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Rafter loading and reactions

Rafter loading and reactions

Rafter loading and reactions

(OP)
I need some info on calcs for a rafter so i can design the supporting members below. my software will not calculate anything over 16/12 pitch. Can anyone help?

RE: Rafter loading and reactions

Perhaps use basic structural engineering analysis and design methods?

RE: Rafter loading and reactions

Bk66440:
If the computer doesn’t work, how are we ever going to get anything done? Did you have courses in Engineering Mechanics (Statics, etc.), Strength of Materials, and Timber Design? If yes, then you should be able to do this problems by hand, following the NDS and the bldg. code; if no, then you probably shouldn’t be doing these types of problems. Why don’t you show a sketch of the roof system so we can actually see what you are working on, with dimensions, loads, etc? Then start to explain how you would go about solving this design problem, and come back here, going as far as you can get, with a second post on this thread. Otherwise, your OP kinda says give me a table of formulas and I’ll plug in some numbers, and see what I get, and still not understand how to actually solve this problem. Get with it.

RE: Rafter loading and reactions

I can help. My fee is $120/hr.

RE: Rafter loading and reactions

maybe redesign the structure with 16" pitch ? c'mon, feel the love!

Quando Omni Flunkus Moritati

RE: Rafter loading and reactions

Its a bit disappointing seeing threads like this !

RE: Rafter loading and reactions

I think proper rafter design is one of the most misunderstood structural concepts out there. The rafters will act in both compression and in bending. The lateral thrust force is often not taken into account. If you factor in collar ties, a simple wood frame is an indeterminate structure with complex lateral loads.

I try to avoid true rafter framing as much as possible. It's extremely hard to properly analyze, and I'm not sure the skill is out there to build it properly.

Sharpen your pencil and get out your statics book. A 16/12 pitch is not going to be much of a problem, just remember to design for all of the horizontal reactions as well.

RE: Rafter loading and reactions

@Manstrom

What is "true rafter framing"
What do you use instead?

RE: Rafter loading and reactions

I don't find it necessary to run collar-tied rafters on STAAD. The methods most engineers use are approximate, and adequate enough to ensure a safe and reasonable design.

RE: Rafter loading and reactions

What am I missing here? Shouldn't even 2D software be able to model a direct stiffness problem like this, obtain the loads, then apply the max loads/calculate stresses by hand?

But as others have said-I know of many homes with collar-tied rafters designed with paper and pencil.

RE: Rafter loading and reactions

Shoot, I know of many homes without collar ties, designed by no one other than the carpenter. That doesn't mean it's the standard to which we adhere, I'm just interjecting a little additional reality into the situation. Some fail over time, some don't. Houses do weird things with internal "non-bearing" walls carrying portions of loads, stresses redistributing, joints relaxing, etc.

And they can be more difficult to properly design than a mid-rise building, in some cases.

Further, many an engineer's been laughed at by construction superintendents when the later is told that something he's done many times won't work.

bk66440,

Return to statics and analyze it. Or just take the horizontal projection of both the rafter and the load and calculate the bending moment that way. Then take the same force and resolve it into it's perpendicular and axial components to get the axial force on the rafter and the thrust on the perimeter beam. In fact, do that on one of rafters you've designed previously using your software and compare results.

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