I have a brochure on I-joists which gives a rating table called “Simple Span” and another table called “Multiple Span.” The span rating is different for a given I-joist depending on the table consulted.
For example, an 11 7/8″ I-joist with a 3.5″ top/bottom chord at 12 o.c has a span rating of 24’11” for the Simple Span and a span rating of 27’1″ for multiple span.
What is a Multiple Span?????
I have a situation where I want to install 41ft long 11 7/8″ I-joists where the span is supported 21 ft midspan by a wall to carry the load of the span. Once it crosses this wall it will continue straight to the exterior wall where the end will rest and that span is about 20ft.
Because this 41ft span is supported about midway, does this make it a Multiple Span and therefore I’m exceeding the rating for the I-joist??
Replies
You're gonna have 20'+ spans twice, you best be thinking trampoline.
Bosshog is the guy to ask and his treatsie on floor vibration is the classic BT resource for this one.
Joe H
Do you realize that the right time to figure these things out is before you start building the house?
I *think* that what you have is multiple and that you are OK for load/deflection, but that you do have a good chance of a bouncy vibrating floor with such shallow depth
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Actually no. We already did the first floor with 20 ft spans of 11 7/8 joists and there is almost next to no vibration at all. It is rock solid.If at first you don't succeed, try using a hammer next time...everything needs some extra persuasion from time to time. -ME
>> We already did the first floor with 20 ft spans of 11 7/8 joists and there is almost next to no vibration at all. It is rock solid. << It ain't over until the fat lady sings.... What's the customer look like? :-)
BTW a friendly word of advice: when looking at span tables, don't use the maximum allowed or near to it.
I guess you got your answer about single and multiple spans but if not look at this:
Single span:<!----><!----> <!---->
„¡„Ÿ„Ÿ„Ÿ„Ÿ„¢
© S0 ¨
<!----> <!---->
Multi span:
„¡„Ÿ„Ÿ„Ÿ„Ÿ„¦„Ÿ„Ÿ„Ÿ„Ÿ„¢
© S1 ¨© S2 ¨
<!----> <!---->
The reason span tables have different span values for mulit spans is that in the above pic a load on S1(span 1) can effect S2<!----><!---->
Matt,
What in the world is this?
Single span:<!----><!----> <!---->
„¡„Ÿ„Ÿ„Ÿ„Ÿ„¢
© S0 ¨
<!----> <!---->
Multi span:
„¡„Ÿ„Ÿ„Ÿ„Ÿ„¦„Ÿ„Ÿ„Ÿ„Ÿ„¢
© S1 ¨© S2 ¨A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
OK - it appeared properly on my screen in the BT editor, but when I hit the post button aparently it didn't work too well. I had created a Word doc with some symbols in it and then pasted it to BT.
Edited 9/24/2009 8:11 am ET by Matt
Thanks for the explanation. You had me pretty well cornfused. An Einstein I'm not.
I'd be interested in seeing what you were attempting.
A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
Hows this:
View Image
Gotcha.
Sort of.
If the load is as you show with the arrows (more concentrated in that area), then the effect might be to raise/lower the plane of the joist on the other side of the 1st floor mid span wall?
I'd better take a look at the different span ratings he showed earlier.
thanksA Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
Then why is the multiple span 27'1" and the single span 24'11"?
Are you saying a joist could span 24'11", but if there's a bearing wall under that joist at the midpoint you can only go another 2'2" overall?I'm confused.k
whew! That's more like it
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New engineering algebra, I'm afraid
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
This is my first project this size. There is a lot I'm learning along the way. So far I been consulting people better experienced than I, but they are not always available when a question comes up that I didn't think of. Also, these plans are done by the HO and not specific. I was charged with figuring it out as I go. We've caught some obvious goofs in the HO plans, some fairly serious. This project involves everything construction related...remodeling, demolition, new construction, engineerig marvels for the roof tie in's, etc. Not to mention, HO is in London and we communicate via email and digital pictures. Perhaps the MOST unusual situation ever between contractor/HO. HO says take your time, just do it right.If at first you don't succeed, try using a hammer next time...everything needs some extra persuasion from time to time. -ME
I've done work for HOs in AK, Paris, London, Japan...time diff makes phone communications hard so digital is far better.But any HO who does not know engineering and design gets told, Just tell me what you want to end up with and leave the details to me. I would not think of letting them dominate the engineering and construction methods.
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
In essence that's what I got here. HO drew up plans and made the foundation for me. It is good the HO is allowing me to do this because he's had problems getting things done right by others before me. The foundation was very bad, large dip it in caused us to have to shim I-joists as must as 1.5" in some places. We have to put in squash blocks yet this week, too. So far the HO is just telling me to get it done right, take my time, and find a way to do what he'd like done. We've been moving forward with that the last 2 wks and doing pretty good I think overall.If at first you don't succeed, try using a hammer next time...everything needs some extra persuasion from time to time. -ME
I'm not sure the answers you were given are right. I'm not sure they're not, either, but they don't make sense based on the numbers you gave in your first post:24'11" for simple, 27'1" for multiple.
If "multiple span" refers to a joist that is supported somewhere midway, and there is a performance issue with that, then why is the listed span greater for multiple?
I think it may actually be the same as "single member" and "repetitive member" in solid wood joist Fb ratings, where there is a bonus factor in floor systems with many joists, but if you're figuring a particular load on one particular joist, you go with the lesser value. Kinda guessing, though.
Having just speculated without checking any data first, I'll try to dig up my I-Joist manual and get some facts. Meanwhile, I'll wait to get reamed... You'd think I'd have learned by now to get facts first...
ok, open season
k
This might help figger this thing out. My experience with I-joists is minimal.
Floor Span Tables: I-Joist Simple Span Applications
To Use:
1. Select the appropriate table based on the project design loads 2. Find a span that meets or exceeds the design span. 3. Read the corresponding joist series, depth, and spacing.
Caution: For floor systems that require both simple and continuous span joists, it is a good idea to check both tables before selecting a joist. Some conditions are controlled by simple span deflection rather than continuous span strength
Design Assumptions:
1. The spans listed are for clear spans between supports.2. The spans are based on uniform loads only.3. Live load deflection has been limited to L/480.4. Total deflection has been limited to L/240. Those spans that exceed ¾” of total deflection are shown in bold.5. A load sharing (repetitive member) increase has been applied. This increase is based on the attachment of a proper deck to three or more joists, spaced no more than 24” O.C.6. The spans are based on the minimum required bearings.7. The tables shown do not include web stiffeners.
Additional notes:
1. The design of continuous spans is based on the longest span. The shortest span must not be less than 50% of the longest span.2. These spans are not evaluated for vibration.3. Bridging, Blocking, a direct applied ceiling and/or bottom flange bracing, though not required for vertical load capacity, can improve floor vibration and bounce.
edit: taken from TruLine Truss, Inc. website- sparta, n.c.
A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
Edited 9/24/2009 3:18 pm ET by calvin
Ok, thanks, so that says the repetitive member factor is assumed and applied.
But what's the deal with the 2' 2" differential as quoted in post 1?
I'll probably kick myself, but I'm still confused.
k
Assuming I'm assuming correctly (and I certainly would have called my supplier by now for the real info, that's for damnsure-remember-I know diddly).........
the more joists in the ballgame, the slightly longer MINIMUM span is allowed. Where's the boss, when you need him?
5. A load sharing (repetitive member) increase has been applied. This increase is based on the attachment of a proper deck to three or more joists, spaced no more than 24” O.C.
that applied increase in this span table from this truss company has evidently been applied already as most joist spans are repetitive.
I guess.
Boy it's a bitch to show your own ignorance on a subject you're responding to...........and believe me, I mean ME.
edit: So I'm assuming again that in the original post and specs given there were two spans shown-one singe member and one for a group of members...............
A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
Edited 9/24/2009 5:59 pm ET by calvin
"edit: So I'm assuming again that in the original post and specs given there were two spans shown-one singe member and one for a group of members..............."Well, that was my initial take on it too. The value difference between "simple" span and "multiple" span is pretty similar to the difference in solid joist Fb's ("single" versus "repetitive") in my chart, FWIW.But that's not what the other guys were saying.I'm still expecting/waiting to be corrected.What manufacturer is your table from? I wonder what manufacturer the op's is from. I feel like if I could see his tables, I could figger it out...k
No manufacturer, but a truss co. from N.C.
I took the first hit on the search and came up with it.
http://www.trulinetruss.com/html/body_floor_span_tables_-_ijoist.htm
Throw in any walls sitting on it and you've got a whole nother ballgame.
Like I mentioned earlier, I would ask my supplier v. horsing around looking it up online. But I don't have this job. Should have kept my mouth shut in the first place, but got drawn in by the intrigue...............and the lack of a definitive answer in the first place................which led to the intrigue part.
Beautiful day today, no?A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
When a joist passes continuously over a center support, there is a cantilever effect. In Matt's drawing, if you ignore the point loads indicated by the arrows, span S1 strenghtens S2 and vise versa. That's why the span can be increased.
Calvin posted about repetitive members, which is akin to the reason many builders like to add blocking between joists: it helps share the load with its neighbors. Span tables are figured for a single member (or joist) but if you have a few joists tied together with blocking or a good subfloor, weight on one is partially distributed to its neighbors.
Now that's a more definitive answer.
thanks. Learned something today.
not bad.A Great Place for Information, Comraderie, and a Sucker Punch.
Remodeling Contractor just outside the Glass City.
http://www.quittintime.com/
Yeah, I know about single vs. repetitive values, I was the one who first posted that.I wasn't sure about the definition of multiple span as per post one, is all.k
So you are saying that due to the cantilever effect you could have one 54' 2" (!!) I joist with two spans of 27'1"?k
Edited 9/24/2009 6:38 pm by KFC
In theory, sure. Personally I keep everything, beams and joists, as "simple spans" because it limits negative effects due to jobsite improvisation. In other words, it's tough to screw up.
Recently Boss Hog posted on a similar thread about the possibility for excessive uplift when loads are applied as in Matt's diagram--another reason why continuous spans aren't always ideal.
Edited 9/27/2009 4:22 pm ET by Mike_Maines
Is that what you were saying?
kedit: oops-your reply and my pestering were in the tubes at the same time. Apologies.edit:I think the 54 foot joist was what was tripping me up. Never seen one of those.Edited 9/24/2009 6:50 pm by KFC <!-- KFC1002 -->
Edited 9/24/2009 8:34 pm by KFC