Jacked Reinforced Timber Joists

[elliottpj]

I was considering reinforcing timber floor joists by bonding a steel plate to the bottom surface of the joist between its bearings. As the joist is already loaded, this situation is different to designing a composite steel and timber beam. The joist will be subject to bending and shear stresses. If a steel plate was bonded to the bottom of the joist would this have a considerable effect on the joist's load capacity or would the joist just fail in compression?

If the steel plate was placed on the bottom of the timber joist, and the joist was then jacked to its original position of no deflection, or even a negative camber, before the plate was fixed, surely this would make better use of the steel plate. You could prestress the timber.

Has anyone seen any research in this area?

 

[elliottpj]

Can I use the principal of superposition to calculate the stresses on a loaded timber beam that has a steel plate bonded to the bottom while the load is applied?

 

[whyun]

sorry elliottpj. i had typed a response and my computer went berzerk on me so i lost it.

i will assume that this is an existing joist(s) you are strengthening using steel plates.

existing wood has compressive and tensile stresses due to current dead load.

case 1: no jacking prior to installing steel plate.
steel plate is added. steel, will not experience any tension until the composite joist is subject to live load. when live load is applied, steel takes tension from it and wood portion will take the original dead load compression plus the compressive stress from the live load.

case 2: jack up the joist and install the steel plate.
when jacking is released, steel will carry tension even without live load. wood compression due to dead load is not releaved. apply live load to get the total tension on the steel. wood carries compressive stress due to live load.

horizontal shear flow showld be calculated to determine the size and spacing of the screws from the plate to the joist.

my question for you: is it 2x joist? 1.5" may not be thick enough to add enough steel to make it efficient.

also, find out where the problem is. shear stress, flexural stress or excessive deflection. most cases, shear may be ok but flexural is overstressed. then i usually sister up a new joist next to the existing - stagger nailed entire length. ends of the new joist need not extend to the support of the existing joist provided that existing joist alone can handle shear. also keep in mind that the existing joist has already "deflected" and the new piece you are adding is "straight".

i apologize if i went too far off tangent. but i hope this helps.

 

[rccon]

If the problem is too much flexural stress or shear stress then, as whyun suggests, sister joists are your best bet. If the problem is too much deflection or vibration from a footfall, try this: put in a row of solid blocking at midspan, very tight. Then put a steel strap across the bottom of the blocking, also very tight and well nailed into the blocking (stretch the steel with angle nails or screws at the ends. This distrubutes the concentrated load of a footfall to maybe 6 or 8 joists instead of just one or two, and gives the floor a much stiffer feel.

 

[dpa]

Hi Recon,

I have the same problem, my wife says it makes her seasick when I walk though the room. My question is can you really get a steel strap tight enough to do any good? Also it would seem the nails have to be the exact same diameter (or even a bit larger than) the holes in the strap to prevent slippage.

Thanks,
Dave Adkins

 

[adk]

My first impression is that this is not a great idea. Bonding steel and wood alone sounds quite difficult and expensive, and I have never heard of it (not that you cannot do it). Please more clearly define the problem with this joist, deflection, shear stress, bending stress, cracking, etc. I agree with the person suggesting parallel beams or joists. Is it truly a joist or a 2x or other solid member? Then the easiest fixt is to nail, lag screw, or bolt an additional member to it. The easiest and most conservative way to fix a defective framing member is to add more members (sounds too simple, but it works).