ridge supported by rafters 4

[duggerdoo]

for sloped rafters bearing on the top plate of a wall on either side of a ridge beam, on a gabled roof...dont the rafters offer vertical support for the ridge. thus the ridge span is the rafter spacing. or is this for roofs with enough pitch that the rafters have a significant vertical component versus horizontal. of course, a light gage shallow CEE or ZEE section may not carry much, but steel beams would.
by the way, the roof Im talking about is of steel welded members. A Free Body Diagram is unneccessary as this is a simple question.

 

[XR250]

Draw a free body diagram of the rafter then get back to us.

 

[human909]

Welcome to the forum duggerdoo. There is a wealth of knowledge available here from a friendly and helpful crew. I suggest you take the good advice given by XR250.

You are correct it is a simple question. Which is why a simple approach of working out the load paths is should be all that is needed. Most of us can do that in our heads. Others need practice by drawing out the load paths. Nothing wrong with that, it is part of the process.

 

[duggerdoo]

didnt see advice. draw fbd? did better, gave a sketch. the loads dont matter (LL+DL), the question remains

 

[mmarlow]

Duggerdoo,

What keeps the "end beam" from moving laterally? This thrust is typically handled with a collar tie.


-MMARLOW EIT

 

[human909]

You are correct. The loads magnitudes don't matter. But the load path DOES matter. Until that is determined the question can't readily be answered.

 

[duggerdoo]

there will be rafter ties (lower third)
but that is not the question...will the rafters provide vertical support to the ridge

 

[mmarlow]

Duggerdoo,


Understood. Your sketch didnt show a tie and a free body diagram would have gotten you to the point that a tie would be required.

If there is a tie, id refer to it as a ridge plate and it is supported by the rafters.

If you have no tie, you need a ridge beam supported at its ends.



-MMARLOW EIT

 

[duggerdoo]

btw,mmarlow, collar ties are located in the upper third of the rafters and provide tension in uplift to prevent the rafters from separating from the ridge.

please address the posed question.

 

[duggerdoo]

mmarlow,
so if there are rafter ties...you say the rafters support the ridge vertically. meaning the ridge beam is sized using the rafter spacings as ridge spans?

 

[mmarlow]

Duggerdoo,

You said lower 3rd previously.

The rafters do not support the ridge in your sketch. The pitch does not matter.

You are correct, it is a simple question. A FBD gets you to the answer quite quickly.

-MMARLOW EIT

 

[mmarlow]

Duggerdoo,

I typed rafter tie when I should have typed collare tie.. but yes.

In typical wood framed construction the ridge plate is supported by the rafters when there are collar ties. I dont know why that wide flange beam at the ridge would need to be there. The collar ties will need to be in the lower third, maybe at the bottom.

That all being said, you might have trouble getting collar ties to work with such a low pitch (it will result in a high thrust).





-MMARLOW EIT

 

[duggerdoo]

Mmarlow,
the latest version of the sketch shows rafter and collar ties.
the W shape was the ridge because I first sized it spanning the length of the building. with end columns.
if the rafters provide vertical support...then the ridge can be a rect.tube and much smaller.
Im not going to detail the whole structure here, so the ancillary questions are not going to be addressed here. just getting a feel for the rafters supporting the ridge is enough today.

 

[mmarlow]

Duggerdoo, when you work through your analysis, you may find you have to lower your ties

Hope this was helpful

-MMARLOW EIT

 

[Enable]

You seem a tad abrasive but I can possibly chalk that up to the difficulties of internet communication. That said, your resistance to drawing a FBD due to the "easiness" of the question is utter garbage and I hope you dispense with that notion swiftly. A FBD is beneath no engineer but it seems like you think that it is something reserved for first year static students. A FBD is the number one way to understand your structure whether it is simple or otherwise and any engineer worth their salt will tell you so.

BTW: had you listened to the advice you would have been forced to think about the support assumptions. Those assumptions in turn will tell you what supports what. Also, you should probably know that all our models are based on such support assumptions and in reality things are, well, different. So practically speaking both things will happen (the ridge will support the rafters and act as a beam, plus the rafters will support the ridge such that it acts as a through-transfer mechanism only..it's all about stiffness in the respective load paths). In more explicit terms, if you make it stiffener as a truss it'll act more like a truss and if you make it a very flimsy truss (pretty much non-existent) your rafters wont be truss members any more rather spanning members to the ridge (now) beam.



CWB (W47.1) Div 1 Fabricator
Temporary Works Design

https://www.enable-inc.com/

 

[MIStructE_IRE]

If it can’t go outward it can’t go down..

I suggest you draw the FBD as noted above.

 

[duggerdoo]

everything is steel. flat bar or small tubes for the ties. they can handle the tension.
your concern is about the the rafter spread or thrust on the end beams/columns...thus your comments about ties. but my question is simply about the vertical support of the ridge, whether the rafters can do the job or does there need to be something else.

my memory of eng tips is that most questions are not explicit enough to get a direct answer. so you get a large volume of ancillary answers and comments until you finally zone in on a direct answer. sometimes that's good, but many times, it is irritating when you're trying to get that one answer... such is life.

I seriously appreciate all of the comments...in the future, I will try to eliminate ancillary questions and answers so that the point of the main question can be addressed.

thanks guys

btw...Im not anti-fbd as one may think. but, there is always more than one way to skin a cat.

 

[WARose]

[blue](OP)[/blue]

dont the rafters offer vertical support for the ridge. thus the ridge span is the rafter spacing

[later post]

everything is steel.

If it's all steel, it's kind of a different animal. Speaking from a wood perspective: The rafters would have to be pretty stiff to do that. (Just eyeballing it.) A 3:12 slope is borderline where a ridge board has to become a ridge beam. (As per the IRC.) If you've got collar and rafter ties....you generally don't need a ridge beam.

But I'd just model it to know what is what. (Since you likely cannot do this by a prescriptive code like the IRC anyway.)

 

[human909]

You seem a tad abrasive but I can possibly chalk that up to the difficulties of internet communication. That said, your resistance to drawing a FBD due to the "easiness" of the question is utter garbage and I hope you dispense with that notion swiftly. A FBD is beneath no engineer but it seems like you think that it is something reserved for first year static students. A FBD is the number one way to understand your structure whether it is simple or otherwise and any engineer worth their salt will tell you so.

Agreed. Understanding load paths is essential and it seems many engineer make it through their study and possibly many years into practice without a great comprehension of these.

I've been training up a fresh graduate recently. I was shocked at his lack of simple understanding of load paths even in basic determinant structure with pinned connections. But I stepped him through it and he improved greatly. I wouldn't say he is a 'natural' engineer but he is diligent and has quickly become a helpful and necessary asset to me and the company.

I'm not implying that any of this applies to the post here. But the resistance to drawing a FBD hasn't helped this thread.

 

[phamENG]

duggerdo - let's back up a second and ask this question: why do you need to support the ridge?

Regardless of the material, the overall rules of physics still apply and if detailed with 'pinned' connections it'll behave a lot like a wood roof, at least in the analysis. In a wood roof, the board is there to help the carpenter put the rafters up and braces the rafters along that line. It doesn't really do anything for gravity loading. Look at a truss roof - most of them don't have anything at the ridge, just some out of plane bracing. So as long as your roof deck is going to span between your 'rafter' beams, your ridge is really only there to brace your rafters at the ridge. That is unless you're hanging something else from it - that's another story. In that case, the rafters could be capable of supporting it.

So the answer to your specific question: if you design it to do it, yes it could.

I'm not sure how long you've been in engineering, but the dismissal of advice from a bunch of engineers who have been doing this for several decades is not a smart move. You asked a specific question, and that's good, but it's important to realize that you're not always going to know the right question to ask. Some of these guys have trained enough junior engineers to know where an underlying problem is based on the higher level question that is being asked. We'd all do well to pay attention to their guidance.

We're here because we enjoy discussing structural engineering with one another, to seek help when we need it, and to offer it when we can. If you need a quick and succinct answer, going to your boss or another engineer in your office may be a better choice. If you want to delve into the how and why of the mechanism and break it down, then this is a great place to discuss it. Hopefully you stick around and contribute to all of our efforts to keep learning.