Need Math Help
I need help to make sure I am understanding something correctly. I have read a residential engineering book and the Engineering for Builders / associated with Taunton book and still am not sure this problem I understand fully.k
I need to ensure sizing of a post and beam for an outbuilding that is gable-ended, insulated attic, conditioned space. The beam will have roof joists hung on either side w/simpson steel connectors and the purpose is to get a larger clear span. I am sizing it for three layers of asphalt shingles.
I understand the formula for sizing a post, it comes down to how many pounds the post can stand before it buckles. I understand the formula for a beam, it comes down to the maximum pounds for an amount of acceptable sag. What I can’t get is how to determine what the loading on the beam, and thus the posts, will be. The books have live loads, dead loads, etc. etc.
So what I’m planning to do to calculate this is figure the weight of the roof joists, and the drywall, and some margin for crawling over it / light storage / insulation as the load on the beam. It seems like the weight of the roof is carried in entirety by the exterior walls and the rafters are in tension with the push-out force of the roof but not transferring any load other than their own weight to my central beam. Am I seeing this straight?
It is 18′ long gable-end to gable-end, and 24′ wide. The beam will be 18′ and support 14′ roof rafters crossing each other at the tails. The basic design is fixed.
remodeler, you are about to spend tens of thousands of dollars on this project, spend the extra $350 or so for a PE or architect to size your posts and beams, small money for your safety, for this money they will also look over your plans and give you other advice that will pay for itself many times over, believe me we dont go into this business to make a ton of money but we do have to eat as well!
note: have all of your plans, drawing, sketches together so you can minimize his time and your money outlay
I gotta tell ya - from your description, I have no clue what you're trying to do.
But from your apparent lack of experience, I'd say you're better off having someone look at what you're doing and design it for you.
Loads on beams aren't calced by figuring actual weights of materials. There are standards and minimums which are typically controlling.
In general, a simple load calc is half the span multiplied by the design load. ie a 24' wide garage roof at 40 PSF total load would have 480 PLF on the walls. (12 X 40) Some people add loading for the overhangs, some don't.
Q: How do you know you're leading a really pathetic life?
A: When a nymphomaniac tells you: "Let's just be friends."
This project is fairly low budget but I want to size things adequately. Basically I am trying to get a large open room with no bearing posts, etc. and using up some reclaimed materials.
I am planning an 18' x 24' outbuilding (no permit required in this jurisdiction) that will have posts at the mid-point in both 24' walls (the gable ends run on these walls) and an 18' beam spanning the building to support the joists which run the 24' length. So I will use 12' joists (+overhang) fastened to this central beam with simpson metal framing connectors.
I am planning to use 4x4 x 8' fir posts and 3- 2x12 x 18' lumber for making the beam. The roof will be 5/12 and shingled. I was hoping if someone could tell me if this is adequate on the post and beam sizing. Yes I'm amateur to this aspect - I do dirt / utilities for a living.
>This project is fairly low budget but I want to size things adequately.
If you aren't familiar with doing load calcs, then avoiding the hiring of a competent structural engineer because of the cost, is being "penny wise and pound foolish." There's a whole lot of risk you could be introducing into your project.
I agree whole heartedly with the other posters. You need to have an engineer check out what you are doing and what your needs will be. That said, your description is a little confusing. One thing is for certain the entire weight of the roof is not carried soley by the exterior walls. The load is split (not necessarily evenly) between the exterior walls, and the ridge. Which I am taking to mean your beam. The load on the beam in that case would be the weight of roof framing, sheathing, and roofing and interior finish materials. Since your post was a little confusing, I assume your ridge beam is sized to properly carry the load of the roof i.e. materials, snow loads with an added safety factor for example if a heavy branch were to fall on it. Plus being strong enough to span the 18 feet, with the addition of finish materials weight. you mentioned a post too, where this post would go would factor into the size of your ridge beam.
The rafters transfer the weight of everything ontop of them and fastened to them to the ridge beam, and wall top plate. Ideally the rafters shouldnot be under any tension from the "pushout thrust of the roof" they should just sit there the only tension they should encounter is the specific tension from the support of interior materials like drywall, light fixtures, and the sheathing, roofing, and snow (gutter cleaning person/ roofer) loads.
Now, this is very important, There is pushout thrust load from the roof onto the exterior walls!! This is due just to the nature of the shape of the roof itself, it wants to flatten out. This is especially true when one tries to open up space in attics for example and starts removing walls and collarties etc. This thrust must be counteracted. Usually by the floor joists of the attic floor, and by the proper construction of the wall topplates. The floor joists must be adequately sized and fastened and spaced to in effect stabilize the exterior walls and keep them from wantiing to spread apart. You mentioned that the tails of the floor joists would cross over the beam, to be fastened. I am sure that you are going to fasten, by face nailing the other ends of the joists to the faces of the rafters thereby adding the critical tensile force that helps to counteract the outward thrust of the weight load of the roof.
With that said, your post is still a little unclear to me and the terminology you use does not seem to be clear. This post of mine is by no means to be an instruction. If I have misunderstood your situation, sorry. The scenario would be different too if for example you are talking about replacing a beam supporting the attic floor joists. In that case the beam would support little weight of the roof and materials other than the floor joists/decking that rests on top of it, and the live or dead loads of what you put on the floor above because the loads from the roof would be taken up at the top of the exterior wall, the ridge beam itself, and the intersection of the ridge beam and the gable end wall. Unless you were in a situation that your ridge was very long, and you needed to put some posts in to help support the ridge which would then in turn bear on the beam. In either case one other important thing to consider is that the ends of the ridge and the ends of the floor beam should be properly supported at the intersection of the beam and the gable end wall.
My advice is to take the others advice and consult an engineer.
Edited 1/22/2003 12:34:15 PM ET by CLWEBB
The push forces are taken into account by the ridge beam, that's the whole point of having one. Ceiling joists will make the roof stronger by adding a triangle but aren't req'd. From what I gather they would make sense cuz you can effectively double the storage space in a building like this and you could delete the ridge beam altogether but now I'm way the hell off topicandstartingtoramble.
My engineer would do this for about $100 (canadian no less )so I'd say go that route too. If you find a good one she'll save you that again in good ideas.
I'm going to semi-hijack this thread, and ask for a clarification.
Ideally the rafters shouldnot be under any tension from the "pushout thrust of the roof" and There is pushout thrust load from the roof onto the exterior walls!!
What's the difference? If #2 is correct and the roof is trying to push the exterior walls apart, don't the rafters serve to keep the walls in place (lacking collar ties or trusses)?
If the ridge is structural there is no outward thrust on the walls. If the ridge is unsupported, the weight of the roof will push the walls out, hence the need to tie the walls together with either collar ties or ceiling rafters.
I guess I should have been more clear in my rambling. I have always subscribed to the ideal that in traditional stick frame construction that, I try to design a roof in which the rafters ideally had no other stresses on them than what they were specifically carrying. Look for example at a basic gable roof the shape basically is an inverted v. With out proper restraining members there is an outward thrust load where the rafters meet the top of the exterior walls. yes the load varies, but there is some thrust load. Basically because we are talking about a hinged structure. That being a structure that is not ridgidly fastened at the ridge. The downward force of gravity on the roof itself can in poor construction sag the ridge and which results in the attachment of the rafters at the ridge to become compromised. We have all seen poorly built structures with sagging roofs. There are other factors that contribute to this too. Usually this condition exhibits itself in rather poorly constructed buildings with undersized members. If for example you replaced the wood ridge and rafters with steel members, properly sized then welded the connection of the rafter to the ridge you would have a much stronger frame and probably wouldn't need a horizantal member such as a collar tie or attic floor joist at all. In almost all construction examples from the cathedrals to our own homes the lateral thrust of certain members whether they be an arch, a vault or a plain old roof rafter must be taken into account. Thats why we use birdsmouths in our rafters and why we tie the walls together with a second top plate alternanting the overlap with the first. Ideally we want to make a structure that before we set a roof on it, is strong enough to counter act the thrust laterally which makes for a much stronger structure. As our buildings get more complex, and we push upward for more vaulted space our walls and other framing members get taller and longer. It is not always easy to resolve these forces simply. Sometimes we don't want to see collar ties or have attic floor joists. That is where engineers make their living finding new ways to hold buildings up and make them do what we want and look how we want. Collar ties/attic joists are still a good idea in most instances, especially in a case where the collar tie is located around mid point to add stiffening to an undersized rafter. Usually it is a non issue because we do like the finished/storage space.
In certain short span situations, the rafters may very well be strong enough to resist the outward thrust for example a simple gable dormer, but in larger roofs in my opinion, it is a good idea to, simply put, tie the bottom legs together. Thats part of the reason that we all know not to cut through the bottom chord of a truss. granted the members are smaller but they all work together to make a basic triangle in most cases, which is one of the strongest structures known.
I do some remodeling my self but am neither an engineer or architect. I am just old fashoined.
Edited 1/23/2003 9:36:09 AM ET by CLWEBB
Edited 1/23/2003 9:39:43 AM ET by CLWEBB
Edited 1/23/2003 9:47:21 AM ET by CLWEBB
I guess my terminology or description was terrible in this post, but I do greatly appreciate the comments people gave me. Believe it or not I have an associates in residential construction. I was pretty proud when I call a soffit by its correct name.
The beam that I am trying to size is *not* the ridge beam. It will be a structural beam running through the ceiling and will have all of the ceiling joists or attic floor joists attached to it. These joists will be attached with simpson metal hardware and the joists will be 13'-6" long - there is a 1'6" overhang of the roof. My original thinking is that the load this beam will support is 1/2 the weight of the drywall on the ceiling and the weight of the joists, plus 1/2 the 10 psf live loading. So it would be sized, along with the posts supporting it, for this weight amount.
Sorry for poor expression of the problem earlier. Sometimes I have a tough time writing a 3-dimensional thing. Anybody know if my thinking on this is correct?
thanks in advance,
No problem remodeler, terminology varies from locale to locale. And even people who are good at visualization, and who are good at written description still have a hard time when puting a picture into words. I know I am no expert in either of these areas nor am I skilled enough to give you concrete answers to a structural issue without seeing it, especially over the computer. This message board can be a lifesaver with small problems, and info, but no one will agree that it should be the bottom line source for advice especially of the structuaral nature.
I would still check with an engineer to be on the safe side if you still have doubts. You said that there was no local building code in your location, there might be one for the county, or you might check to see if you can find out the answer to your question based on the building code for your area of the country. Which in the long run it would probably just be easier to ask an engineer. Call up some and explain the aspects of your situation and see what they say if they are not interested well there tough luck try another one. there services will probably be reasonaably priced especially at this time of year when they might be slow, and they could even possibly advise you on a way to do what you are doing better, stronger, and maybe cheaper. With an engineers help you will probably end up with a bettter building.
2 items caught interest:
a. "using up some reclaimed materials". excellent, keep your costs low!
b. "no permit required", my kinda job!
So, in a lighter vein and only a little bit tounge in cheek, the below is how most stuff was built a few hundred years ago and typical backyard technique today.
Anything built that doesn't need a permit just gets built - no calculations except in head, no drawings, etc. Shape and size depends on what's currently in the recalimed materials pile.
If it wiggles when you shake it nail on some more triagular braces. If it bounces when you jump on it nail on some more sisters. If the 18 ft beam creaks when you hang an old Chevy 350 from it, nail on even more sisters.
If it falls down when it snows 3 feet rebuild it <G>
60 years of practice and a few things falling down and you can do all the calculations in your head.
PS: Don't let the grandkids play in it until after a few snowfalls and an earthquake <G>
Right on Junkhound, I do agree alot with that aproach, and I also live outside town jurisdiction so alot of my own work is, lets say, "unencumbered."
Edited 1/24/2003 5:57:16 PM ET by CLWEBB
you know , sooner or later, someone's gonna come up with "The rural Building Code"., then whud are ya gonna do?? , <G>
Non-PE folks are reluctant to provide info for which they or you could get into trouble. PE folks are reluctant to provide info for which they are not paid.<G>
Please run these figures by a 'paid' PE. No guarantees given.
Beam length 18'
W=50# X 24/2 = 600
use 2- 1 3/4 x 18" LVL's
ECCQ44SDS2.5 post cap (I like using the screws, rather than bolts or nails)
similar post base for your application (wood framing or concrete embedment. Look it up or ask the PE.
Edited 1/25/2003 8:37:41 PM ET by Ralph Wicklund
Start by measuring and drawing. If you do that accurately and thoroughly enough, you may well be able to find an engineer who'll do the job via faxes, saving you the expense of a site visit. Most building departments will insist on an engineer's wet stamp before you get a permit.
I did the measure and draw stuff, and got a book and did the calculations. The city wanted the wet stamp, so I took the whole works to an engineer who also teaches at a junior college. If you're interested in learning, you may also find an engineer who would play professor and grade your homework. Mine was essentially correct, but way too much detail. And my engineer suggested ways to beef up other parts of the structure for better earthquake resistance. Do you really want to bet the house on getting your homework 100% right the first time without the professor?
As for the weights to use, it's the sum of the actual weights of the materials plus a standard amount for live loads. What those amounts are will be specified by your local building code. Typically you might have 40 pounds per square foot for ordinary residential space, 10 psf for the attic, 20 psf on the roof, 125 psf for a library .... But you have to use the numbers that apply in your jurisdiction.
This is the kind of issue that generates a lot of confusion when non-professionals are involved. And I'm not an engineer either, but what the heck. (I agree withthe others that a real engineer would answer this question quickly for you, with a little handheld calculator.)
I did have a similar situation on a remodel project a few years ago - 18' ridge beam, gable ends, the overall width was about 26'. Anyway for this area (heavy snow load possibility - 50 psf) we had to use a glue lam - about 6 x 12 (could have used 2-1 3/4" x 14 microlams) for the ride beam. I don't know what snow conditions you might have, but for a 24 x 18 building you've got 432 sq ft of roof area to support, and 50% of that will be carried by the ridge beam. If you use 40 psf for a common load rating, you've got 8,640 lbs to support. My pocket carpenter's helper shows that 3 - 2x12s won't do that. Also, the 4 x 4 posts won't adequatly support the beam, as some of the beam will overhang the sides of the 4x4. I always put the same width of post under a beam.
Good luck, and remember gravity: not just a good idea, its THE LAW.