Calcuating a structural ridge beam for a shed
Paul_Z
| Posted in General Discussion on
Hello All,
I am building a storage shed for myself.
The exterior dimensions of the shed are 40′-0″ wide X 13′-0″ deep.
The walls are 2″x4″x10′ stud framing with 1/2″ plywood sheathing.
There are minimal window and door openings.
The roof framing will only hold the load of some extension ladders hanging from the framing.
I live in a low snow load area in central Maryland.
Can anyone help me with design load for a structural ridge beam using LVL’S and the sizing for the roof rafters
Thanks
Paul
Replies
Paul
Rafter span tables say you could use 2x4's on 16" centers, but I would go up to 2x6 if for no other reason than it might have a good chance of holding up to nailing.. You don't mention the pitch.
For ridge beam sizing I would go to my lumber supplier (not a big box) and give him the numbers. They will send the info to their lvl supplier who will figure your sizing.
You can shorten the size by breaking up the length, which would seem possible in a shed, but maybe not so in a drive thru carryout.
By the way, something 40' long deserves the title-Building.
structural ridge beam
Paul, I'm sure all of the technical and engineering advise you get here will be backed up with witten unlimited warranties and full liability insurance coverage. Just in case it is not readily forth comming, you might make a formal request just to be safe . : )
As well intentioned as your request may be, you cannot expect an answer with the limited information you provided. Also you have no idea as to qualifications of the person(s) or the validity of the information you may receive.
As has been mentioned before, go to a lumber dealer that has access to an Engineer or an Engineer of your chosing for assistance. A 40 foot span is pretty long, expecially if you are using wood/timbers.
unlimited warranties and full liability insurance coverage
give the guy a break, it is just a long shed....almost appears it is to be open along one side.
although I do wonder of the utility of just 13 ft deep witha ridge, and why a beam would be needed when just 2x4 rafters and a tie would suffice.... could see one side fully open and the 40 ft beam across the facing side..
ridge beam
My statement about warranties and insurance was tongue in cheek as I attempted to indicate by the smiley face at the end of that statement. I'm sorry I didn't make that clear.
However I feel that the balance of my post is born out by your post which was primarly a question as to how and what he was actually proposing to build.
Which way is the ridge running?
Of course, 20--odd foot rafters would be problematical too. Perhaps the best approach would be to actually make this a shed, with no ridge to worry about.
HUH
I'm envisioning a 40-ft by 13-ft shed roofed structure open on one side. The "ridge beam" will need to be a really deep 8-inch wide gluelam.
Ridge Beams
It does sound like the author of the thread need to lay out a bit more information about the building in order for people to help him out.
That said I would like to piggy-back on this thread. I beleive typical glulam beams have a deflection rating of L/180. Now if you use a 20 foot beam and you get the maximum deflection that works out to about 1 and 1/2 inch sag in the middle. If that is the case any rafters affixed to or attached to the top would drop at the ridge and the tails would then slide out by a similar amount. That is a lot of push out. Not that it would be a structural failure issue, but sure would be unsightly. And if the wall that is pushed out had second story floor and wall up against those tails it seem to me you would ahve a mess with that wall getting moved. Am I missing something here? Even if you get an L/360 deflection rating you still ahve about a 3/4 inch kick out. Is this a problem?
Are you sure of your calcs?
There's probably more to it than you think.
If no one can answer this here, why not contact an lvl or lam. beam company for an answer to your question and maybe post back what you get.
There used to be a couple engineers/architects around here. And no, I don't mean the kosher steel boy.
Deflection
Thanks for a reply, Calvin.
Am I sure of my calc? Well 20 feet times 12 inches equals 240 inches. If you look at L/360 then you have 240 divided by 360 which equals about .7 inches. Therefore, if yo have that deflection at the center of the beam and you have a rafter attached at that point, the rafter at the ridge drops by .7 inches. You know the rafter length remains constant as the rafter drops so the tail has to move out. Using a squared + b squared = c squared (where c stays constant) you get roughly the same kick out as you get drop at the ridge.
Now, do I know what I am talking about? Not entirely, but I beleive I am on the right track, and need more information, so I posted here.
As for going in to talk to the LVL / Glulam guys, it is a bit premature for that. I am designing my own house ... Oh, I hear the groans. And believe me I understand the groans, as I have seen a bunch of owner-designed homes where it is clear they had no business doing such a thing. I am confident I am not one of them and do plan to get some review help (Designer, Arch, Eng ???).
What I am trying to do at this point is figure out the areas in which I need the most help and or review and to pretty well have worked out the bugs before I proceed. I would like to have a great room with a cathedral ceiling with a 22 foot ridge (a bit more than my example). And I would like to plan for a heavy snow load. The county has a 60 pound load, I would like to build to 80. I know this area, and 80 lbs is going to happen at some point and I don't want to shovel this hog. Therefore, I am looking at something like 30,000 pound load (live and dead). A glulam of roughly 6" x 18" is in that neighborhood. However, if a 3/4 inche deflection could result under 60 or 80 pounds of snow and the push out is 3/4 inches, either that works in such structures or I need to go to a oversized ridge so as to never see 3/4 inches of deflection.
Just thinking that someone around here would have a few educated thoughts.
Well, my experience is about all the education I got.
and that too should be taken with a grain of salt.
But, I'm thinking the whole system-the ridge, rafters and plates along with the span and pitch make a bit more difference when assembled together.
Therefore, your single member deflection using your calcs might not be entirely correct in the answer.
You know the pitch you want. You have the span and I'm assuming the size (what size rafters are you designing for this extreme snow load?) and spacing of the rafters (I-joist rafters)................and the length of the ridge. Take that drawing along with window openings and point load area details (for under that beam) to your local yard. They will fax it to an LVL company and they will figure the beam necessary (perhaps not if your wishes scare'em as far as load). OR go to an engineer.
Remember, a steeper pitch means less downward force. No?
According to your calcs, if you take the size beam you are talking about and put it on blocks 22 ft apart and load it with 30,000 lbs in the middle, it'll deflect 3/4 of an inch? What if that 30,000 lbs is divided equally at 1360 per foot, which it will be. And part of that is picked up by the rafter and it's connection at the plate? Is that in your figures?
Again, I'm no engineer-but something seems wrong with the numbers. Please help me out here.
"a steeper pitch means less
"a steeper pitch means less downward force."
Actually it doesn't. I'm not sure why you would say that.
I'd say it because I'm ignorant.
So, how bout this. Steeper pitch, less outward thrust.
That would be true (the second statement -- I'll leave the first one alone).
So, then is that the reason that the Swiss build those..........
STEEP pitched roofs?
And how about some answer to the deflection he's proposing of 3/4" on the ridge beam and the resultant bow in the side walls of 3/4".
A 22' ridge beam isn't out of the picture, nor does it have to be HUGE................
does it?
Is there anyone here that can definitively figure out what is necessary for that 22' application and/or can say whether his total load of .............what was it-30,000 lbs.........isn't spread out over the complete structure-Ridge, rafters, wall plates? and if so, does this then limit the deflection he's talking about?
thanks.
My gut injuneer feel is that there would be about a 1:1 deflection for a 12/12 pitch.
The Swiss have the steep roofs presumably to shed the snow ... and because they like the look of steep roofs ... and because it gives them space for a loft. But mostly because that's the way their grandfathers did it.
Accurately figuring the deflection for an evenly spread load in this scenario would be a bit tricky. One can probably approximate it by taking half the total load (half would be borne by the walls) and assuming that that load was evenly spread along a single beam, then compute the deflection of that beam. This would be a worst-case number, though, since it wouldn't account for the stiffness of the roof or the amount of weight borne by the gable ends. Nor would it account for the horizontal resistance of the side walls.
Somewhere, though, it should be pointed out that a deflection of an inch or so at the top of a 22-foot-long wall would hardly be noticed.
Deflection thesis
I believe typical glulam beams have a deflection rating of L/180.
Deflection limits are confusing. I'd bet that most guys who design trusses don't completely understand the subject. But stick with me a minute and I'll tell ya everything I know about roof deflection.
Keep in mind there are two different deflection limits - Live load and total load. Live loads are temporary, like snow and/or people working on a roof. Dead loads are permanent, like framing, plywood, and shingles. The total load deflection limit is for both the live and dead loads added together.
If you look at a load chart for roof framing or roof beams, the typical deflection limits are L/240 for live loads, and L/180 for total loads. IMHO that's not nearly enough. But that's the typical standard. Computerized beam design programs have the same defaults.
Let's say you go to a lumberyard and ask them to design a beam. Tell 'em right upfront that you want the beam designed for L/360 live and L/240 total. They might argue with you, but they should be willing to do it.
Also tell 'em you wanna see a printout of the results. Somewhere on that printout it will tell you the limits that the program used, as well as the actual live and total load deflection for that particular beam.
f that isn't clear let me know and I'll take another crack at explaining it.
If you look at a load chart for roof framing or roof beams, the typical deflection limits are L/240 for live loads, and L/180 for total loads. IMHO that's not nearly enough. But that's the typical standard.
I agree. I'm an architect that does a fair amount of beam and floor design and I tell my clients designing a floor to meet code and designing a floor so your plates don't bounces around on the table as you walk accross the floor are two entirely different things.
Doug
Deflection
Thank you and everyone else for replying.
I am on your page. But am I right, if you get deflection at teh ridge it will result in push-out at the tails of the rafters? I am guessing you are agreeing, and that you are saying if you size the ridge so as to have "accpetable" deflection under a snow (live / temporary) load then the movement at the wall is acceptable.
If there's deflection in the ridge beam there will be outward thrust forces at the rafter tails. The typical way to counter act that is to install collar ties. If you can envision two rafters in section forming an inverted "V", the collar ties are positioned to make the letter "A" out of the the assembly. These can be rather thin members (2x4's or 6's) because they're in tension. Ideally, they're positioned below the upper third of the roof where they're most effective but they work surprising well in the top third but the lower they are the better. The ultimate collar tie can actually be the floor system if there's a single span with no bearing walls, stair openings etc. to complicate matters.
Doug
Cathedral Ceiling and Deflection
Doug , etal,
Yes, collar ties are typical; however I am talking about a cathedral roof structure. Not sure I mentioned that as I started my hi-jack of this thread with a general question about beam deflection.
Someone else mentioned that in a 12 in 12 pitch a 1 inch deflection in the ridge would be about equal to the push out at the wall. At a purely basic level using a squared + b squared = c squared where c being the rafter length which remains constant a change or drop in the ridge roughly equals the push out at the wall. This poster suggested that a 1 inche delfection would likely not be noticed. Clearly I don't think you would notice a 1 inch deflection in the ridge, but if the walls moved out at the top by that amount I think you would notice it and if you have windows in that wall they might not operate.
So I guess that becomes the question. There is deflection under load, how much deflection can one tolerate.
One thing I am thinking about is that in my design idea, the cathedral ceiling section of the house runs perpendicular to the "main ridge". The cathedral section is partially within the main body of the house with some 10 feet jetting out from the main body of the house, forming a stubby T (the foot print). So, as such the walls of the cathedral section of the house are partially exterior walls and partially interior walls. The interior portions would therefore be buttrussed (spelling) so to speak by a second story floor system. This seems like a good thing. However, the rafters would have to be nailed to the floor system at that point so the fastners would be carrying the trust of the push out deflection. If there were a second story wall associated with those rafter tails and if that wall were a tiled shower wall, and if the rafter tails moved by 1 inch or 1/2 inche I wonder if that would be a problem.
Ok, see an Engineer. But it is interesting and fun to hear of the knowledge on this board.
Thanks for the fun.
do right........
would you please take your drawing and submit it to the lumberyards beam supplier and get an answer. I'm getting tired not knowing the correct answer.
'thanks.
Calvin
Are you funning or getting a tiny bit snotty? Thanks for your comments. I get it you are done with the train of the theoritical discussion.
Tiny bit snotty? Surely you jest. I don't get "tiny bit'.
I'm all for theoretical, but this one can have an answer and it's been suggested at least one place to try. When I needed a structural ridge, I asked at the yard. They fax'd the drawing and back comes a spec with back up for submission to inspection.
Now, I imagine a separate engineer could be paid to draw the details and assure you that 1)- your ridge sag would be X with a certain size rige, and could be less with another size. And 2)- the resultant sag on your plate line.
It's already been pointed out that load on that length is picked up by each end, so calculating the overall load in the middle is not quite right...............
But, I'm curious and just a dumb carpenter. When it comes to building-I put up whats required by design and permit. I'm not schooled in the why.
Snotty? no, just getting old coming back to this thread with no educated info being posted-just a lot of hypothesis (including what I shared). I am interested in how the loads are born and whether the whole structure takes part in holding it up.
thanks.
one more point
you say 240 total. 240 is still alot of deflectiona and therefore potentially a lot of movement at teh rafter tails. Perhpas you are saying that since a portion of that deflection is dead laod, then you will not need to look for that portion of the deflection at teh rafter tails, because if will occur one time over the course of construction. Then the only movement you will get during the live of the building will be the 360 snow/live laod.
One other poster suggested that the attachments at the tails would help support the load at the ridge.. This makes sense if the connectors were 100% perfect. In theory if the connectors were perfect and the walls could not move the ridge would support nothing, but that is asking alot of a few nails or brackets at teh rafter heels. NO?
Bottom line is, I understand that an engineer is improtant here and or the lumberyard. Agreed, I am just trying to figure out roughly what is going on and what to ask for.
Thanks again.
Don't know about central Maryland ( typically that's Frederick, Carroll and Montgomery Counties) being a "low snow load area". Can't recall the code snow load requirements for there but in Fairfax and Loudoun counties, just south on the other side of the Potomac River, it's 47#/SF. We would get 24" wet snowfalls a few times every decade. And that's not figuring on drifts. There's a real (no certain) possibility of the downwind side of the roof getting 5' of the white stuff every few years.
It seems we have a disappearing OP with all this great advice and all but if he's still here, with 40'X13' I'd go with engineered roof trusses at 24" oc. spanning the short dimension. With only a 14' truss (I just assemed 6" overhang) a crane wouldn't even be required.
Doug
Structrual ridge calculations
Doug
Thanks for your response and all of the other responses from others. I did not intend to be a disappearing OP. I have been going 16 hours a day with work every day since my origional post. I only had a few minutes to read some of the earlier responses until tonight.
I attempted an original post that included a lot of the information that the other respondants thought were missing from my post. Unfortunatly when I tried to send the post everything disapeared and I had to start over. My second post was a lot shorter and left out some of the important information.
To answer some of the questions. The shed is 40' wide x 13'. The shed is not open in the front. The shed is a fully enclosed rectangle. The structural ridge beam was to run the full 40" wiedth of the shed. The pitch of the roof is a 5" in 12"
I am sure some have wondered why I have chossen a 13 ' deep shed. I know that this is an odd number.
The shed is being built in a commericial parking lot of a property that I manage. I can only take up 4 parking spaces therefor 40' in wiedth. I do not want the shed to stick out to far into the parking lot because dumpster services will have to back a large truck up beside my shed and will need plenty of room to make sure they will not hit my shed. If I was to buid this shed in my back yard I might have to live in it.
I thought of a structrual ridge so I could hang my ladders up out of the way. With the 10' ceilings I do not think this will be a problem to hang the ladders below the plane of the 10' ceiling.
Calvin was the first to respond to my post and he mentioned that I should consult my local lumber yard for the calculations for the ridge beam. I imediatially thought why did I not think of this. I have done this so many times on other jobs for my customers.
I talked to my engineer at the lumber yard the next day and explained the scope of my project. His response was that any type of lvl engineered wood structural ridge beam in the length of 40"'without steel being involved would not be feasiable.
I weighed my options and decided that a conventional gable roof structure with collar ties or ceiling joist will meet my needs. If I forgo the structural ridge beam that will yield very litte extra overhead space ( due to the short run of the rafter) to hang ladders and other materials, I can increase the rafters to 2 "x1 0" with collar ties or ceiling joist, and add R-30 insulation to the ceiling of the structure. I think that this is a good trade off in the savings of the structural ridge) since I hope to add heat to the structure in the neer future.
As for the low snow load comment that I mentioned living in southern Montgomery County, Maryland. You are right . We do get a 24" snow fall every decade or so. Last year was a very good example.
Thanks
Paul
So why isn't a shed roof an option?
And what about engineered roof trusses? I've generally found them to be a cheaper assembly then stick building a roof in the field. And with a 5/12 pitch over 13 feet, getting usable space up there certainly doesn't appear to be a concern for you (which trusses generally won't permit) as the center will be only about 30" high.
And they'll be fine for hanging ladders from. Garage doors are routinely supported by roof trusses as well as HVAC units.
There's a reason why engineered roof trusses are used for the vast majority of production housing. They're a very inexpensive way to structure a roof.
Doug
One could always go for scissor trusses, for a little more overhead room.
And all he needs is 19 of them and the two gable trusses. And he gets the gables framed up "for free" in the process to boot!
Doug
Why not use trusses?
If this is not an open shed you will have headers of dorrs and windows on the 40 foot side of teh shed. Trusses would be so easy to acquire and to set for a 13 foot span. As for hanging ladders, etc. , trusses can be designed for loading of the bottom chord.
ridge beam
Why not forgo the ridgebeam and ceiling joists and rafters and get trusses, 13feet wide with a 6/12, I am sure if they are constructed with 2x6 they can handle any ladder load and/or snowload for your area. Much cheaper too.
Well duhh!!. Look above. I think that's what some of us have been saying. BTW, where's the OP??
Doug
Heck, cheaper still would be to give this shed a shed roof.