Rafter Spans and Purlin braces

[Uncle Bob]

I’m working on an amendment to require rafters to meet minimum span requirements without the need for purlin bracing. In the last year I have not seen a home built that could not be built without the necessity for purlin bracing.

Before biting my head off please here me out? I’m in Tornado Alley, and if you watched the news lately you know that we have been beaten up pretty bad in central Oklahoma. One of the problems I see contributing to the destruction is the use of 2 X 6 rafters necessitating the use of large double LVL beams to support the purlin braces; when they could have used 2 X 8 or 2 X10 rafters and not have had to use purlin braces at all. Imagine these huge beams stretching across the attics being hit by a tornado which use them as a tool of destruction. In some cases I’m talking about double 16” LVLs stretching some 20 odd feet and more in the attic.

Before I go off the deep end, do you see any problems with what I’m trying to accomplish. I’ll not take any suggestions or criticisms personally.

Thanks,

UncleBob

 

[pwood]

ub,

we are running out of trees large enough to make decent sized lumber. i inspected a fire rebuild the other day that required 20' 2x12 #1 for rafters. The guy with the grading stamp must have been bribed or blind, real garbage.

 

[Rider Rick]

Uncle Bob,

Why not use 2x6 for the purlin bracing in place of beams?

 

[Rio]

I was reading an article a while back about hurricanes and how to keep houses intact and the gist of it seemed to be to have good connecting hardware from the rafters to the foundation. This included hurricane ties from the rafters to the plates and studs, sufficient A.B.'s from the sill plate to the foundation and possibly some sort of connector from the studs to the sill plates instead of just relying on the end nailing of the studs.

It also mentioned that one of the biggest problems was with the plywood sheathing getting blown off and once that happened then the suction pulling the house apart. It said that if ring shank nails were required that alone would prevent the vast majority of roof sheathing from being ripped off. I remember thinking to myself that it made sense from having to try and pull ring shank nails out of lumber and ending up breaking the nails before they would come out.

 

[jar546]

UB,

I would think that changing the wind speed designation would have more of an effect. The issues as I see see them, and I could be wrong are:

1) Uplift requirements

2) Sheathing fastening

3) Improved gable end bracing

That is is just for the roof.

 

[Uncle Bob]

Jeff, I would really like to see the wind speed category change for us. It would be a help; and as some have stated even going so far as to adopting the Florida code requirements for high winds. thanks, Uncle Bob

 

[MASSDRIVER]

Nothing seems to keep a tornado frm ripping a house apart. Instead of lighter projectiles, regulations would simply require larger lumber attached to those larger purlins. I just finished a roof structure yesterday using 2x8 rafters 16" oc spanning 17' 6", over 2x8 joists 16" oc, purlin required one nominal size over the rafter on struts same as rafter size. Full width shear on attic wall to shear walls running 42' width of building with all the required hardware and transfer. On a 4-12 pitch ranch house.

Gets kind of silly sometimes.

Brent.

 

[jar546]

Nothing seems to keep a tornado frm ripping a house apart. Instead of lighter projectiles, regulations would simply require larger lumber attached to those larger purlins. I just finished a roof structure yesterday using 2x8 rafters 16" oc spanning 17' 6", over 2x8 joists 16" oc, purlin required one nominal size over the rafter on struts same as rafter size. Full width shear on attic wall to shear walls running 42' width of building with all the required hardware and transfer. On a 4-12 pitch ranch house.Gets kind of silly sometimes.

Brent.

Not all tornados are category 5. It is the majority of the tornados where an increase in the wind zone would help exponentially. There is a lot more other than lumber size involved with an increase in wind zone category.

 

[MASSDRIVER]

My point, more clearly, is to see stronger, more lightweight, more "elegant design" from the engineering crowd rather than the-more-the -better approach, which is uninspired and wasteful.

There is some very good, minimized engineering out there that I would like to see more of, and I'm sort of following that in with the OP's thoughts.

My concern is that we may head down the road with upsizing rafters, followed by upsizing everything associated with it for lack of intelligent engineering.

Brent.

 

[Uncle Bob]

Thanks for all the input,

Jeff and Massdriver, I think we are on the same page here.

As of tomorrow, July 1, I will have been working here for one year. When I came here local builders were submitting, as a “set of plans”, one room layout and two elevations (front and back). Now, thanks to new management, who understands what this old code preacher is talking about, I am getting a full set of plans.

After two weeks on the job, I figured I would be here two, maybe four months max. I don’t work for the money; so, if you don’t want to play by the rules don’t hire me. But, I digress. Thanks to the backing of a new management team that has the political skills, as well a basic knowledge and experience with codes; I now get a complete set of plans, including truss layouts and specs.

For the first time in my chosen profession; I’m stepping out of my box and attempting to assist builders with producing a safer, stronger product; and I think it will cost less than the old way of throwing bigger massive engineered lumber at it. Please keep in mind that being a “builder” here doesn’t mean you have any knowledge of construction; it only means that you have money or credit; knowledge is not required; although I am blessed with a few knowledgeable builders.

Living in Tornado Alley has opened my eyes to some very alarming and yet hopefully preventable catastrophes. So, my first step is to attempt to adopt a code requirement that prescriptive methods are required, where financially (without raising costs unduly) and structurally practicable. That would mean that engineering is not optional where code requirements are prescribed (no substitutions).

Here is an example using 2009 IRC, Table R802.5.1 (1):

Some builders here use 2 X 6 rafters (a practice used when they were allowed to use ceiling joists and rat runs as purlin brace supports) in areas that, by code, require an engineered beam in the attic to support the braces; where they could use 2 X 8 or 10s; and not need the massive engineered beam strung through the attic to support purlin braces, eliminating the requirement for purlins altogether; and taking the double LVLs out of the attic.

I may be out of my environment here, and that is why I seek your input as stated in the OP.

Thanks,

Uncle Bob

(sorry for not spacing the paragraphs, but my computer won't let me)

 

[jar546]

Rafter Spans and Purlin braces

My hat is off to you UB for what you are doing to change the quality of construction and improve safety for the residents of that community

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