P&P Cross Bracing

[DirtyLittleSecret]

Anyone got some good advise regarding cross bracing for concrete "post & pier" floor structures (ie: joists to beams)?  Curious whether my joist hangers and hurricane ties are sufficient or whether it would be good to add some cross bracing (and what's the preferred method).

[DirtyLittleSecret]

wow, nuttin'?

[Don_P]

I think I lit that fire, I was hoping John or one of the more experienced with this method had a common method. You all are adressing vertical loads pretty well but it doesn't seem that much thought is given to lateral loads. Typically the ends of girders are fixed to prevent rotation by dropping them into pockets or framing and blocking them to a rim. I've not seen signs of much support in this area. Is your beam saddle rated for this type of load? There is some type of restraint specified at intervals along some beams, especially as they get taller and narrower. One type is a steel strap well attached to the joist on one side of the beam that runs down under the beam, attached there, and then back up to the joist on the other side of the beam where it is attached again.  This is all to prevent the girder from rolling over in a lateral (sideways) load like a high wind or quake.

This is one typical table I've seen various places;
At a depth -breadth ratio of 2:1 or less no lateral support is required
3:1 to 4:1 The ends shall be held in postion as by full depth solid blocking, bridging or hangers (the strap I described above is the hanger they are talking about here), nailing or bolting to other framing members or other acceptable means
5:1 additionally one edge shall be held in line for its entire length
6:1 Bridging, full depth solid blocking or bracing shall be installed at intervals not exceeding 8'
7:1  additionally both edges shall be held in line for their entire length

My take on it would be that a 1/4" steel plate beam saddle that came well up the sides and was well embedded into the piers would take care of most situations as I think generally you are in the 3:1-4:1 situation and with the floor on one edge you're good to 5:1. I wouldn't trust lightweight metal connectors to restrain a beam from rolling.

This is an engineer's territory  .

[John Raabe]

I have rules of thumb for the slenderness ratio in the overall article on Post & Beam Foundations, but those are about the posts.

Once you get up to the beam itself you would normally build the floor on top the same way you would on a concrete foundation with a sill plate - joists on the beams and nailed into the blocking or rim and toenailed into the beam w/ 16d nails. See "framing floors" p 81-83 in Wagner's House Framing. You would also want to extend the structural wall sheathing down below the bottom of the joists to nail into the beams to provide a continuous diaphragm connection between the beam, the floor and the wall.

In high wind or tornado areas your inspector may want additional bracing, straps or other hardware. The same framing article above will show you how to do bridging and blocking for long span joists (if needed). That will not be a factor for uplift or racking resistance however.

If you have very tall posts or piers then you should consult a local engineer who can design cross bracing to meet your local design loads. They can size this at the same time as the floor/wall/beam connections.

[Don_P]

The slenderness ratios for columns are to prevent buckling. Googling Euler Buckling will yield a wealth of information on that. Dr Calvert's paper online is a very good one.

Yes the wall sheathing is a diaphragm. The sheathing does not cover the girders in the photo's I've seen and I've not seen anything written about reinforcing them from overturning.

In typical foundation systems there is a continuous perimeter wall with the floor diaphragm directly bolted onto it. Nothing to roll over, it may slide as a unit, which the bolts must resist. If there were girders sitting atop the foundation walls and the floor system was just sitting on that, as you can imagine, I could walk up and push on the floor until the girders tipped over. Something has to resist that tipping force. That same wind base shear is acting on the piers trying to topple them. A typical floor diaphragm, acting as a unit transmits this force to the end walls which disperse the load along their length, again nothing to topple.

[Don_P]

This is one version of what i had in mind;
http://strongtie.com/products/connectors/VB.asp

[Don_P]

This is a way to get good lateral strength in a foundation, its prescriptive, without having to use masonry. A treated wood crawlspace on a gravel trench footing.

[John Raabe]

I have a chart of options for this type of foundation in the Universal Cottage plans. There are different types of joist to wall framing connectors that are used depending on the unsupported back fill height of the PT wall. This can be a good option to pouring concrete for either a crawlspace or basement - especially a daylight basement where you will be insulating and wiring in the studs. In a concrete basement you have to furr out a new frame wall to do this.