then connect everything with SDS lag screws of appropriate length so that each member is penetrated at least 3/4 of the way through. If you got 5 or 6 SDS screws in each the rafter and joist, you’d be golden.

By: snowman

snowman — Wed, 26 Oct 2011 19:01:18 +0000

Tim,
I’m in Maine and I’ve discovered a potential disaster in a 24 x24 garage, built in 1991, on my father-in-laws property. The code calls for 50PSF snow load and the garage is built almost exactly as your example above: Stick built 24 foot wide (door on gable end), 24 foot 2×6 ceiling joists on 48″, 13 foot 2×6 roof rafters on 24″, 5:12 pitch, all lumber #2 SPF. Roof has plywood sheathing and 1 layer asphalt, the ceiling is unsheathed. Note the 24′ ceiling joists are every other rafter with 2×4 high collar ties on the odd rafters (useless), rafters meet at peak on (non structural) 2×6 ridge beam. Walls seem OK: 2×4 on 16″, sheathed in T111 with double top plate. Each rafter heel is secured to the joists with only 4 12d nails. Front 4 rafters have been elevated about 6″ to clear GDO drive rail (so they are low collar ties). There are 2×4 wind braces at each gable end. There is sag evident in the joists, a little sag (1″) in the mid ridge but the walls have not bowed out (yet!).

I want to retrofit this structure before our luck runs out. We can’t add any mid-span posts because we need the clear space.
After I add the missing odd rafter ceiling joists, I am considering either retrofitting a stick built Pratt truss as shown here: http://www.public.iastate.edu/~mwps_dis/mwps_web/plans/truss_24.pdf
(I need to know if the current heel connections are feasible in this truss design)

OR would I be better off by sistering 10′ (2x?) onto the rafters? If that is the best way, how should I reinforce the joist heel connections?
Utimately we want to use the ceiling area for LIGHT storage (< 10PSF). But I recognize that we have to support the potential snow load as a first priority!
Thanks, snowman

By: Tim Garrison

Tim Garrison — Tue, 07 Jun 2011 14:16:30 +0000

To size a beam that supports the middle or so of rafters, I’d use ProBeam by ConstructionCalc. The span is the length of the beam, which could be long if it goes from gable to gable. The load is a uniform load over the length of the beam and the tributary width is half the distance to the wall on one side + half the distance to the ridge beam on the other. If there is no ridge beam then the trib width is half the distance from exterior wall to exterior wall where the rafters bear. Dead load will be about 10 and snow load will be what it is in your neck of the woods – around here it’s 25 psf. That’s all there is to it.

By: kjward

kjward — Tue, 07 Jun 2011 12:44:11 +0000

hi tim-

it is a term also used to denote an intermediate mid-span-ish support for over-spanned rafters. the mid-beam added as shown in the last sketch above could be considered a purlin. they are usually supported by braces to an interior bearing wall, much like the kickers in the next to last sketch above.

my question is how to size the beam to support the rafters mid-span when it is only supported at its ends?

thanks tim