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I-joist and supporting walls question


wolfmanyoda's Avatar
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07-23-08, 04:53 AM   #1  
I-joist and supporting walls question

Hello, I'm getting ready to do a 2-story addition to my home. I want the downstairs to be one open room, 21' x 24'-6". According to the charts, a PRI-60 11-7/8" joist can span 22' at 12"OC without support in the middle so I'm good with my 21' span.
I want to put 2 bedrooms and a bathroom above this. I was looking at some other documentation and it said that upstairs walls should be above downstair walls, but I will have no downstairs walls to do this.
Since I'm using trusses for the roof the walls upstairs will not be supporting walls, so can I get away with having no walls downstairs and non-load bearing walls upstairs?
Thanks.

 
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Michael Thomas's Avatar
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07-23-08, 06:44 AM   #2  
Yes, in theory it is possible to do this. However, there's sometimes a world of difference between the numbers you read off the span charts - which reflect the minimum size of the structural members which will safely carry the calculated load - and the construction techniques that will produce a floor with sufficient rigidity so that it feels comfortable and secure (for example, the occupants do not experience excessive "bounce").

IMO this is one of those cases where you want a design professional - such as an architect or structural engineer experienced in residential construction - to perform the calculations needed to define the actual load carried down to the second floor floor structure and then design a structure that will not only meet the code minimums will also be a comfortable to use.

 
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07-23-08, 10:17 PM   #3  
What Mr. Thomas said is so correct. Consult an architect and/or a structural engineer to be sure. There are many calculations to be made not just span. You don't want to finish your project and find you need to spend the same amount again just to repair "short cut" problems. Also, your building dept will need these calculations/drawings to permit the project. Good luck with your project.
Thanks for listening.

 
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07-24-08, 03:47 AM   #4  
Thanks guys, I'll look into finding someone to check my plans out.

 
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07-25-08, 05:11 PM   #5  
I'm going to play devils advocate here and people will probably be upset with me. my whole thing is, that ultimately the reason people diy is to save a buck and at times consulting a pro is neither needed or economic. I think this is one of those times. Too many times people come here for help and we refer them to an architect or engineer. well...they don't need us to tell them they're in over their heads.

anyways, there is nothing difficult going on here. The exterior walls are carrying the load, and floor joists need to be selected that can span the specified distance. After you make your selection, your plans examiner will tell you if it will work or not. That's it...no paying a pro $80/hr.

I work for an engineer. There is no smoke and mirrors with something like this. The span tables that are available online for TJI trusjoists or southern pine (just a few examples) are the same ones we use. If the span table says a 12" deep joist will make a 22' span, then it will. The only consideration is the deflection limit:
L/360 (code minimum) you'll have a little bounce
L/480 (recommended) less bounce, pretty sturdy
L/720 darn stiff. necessary for natural stone tile
ALSO, the 2nd floor partition walls (non bearing, standard 2x4 framed/drywall) are negligible in the selection of joists, and they do not need to be above 1st floor walls.

 
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08-01-08, 10:49 AM   #6  
OhioDraft--thanks for the comments, especially about the 2nd floor walls not being above the first floor walls. That had me a bit nervous.
I've decided to save the money and not hire an engineer. I'm going with my I-joists that are rated for 22'.
Now I have a new question, they delivered the joists almost 2 weeks early. I know I should keep them dry, so they are still bundled together in my driveway laying atop some 2x4s.
I put a tarp on them to keep the rain off, but will moister condense under the tarp and damage the joists or rimboards?
Thanks.

 
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08-01-08, 11:44 AM   #7  
keeping them covered isn't a bad idea, you should be fine. as long as they aren't wet when you close them up with the 2nd floor subfloor and 1st floor ceiling, in that way you would be trapping excess moisture if they aren't dry at that point.

 
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08-01-08, 11:52 AM   #8  
Sounds good, they were dry when I covered them up so all should hopefully be well.
Thank you.

 
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08-01-08, 12:29 PM   #9  
Posted By: OhioDraft After you make your selection, your plans examiner will tell you if it will work or not..
At least around here, depending on the plan examiner and the AHJ to verify correct design and construction could be a BIG mistake; a good part of my life consists of someone pointing at something that's clearly wrong, stamping their foot, and insisting that; "It passed inspection"...

..... and probably, it did.

This morning I was standing under a ceiling with water pouring out of a recessed light above, with the builder telling me that he did not need to fix the shower in the master bath above because it was "Built to code, has passed city inspection, and turning the shower head so water goes on the bench is not "normal use"", that the balconies with their ledgers nailed to the building and the joists toe-nailed to the ledgers and rim joists had been approved by the city and had passed inspection (I checked with the city, they indeed had), and that the loadside panel in the garage with the neutral bus bonded to the panel "Is per code and as required by the city inspector No correction will be made". and so on.

Happens all the time.

And I don't want to sound cynical, but people attempting to save money by depending on an overworked, understaffed AHJ for design approval and quality control is one reason why I will never be short of work.

But, hey.... I'm just the guy with the shovel *following* the elephant - you get the choice of whether or not you bring it home in the first place.






 
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08-04-08, 03:21 PM   #10  
1st, I'm leaving the city inspectors out of this, they bear no relevance to the OP and the questions he is seeking answers to. As for that balcony, the plans that spec'd that type of construction wouldn't have made it past our PE. I dont know what kind of plans examiners the city of chicago employs? Here in cincy you have to have a professional degree, be it engineering or architectural. So obtaining the approval of this individual is not much different than seeking the advice or full blown services from a design professional anyways. why pay for it on something that is relatively straightforward.

 
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08-04-08, 05:29 PM   #11  
Posted By: OhioDraft Here in cincy you have to have a professional degree, be it engineering or architectural. So obtaining the approval of this individual is not much different than seeking the advice or full blown services from a design professional anyways. why pay for it on something that is relatively straightforward.
For starters, because the AHJ's plan and building inspectors have no skin in the game – every place I've ever worked municipal plan and building inspectors have no professional or personal legal responsibility for their mistakes.

None.

Nada.

Zip.

On the other hand, when a PE or an Architect stamps your plans (or, for that matter, when I sign my report) we assume strict liability for our mistakes.

And if we are in our right minds we carry (and pay dearly for) Errors and Omission insurance to cover the cots of judgments against us should we neglect.

Think this makes no practical difference?

Ask the people who attempted to sue the City of Chicago and the municipal inspectors in the case of that deck collapse a few years back – the Illinois supreme court just reaffirmed that the city and its inspectors have total immunity from liability no matter how badly they botched their jobs… the survivors and their relatives are reduced to suing a bankrupt contractor and fighting the building owner's insurance company.

Then, consider that the plan inspector's standard is the building code - not "ideal", not "industry bast practice", not "the way I would build it for my daughter", but the building code: the absolute lowest standard to which you are legally allowed to build something. The plan examiner knows nothing about your plans or preferences or standards, he or she just reads span tables that attempt to guarantee that under "normal' loads the floor won't make the average occupant queasy.

Combine that with the fact most places that money is tight, taxpayers in revolt, and most AHJs are increasingly way overworked and understaffed...

------

Tomorrow morning I will be standing in a 1.75M townhome, passed by the plan reviewer and the municipal inspectors, getting paid to watch a general contractor tear up the defective work mentioned in my last post and figure out how to straighten this mess out - because it has dawned on the buyers that otherwise no one is providing really effective oversight.

And you know what? I feel like it's dirty money - I would much rather inspect properly built construction, even though that makes it much harder for me to look good to my clients.

But the fact is that builders, developers and homeowners depending on the AHJ for professional services - be they design, oversight, or inspection - are one of major reasons I will never run out of work.

 
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08-04-08, 06:54 PM   #12  
well, i'll attempt to keep this on topic once again. The OP wanted floor joists sized for his above-garage finish space. Well, that's pretty straight forward barring any usual loading circumstances. Meaning, over 40 psf, which is usually not exceeded unless your putting pool tables and jacuzzi's everywhere.

We drew up cd's for thousands of drees homes in the midwest over the years (biggest homebuilder in the nation), using nothing but the southern pine span tables, located for free, on their website. If you have a sense on how to use them - as in using the recommended deflection limit, keeping a few feet of cushion, etc. your golden. The result - an effective, no bounce floor. I just don't see what you think modern day architects do to size floor joists. I'll tell you for sure that they don't recreate the wheel. They do exactly what I explained above. end of story.

just a closing thought - these forums become pretty useless if you send everyone with a legitimate question running for an A/E. Anyone with half a brain that comes to ask a question of us knows the knowledge gained here is a basis for which to expand on. Especially if they are going to actually attempt a diy, you must assume they are at least competent enough to get a second opinion from someone that they can actually talk to in person when it's a significant project.

P.S.

in the case of that deck collapse a few years back the Illinois supreme court just reaffirmed that the city and its inspectors have total immunity from liability
Well if your talking about the one in lincoln park, that would be because there were no plans, there was no permit.

 
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08-04-08, 07:54 PM   #13  
Posted By: OhioDraft We drew up cd's for thousands of drees homes in the midwest over the years (biggest homebuilder in the nation), using nothing but the southern pine span tables, located for free, on their website. If you have a sense on how to use them - as in using the recommended deflection limit, keeping a few feet of cushion, etc. your golden. The result - an effective, no bounce floor. I just don't see what you think modern day architects do to size floor joists. I'll tell you for sure that they don't recreate the wheel. They do exactly what I explained above. end of story.
Generalizing from new construction - where design staff is working from scratch "from the ground up" - to remodeling work - where you are adapting existing structures which may never have been designed to accept additional loads where you need/want to impose them - will get you in BIG trouble in my business.

-----------

Your experience leads you to suppose that simple things generally get designed and built right. My experience is that they are often wrong.

Real world, I constantly encounter significant structural problems in both new and existing construction, and I just eyeball this work, I'm not performing calculations and I'm not licensed to give design advice.

Some of it is the result of homeowner or contractor "guesstimation", some of it slips though the approval and municipal inspection process, and some of it is jut plain nuts and it makes no sense at all that it happened.

I'm fine with DYI work - I did at least part the work on every system during the gut rehab of my current house except the HVAC - I just don't enjoy bending tin.

An architect approved the structural design.

I sleep better that way.

YMMV.

 
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08-05-08, 12:33 PM   #14  
FYI

Architects use span tables...structural engineers calculate loads, moments and deflections.

just a little A/E humor....actually, it's somewhat true b/c engineers typically think it's exciting to sit around calculating beam deflections while architects are only worried about going out to dinner with their girlfriends...or boyfriends

One comment:
If you're running any of the upstairs walls along the length of a joist sized for normal floor loads (i.e. not across several joists) you'll have deflection unless you're beefing-up the joists beneath the walls. Walls add roughly 40lbs/lf along the joist for a total of 880lbs in your case.

Probably not much deflection, but probably enough to notice a nice "joist-sized" impression on your first story ceiling sheetrock in year 2 or 3 under normal settlement (something a lot of mass home builders don't really worry about). That's the reasoning behind putting the upstairs walls above the downstairs ones. First floor walls typically always have double or beefed-up floor joists beneath them...or they're resting on a slab.


Last edited by unccivil; 08-05-08 at 01:11 PM.
 
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08-05-08, 03:41 PM   #15  
I'm not an engineer or an architect....I wish I was though..$$$

If you are going to put the bathroom anywhere but along the outside walls, then be prepared to replace any tile that you installed. Engineered I-joists are a thing of beauty for spans, but they do bounce and vibrate. The closer you get to the outside limits of the span chart, the more they will bounce. And I mean bounce. If you framed a floor out of conventional d-fir and then did one with I-joists, the d-fir would be sturdier every time. Every time I was involved in a project where I-joists were used, we always spaced them... 1. closer together than required, 2. always doubled them up under walls and 3. used the joist with the widest flange. The only exception is where you have an open room (like a playroom etc) and are not worried about slight bounce. The wide flange also helps with hardwood floor nailing and the hardwood floor is helpful to add some rigidity to the floor.

There are huge advantages to I joists but you still have to use an old school approach to installing them.

 
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