Making A Joist A T - Building & Construction - DIY Chatroom Home Improvement Forum


Go Back   DIY Chatroom Home Improvement Forum > Home Improvement > Building & Construction

CLICK HERE AND JOIN OUR COMMUNITY TODAY...IT'S FREE!

Reply
 
Thread Tools Display Modes
Old 10-04-2010, 01:39 PM   #1
Member
 
Join Date: Dec 2008
Location: Fairbanks, AK
Posts: 1,847
Rewards Points: 1,000
Default

Making a joist a T


Situation: A person needs stronger floor joists and can't add much height. Sistering the same size joist will double the load, but is a lot of wood. Question for an engineer: What can be gained by adding strips of plywood, say 3" wide by 1/2" thick and overlapping joints, to the bottom of the joist, making a T? Since the top of the joist and floor are already a T, really, wouldn't this make an I beam out of the joist, and hence increase it's load carrying capacity? If so, by how much? Dan: crunch some numbers on this one, pls!

Advertisement

jklingel is offline   Reply With Quote
Old 10-04-2010, 11:34 PM   #2
Member
 
jlhaslip's Avatar
 
Join Date: Dec 2009
Location: Canadian Rockies
Posts: 1,280
Rewards Points: 500
Default

Making a joist a T


a waste of time and material.

Advertisement

__________________
Quote:
Originally Posted by Bud Cline View Post
Go ahead and apply for a variance, those guys at City Hall can use a good laugh.
jlhaslip is offline   Reply With Quote
Old 10-05-2010, 12:26 AM   #3
Member
 
Join Date: Mar 2009
Posts: 780
Rewards Points: 500
Default

Making a joist a T


It will add virtually nothing extra to the load limit or rigidity.
hayewe farm is offline   Reply With Quote
Old 10-05-2010, 02:34 AM   #4
Member
 
Join Date: Dec 2008
Location: Fairbanks, AK
Posts: 1,847
Rewards Points: 1,000
Default

Making a joist a T


And why would it not help? When a joist sags, the bottom is in tension and the top is in compression. It seems that plywood down there would have to reach its tensile strength to fail. The joist would then be the I beam's web, the floor and the new plywood being the flanges. Same principal as a steel I beam or a TJI/BCI, no? Maybe I'm missing something here. Wait. I AM missing a few things; screws, but that has been that way for decades.
jklingel is offline   Reply With Quote
Old 10-05-2010, 08:58 AM   #5
Civil Engineer
 
Join Date: Mar 2009
Location: Boston
Posts: 4,308
Rewards Points: 2,194
Default

Making a joist a T


You can stiffen a joist this way, however the key issue is horizontal shear that develops between the old joist and the new plywood. This has to be addressed correctly via the use of an adequate number of fasteners, possibly glue, between the plywood and the joist. I have never actually seen this done in practice on a wooden structure. You see it all the time on bridges, where the beam has a welded on steel plate on either the top flange, the bottom flange, or both, which deepens the section and adds to the moment of inertia. On old railroad bridges you can find cover plates that are riveted onto the original steel channel.
Daniel Holzman is offline   Reply With Quote
Old 10-05-2010, 11:53 AM   #6
Member
 
Join Date: Dec 2008
Location: Fairbanks, AK
Posts: 1,847
Rewards Points: 1,000
Default

Making a joist a T


Quote:
Originally Posted by Daniel Holzman View Post
You can stiffen a joist this way...
Dan: Thanks. I suspected this would work. It would certainly be a lot more time than money, but sometimes we have our resources distributed that way. When I get time (ha ha) I am thinking of mocking up a floor w/ 1x4s on a sheet of 1/2" plywood and put a load on it, then add bottom strips, just for kicks and giggles. It may not be enough stiffening to dink w/; shall see.
jklingel is offline   Reply With Quote
Old 10-05-2010, 01:05 PM   #7
Chicago, IL
 
Michael Thomas's Avatar
 
Join Date: Jan 2008
Location: Chicago, IL
Posts: 1,037
Rewards Points: 500
Default

Making a joist a T


jklinger,

Plywood won't help, however steel strapping might, see pp: 94 here:

http://www.google.com/url?sa=t&sourc...WiqeIw&cad=rja

- or -

http://tinyurl.com/2by5dhx
__________________
Home Inspections, Infrared (Thermal Imaging) Leak Identification and Inspection Services, Roof, Attic, Building, Basement and Foundation Moisture Intrusion and Water Leak Inspections, Troubled Building Consultations - Serving Chicago and Suburbs http://paragoninspects.com/

Last edited by Michael Thomas; 10-05-2010 at 01:10 PM.
Michael Thomas is offline   Reply With Quote
Old 10-05-2010, 03:22 PM   #8
Licensed P.E./Home Insp
 
Aggie67's Avatar
 
Join Date: Dec 2008
Location: New Jersey
Posts: 587
Rewards Points: 500
Default

Making a joist a T


Plywood is one of the last wood-based materials on the planet that I would specify for this. The ply's are oriented 90 degrees off every other ply, so half the material is oriented in the weakest plane. These perpendicular plys contribute very little to tensile strength. My money is on the plys running perpendicular to the joist failing the first time you load it (which will put even more stress on the plys running the right way, which in turn will lead to those plys rupturing).
Aggie67 is offline   Reply With Quote
Old 10-05-2010, 04:40 PM   #9
Member
 
Join Date: Dec 2008
Location: Fairbanks, AK
Posts: 1,847
Rewards Points: 1,000
Default

Making a joist a T


Mike: good link. thanks.
Aggie: Good point on the plywood. The reason I thought of ply is because that is what (maybe) TJIs/BCIs are made of. That said, are the flanges on BCIs/TJIs all parallel laminations, or do they rely some on the glue? Maybe OSB would work better than plywood... till it gets wet. Looking at Mike's link, those steel straps are probably the easiest way to get at least some help, esp if you criss-crossed them and had two in there. OK; I'll work on another idea.... later. j

Advertisement

jklingel is offline   Reply With Quote
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Dryer duct hole thru band joist & floor joist justplumducky General DIY Discussions 12 10-05-2010 01:31 PM
Supporting joist under cracked tile JT31 Building & Construction 2 08-26-2010 09:00 AM
Jacking and Sistering Floor Joist Captain.Sassy Building & Construction 17 08-11-2010 02:30 PM
Space between joist and beam on deck schveiguy Building & Construction 11 09-08-2009 10:15 AM
one rotted deck joist to replace, need advice dougq Building & Construction 10 07-07-2009 03:18 PM




Top of Page | View New Posts