Shorter Joist Span on 16X26 Cabin

[Chutz]

Let me preface this with a caveat... I've been around the construction industry for over 30 years and am used to working around large houses and such.  As we're getting older we're going to tackle this studio/cabin ourselves and want to make it as easy as possible. You know...the mind is willing but the body protests!   :'(

Anyway, the design calls for TJI's which are fine, but I as thinking instead of running a row of posts down the center to shorten the joist span to 8' which would make for much easier handling.  Seem very logical to me, but is there something I'm missing here? I'm assuming I still need the beams to be 6X12. Ran the beam calculator and I think so...Any suggestions are welcomed.

Chutz

[PEG688]

HUH?  You could  put a beam down the center , but don't order shorter TJI,  you want to minimize joints that the  one of the beauty's  of TJI .  I would strongly recommend the center beam , John has said his calc's  for joist / rafters are sort of the max span you could use , with TJI's I aways , well almost always . recommend a shorter span than the spec call for , reason being TJI's tend to bounce when you walk on them at there max , and  a bit under the max span called out, part of the sales spiel is longer span , don't buy it at face value .

Your beam size sounds big ? What the post  spacing OC ?  Smaller beams easy in a crawl space ,just put more posts in (closer together / OC ) more post  = less span.

G/L PEG

 PS Another trick ,if your using a full foundation , or a beam on the ends , is to drop / hang your joist , it lower the building making access easier , you mentioned you are older so , less stairs some times is a consideration , the dropped joist generally save you two steps .

That system does cause some  issues with plumbing / wiring , / venting as it does block the path down to the crawl space Thru the bottom plate , the beam / foundation gets in the way. So consider that as well if you go with a dropped floor system.  
 

[Chutz]

Wow, Peg...I appreciate all of your comments. I see you're on the island...I'm out in the Key Peninsula, near Vaughn.  Howdy, neighbor!    Anyway, it shows me that I didn't expalain myself well. Here's what the deal is. I'm using John's Victoria plan...and making the smallest one. This is a studio adjacent to the house and will eventually have a hallway connecting the 2.

On sheet 3 he gives spec's for a post and pier foundation. The beam sounds big to me too, but here's what I'm doing ...  16'X24', spacing the posts/piers @5'10-1/2 o/c will run 5 posts down each side. So, mighty short span that way but if I go one less the span is too long.  On the specs it calls for a 6X12 down each side to rest or hang the TJI's from.  

What I want to do is use lumber...NOT Truss joists.  (forgot to mention that)  But instead of spaning the 16' I want to run another set of posts/piers down the length so the span is reduced to 8' for joists.  Obviously will have to be longer for a lap.  This will make a much stiffer floor, but am I going way overboard?  That 6X12 beam seems huge!

Here's a CAD drawing I did of the foundation idea.  


<<682x412 pixels image>>

Thanks so much for your time!
Chutz

[PEG688]

Yer welcome neighbor I'm not a big fan of post and pier foundations , I  don't do that many , as well,  there just not that common in the real const. business side .

That being said John has a lot of these out in the field holding up his designs , BTW I do not have any of John's plans so I shoot from the hip / out of field experience when I give all this free advise


What seems odd is a post smaller than the beam  , for looks mainly and to provide a solid lap joint connection , I like to see the post larger , of course the beam itself  [highlight]has[/highlight] to be [highlight]adequate in size / strength / species of lumber[/highlight] . I like the look and feel , so to speak of a lap joint thru bolted beam to post connection. I am sort of a throw back / old school carpenter who thinks most , [highlight]not all [/highlight], of these Simpson connectors are replacing good craftsmanship / building practices .  I do know the reason for it , arch / building dept / eng. cannot be sure a "good craftsmen/ carpenter" will be onsite , therefore they spec a such and such anchor / connector / bracket etc , in lew of old school / sound building practices.

So if John spec'ed it and the build Dept ,(if you have a permit) approves it that's the bottom line . I'd go 4 x12 beam / 6x6 post lap jointed into a big foot pier w/ ZMAX post bracket , a bigger one like this :

 

With all our wind and those pesky earthquakes , lahars , etc  ya want that building tied down good , see the thread above about the cabin blown off/ down up on Vancouver Isl. , up in the first section of the forum .

So that's my piece , at least to this point of understanding , John may come by and add some of the Arch. mumbo jumbo / ju ju as well  

[Chutz]

Thanks for moving this to where it belongs.

No plumbing in this studio since it will attach to the house, just some wiring for lights and outlets. Small woodstove in front corner for heat. Opening skylight for airflow/ventilation.  I keep forgetting to add the 'details'.  There's a link on John's web site to someone who did a similar plan with posts down the center. This is teh size I'm doing. Just trying to get the sizes worked out for this one.  No need over building but want to make is up to code.

http://www.countryplans.com/lemay.html

Chutz

[John Raabe]

Now that I once again have power I will jump in here.

There is nothing wrong with running a center line beam to use smaller floor joists. For a floor joist of less than 8' span you could use 2x6 DF-#2 with a max spacing of 24" o/c. The centerline beam carries only 8' of load from the main floor. Thus for a standard floor that would be 400 plf for the center beam (8' x 50 psf). By doing this you have also cut down the load to the sidewall beams, but they are still carrying 4' of floor load (4' x 50 plf) plus half the roof load (8' x (LL + 10 psf DL)). Since I design for a national audience I have to be able to handle a snow load heavier than our wimpy 25 lbs! (I use 50 psf but you don't need to). OK, once you've traced the roof load to the wall you need to also add any loft load you will (or will not) have. That is 8' x (30 psf LL + 10 psf DL) = 240 plf.

Doing a back of the envelope calculation using local roof loads (and a loft) I come up with the side beams carrying 720 plf (pounds per lineal foot). This is not adding in the weight of the sidewalls and would be light for a snow load site. But, you could use that number to size the side beam for different spans and pier spacings.

Hope that helps.

PS - I use TJI's because they are light, stiff (for a 16' span), straight and they give you plenty of depth for a well insulated floor. Also, you end up drilling fewer piers. But 2x6s are fine too. Also note, as PEG has, that P&P foundations are not ideal for earthquake areas. A full perimeter foundation (concrete or wood) gives much better anchorage to the ground and better resistance to sidewards forces. You've probably already seen this review of Post and Pier Foundations. The concrete tube pier would be strongest for our area and then you would want to brace and triangulate as much as possible to the post or beam that sits in the bracket. Tie this into the floor and wall system with metal straps as well. And be sure to use plywood structural sheathing (or OSB - but I would do plywood in the damp NW).

[PEG688]

So there you have it , mombo jombo from the Architect.   Thats why they make the big bucks   They tell ya stuff and ya come away wondering what they said  

Weathers been a Bit..!! Eh John

[glenn kangiser]

8' x (30 psf LL + 10 psf DL) = 240 plf

Did I do something wrong -- I was thinking 320 plf ?

[Chutz]

OK, that all makes good sense to me. I've figured loads before but it's been many years...  :-/

What size beam should I run down the outside and center now?  I'm usuing 2X6 walls....code for WA state basically.

Thanks!!
Chutz

[John Raabe]

Thanks Glenn - I need to erase that number on the back of the envelope  

Floor= 200 + roof= 280 + loft=320 for a total of 800 plf. A 26' long building might have 4 piers on each side. The span would be (26/3) or 8'-8". I will do a quick beamcalc run but I'm not doing your engineering you understand. I'm not an engineer and will deny all knowledge of this matter!  

That said, a Doug Fir #2 beam 5.5" wide by 10.59" deep will span 9' at that loading and with 1/240 deflection. I would say use a 6x12 timber to be conservative or a built-up continuous girder of 4 - 2x10s if that was easier to do (the program wants a 6" x 9.71" beam but that is for a simple span and since you would be continuous over several piers the actual installation would be stiffer than the simple span calc assumes). These beam spans are limited by deflection not strength.

The centerline beam is only carrying 8' of floor load. You only need two 2x10s for that girder.

[Chutz]

And I'm GLAD you're not an engineer...Know way to many in my years!...lol   I'll try to find my beam calculator...it's burried here somewhere! Thanks as always for the help.

Chutz