Cuckoo for Cocoa Puffs or Practical? Reinforcement for wood joists.... 1

[luftweg]

I was reading some posts on some other forum, and I noticed the following discussion, arguing for the use of strips of steel to be afixed to the bottom flange of wooden joists, in order to increase capacity or decrease deflection.

From what I gather, this seems theoretically okay, although practically, I'm wondering about a fastener schedule and the field success for such an undertaking.

I've searched posts on THIS forum and haven't found anything that talks about this...

What do you think?

Here are the important excerpts from this discourse:

"... strapping the bottom of the joists with steel after jacking out the sag can produce a very rigid floor.
Steel strips cut from #10 steel (about 1/8 inch thick) 1.5 inches wide are fastened to the bottom of the joists using screws at about 8 inch centers.
...
The straps do not have to go under the bearing surface of the joist at each end, but can stop a few inches away. A 1/8 inch thick by 1.5 inch wide strap is the equivalent of adding a massive timber to the bottom edge of the joist..."

"...For the joist to sag from the straight position the steel strap will need to increase in length, and that is not going to happen. The steel strap performs the same function as the bottom flange of an I-beam. It is loaded strictly in tension.
Attaching the steel to the side of the joist will produce a buckling failure and result in loading of the fasteners in withdrawal (the weakest direction) as they try to prevent the steel from buckling away from the joist. The fasteners holding the strap on the bottom are loaded completely in shear, the strongest direction..."

"...I am an engineer and have designed and installed a number of steel strap systems to stiffen floors. The first time took a bunch of calculations to determine fastener schedules and such, but wood is a nice material to place nails and screws into. Unlike metal to metal joints were deformation occurs in the fasteners weakening them, deformation in pinned (the engineering term for nailed or screwed) joints, the wood deforms and increase in bearing capacity from any slight crushing that occurs since the load is spread out in the wood over a larger area...
It is that ‘slight deformation’ that allow an I-beam made from wood, metal, or a combination of the two to bear loads.
...Even a flitch design (a steel plate between 2 pieces of lumber) requires as a minimum clenched nails, and I would never allow those. I require through bolted fasteners in tightly drilled holes. A reliable flitch beam con only be made by clamping all the layers together, and then drilling the fastener holes. This eliminates any tolerance stack up in the drilled holes.
It is possible to fasten steel to one side of a lumber section and through bolt, but the number of fasteners required to prevent buckling often results in weakening of the beam. The loads under each and every fastener on the wood side must be examined for bearing pressure and held to below about 30-70 PSI (depending on the actual wood species used). These joints also have a greater problem with loosening of the fasteners from dimensional changes in the wood weakening the beam. This effect is present I a full flitch, but is not as serious since the wood itself acts to spread out the loading and prevent buckling over the entire area of the flitch, instead of being concentrated strictly at the fasteners..."

"...If the headspace is available I prefer wood, if headspace is tight #10 steel straps 1.5 inches wide by 90% of the span attached with 2 inch screws every 6-8 inches (holes for the screws in the steel must be very close to screw shank diameter and staggered slightly) prduces a very stiff floor..."

http://ths.gardenweb.com/forums/load/repair/msg0915531723856.html

thanx,
K

 

[structural01]

Normally, what I've done to stiffen or strengthen the bottom chord of a wood truss is simply attaching another 2x's to one face or both face of the truss (flatwise) with penny nails or wood screws and staggered.

 

[boo1]

I dont like the steel strap idea. You will have trouble attaching sheetrock or any covering to the bottom of the joists. Structural01 has a better idea or consider an angle bolted on the side of the beam.

 

[AsianEngineer]

Can't you just sandwich 2x to the side of the joist to nail sheetrock to?

 

[luftweg]

In this particular instance, the ability to nail to the bottom flange of the joist is not important, and as AsianEngineer said, a 2x4 could still be attached to the side of the joist for the purpose of any attachments, if needed...

The big question here is whether the structural claims of steel straps to the bottom of the joist are real, or if the fastening mitigates the theoretical benefit...

K

 

[TDAA]

Why would this be such an odd concept? It is not as though composite sections are a new thing. Look at reinforced concrete. It is all about putting the steel in tension to improve the other material. Also similar to bolted up/welded thickening portions on steel members.

I guess the thought partof it is kowing the interaction of the wood and steel, as in the connection. Otherwise it is a mater of knowing the properties of the materials, the cross sections, and the locations.

 

[cvg]

I think the key is in the context of the original post. It is being used to "stiffen floors" and presumably to rehab existing bouncy floors. This could be used prior to installing a tile floor or to repair damaged joists where plumbing or mechanical contractors may have cut or drilled through the joists if there isn't another better option. I would not propose to use this method in new construction to either increase joist spacing or to reduce the depth of the joists as it would be easier and cheaper to just use a wood joist.

 

[bvanhiel]

I'm not a structural engineer (I do product design) but I played one on my house. I bought an older ranch a few years ago. One of my pet peeves regarding older houses is floors that arn't level, and this house was no exception. I rented a rotating laser level and mapped the height of the entire floor. My wife continues to use this as evidence that I'm insane.

In addition to some larger sink due to an undersized flitch plate, I also found where most of two joists had been cut away from the bottom for plumbing and "sistered" with pieces only 18" long.

My fix was to use a 4' piece 1-1/2X1-1/2 1/8" think angle iron that was lag screwed to the joist. What had been a dip in the floor big enough to trip the unwary is now perfectly level.

I can't say what the industry standard solution is, but I can say that it worked for me.

-b

 

[d2TY]

Flitch plate design (not used much due to the labor involved, and the more economical glue laminated lumber) as I was taught:

I tot = 2 (Iwd) + nIpl

The transfer of the load through the flitch plate should be shared by the two wood members and the steel plate between them.

Flitch plate design was created for beams, joists would not require the expense of bolting 1/8" + thick plates.

Labor would be the factor.

In existing floor capacity, contractors usually would prefer new beams to cut the span (if possible) to such an expensive operation as reinforcing each and every joist, so it does seem a practical solution.

 

[d2TY]

oops, add a not before the seem a practical solution statement above (where is the edit post feature?)

 

[luftweg]

I think the only places that this type of retrofit might even apply is:
if there is no room beneath to put a midspan beam in, or if the ceiling height below is too low to use deeper sister joists or flat 2x's added to the bottom flange...
OR, if standard sistering (with the same dimension lumber) would be inadequate

However, that may mean that there ARE a good deal of times that a thinner, less intrusive intervention is desired/needed.
For instance, if the space below the joists is intended to be used (or is already being used) as a living space, and if the height from the floor to the above joists is already only barely meeting code, one might not have the luxury of using the more 'straight-forward' remedies.

BUT, with the procedure of adding the steel plates themselves, there are other things that might be problematic:
The joists might not be linear enough; many joists might be too wavy, and the 1.5 inch plate might end up not being sufficiently centered under the joist in some areas along the span.

Perhaps it would be better, if needed, to sister the joists with the same size joists, and then add a 3 inch plate wide, 1/8 inch thich (actually a thinner plate could be used in this case, since it's wider, no? say 14 gauge?)
That might ensure that the enough of the steel is directly below joist (have and idea: how about 3.5 inch steel channel, unpunched?)

K

 

[MikeHalloran]

My Dad visited his new house often while it was under construction, in 1950. He got into an argument with the contractor when he caught them only nailing every third row of t&g hardwood flooring boards. Of course they refused to change their ways, and assured him the floor would never squeak.

The floor squeaked immediately, and continued to do so for 20 years, when he decided to do something about it. Until then, he'd never had money and time available at the same time.

His plan was to sister the joists with identical joists, but he was stymied by how to get full length sister joists into place in a completed house. He had figured out lots of ways to do it, but they were all labor- intensive and would have caused a lot of collateral damage. Having exhausted _all_ other possibiliites, he asked for my opinion.

So, I told him the secret that I actually learned in engineering school; you only have to sister the middle half.

He was unconvinced until I found a practical example. He had been associated with construction of a public building with a too- flexible roof, and the consulting engineer for whom I had worked in high school was engaged to fix it. He had welded half- length doublers to the bottom flanges of the roof beams, which stiffened it up just fine.

The solution we finally arrived at was half- length doublers cut from 3/4" plywood, glued and (wood) screwed to both faces of the joists in high traffic areas.

Still nailable, drillable for added wiring, etc., and still effective, now 20 years later.

;---

WRT to the metal flange nailed to the bottom of the joists, I don't care for it not being nailable for finish ceilings, but I have another objection. Green softwood conforms to nails okay, but the seasoned lumber you'll find in a house that's dried out a bit may just split from having a row of nails driven into it. Then you'll have to sister the joist anyway, and maybe replace it.



Mike Halloran
Pembroke Pines, FL, USA

 

[luftweg]

"...Green softwood conforms to nails okay, but the seasoned lumber you'll find in a house that's dried out a bit may just split from having a row of nails driven into it..."

I don't think it would be advisable to nail the steel strap to the joist, so it would be drilled and screwed (and glued?) anyway...
Drilling will allow for more precise connection between the screw shank and the holes in the steel -- in order that forces be transferred more efficiently, and to avoid any slippage back to the sag (?).

I also feel that sistering should be drilled, glued, and screwed...
Somehow I think that by getting such an intimate connection between the sisters, the combo would be stronger than the some (any defects in one joist, would be compensated for by the sister, and vice versa -- much like in any other 'lamination')

Plus, if the above floor has tile, the trauma of hammering nails could crack things up (of course, the process of jacking the joists straight won't help in such matters either).

K

 

[luftweg]

"...Somehow I think that by getting such an intimate connection between the sisters, the combo would be stronger than the some (any defects in one joist, would be compensated for by the sister, and vice versa -- much like in any other 'lamination')..."

There is a typo in the above paragraph; replace with:

Somehow I think that by getting such an intimate connection between the sisters, the combo would be stronger than the same 2 pieces unjoined (because any defects, or changes in grain in one joist, would be compensated for by the sister, and vice versa -- much like in any other 'lamination'); the 2 sisters would function as one joist.

 

[daveh6729]

If anyone still cares about this post...I have specified 2x4's of the same grade as the joists epoxied to the bottom of existing joists with West Systems or equal two part epoxy (with some filler added to thicken the mixture), and this greatly increase the section modulus, and momnet of inertia. Nails or screws are used only for clamping pressure...

 

[luftweg]

Supposedly, the steel straps offer even greater strength/stiffness potential.
And, one big reason to using the steel is due to height concerns...
Adding the 2x4's (flat, I assume you mean) requires 1.5"; adding the steel would require like 1/4 to 3/8" absolute max...

Not just for the clamping pressure, would not the screws also benefit in limiting some creep of the epoxy over time?

K

 

[BigInch]

Using steel straps works and works very well, not only for wooden trusses or deep wooden beams, but for ancient stone construction as well. You should see the number of structures in Europe and the mideast that have been refitted (probably several hundred years ago) using wrought iron reinforcement straps bolted across cracked wooden and stone constructions.

With wood, one must pay attention to minimum C2C spacings for bolts and, for that matter nails.

Bearing allowables of wood where the bolt shank makes contact are often critical.

Now go find a copy of the "Timber Construction Manual"


Going the Big Inch!

http://virtualpipeline.spaces.msn.com