lateral bracing

[astidham]

Hello all. I have installed my bracing the direction of the 30' side of the house with 4x6 lumber. what is a good way to brace the 20' width and what lumber?

[John Raabe]

You can triangulate braces from the floor joists to the beam posts or to the beam itself.

[astidham]

Thank You John, can I use my scrap 2x6s for this?

[John Raabe]

Yes, if you made a sandwich with 2x6 on both sides of the joist that should give you plenty of room for nails and a good surface area on the post or beam. You might do this where you can bear a longer brace to the post and then use single 2x6 braces to the beam.

[jfmann]

Essentially, you should brace each short end of the building with an adequate foundation element (under each exterior wall & edge joist).

The entire house will distribute lateral loads (wind, seismic) to the exterior walls. For wind against a long wall, wind force is distributed to each short endwall. 

The endwall will act as a braced-wall panel (also known as shearwall). Wind force against the long wall is distributed (vertically by wall studs) to the roof and to the first floor. Both roof and first floor sheathing act as a diaphragm .........distributing wind force to each endwall.

Net result is that you must brace each endwall..........parallel to length of the wall. Therefore, you need a foundation element under each endwall that has adequate in-plane capacity to resist the accumulated wind force (against half of the long wall). For interior areas of the US........not in "special" wind zones............total wind force to be resisted along each endwall is about 4,800 pounds, assuming 8-foot high walls (and 40 foot length of long walls).

A block foundation wall (about 8 feet long minimum) should provide adequate lateral bracing capacity. The block wall should be built like any standard foundation wall..........although you might get away without a typical footing (especially with 12-inch block).

Edge joist must be connected to top of foundation wall with adequate connectors to transfer the 4,800 pound wind force into the block wall. For an 8-foot long foundation wall, uniform shear force is 600 pounds per linear foot (PLF), which will likely require at least one strong connector per foot.

You might think that the existing concrete piers should be able to resist wind force against the long walls. However, there are two problems with such theory.........(1) Wind distributed to the roof will be transferred (by roof diaphragm, or maybe ceiling acting as diaphragm) to each endwall.........(2) Wind in the first floor diaphragm will twist the rim joist (and even short posts) before load can be resisted by concrete piers.


Also.......since you are using individual post foundations on the long sides only.........and there is no (apparent) girder supporting midspan of first floor joists (20 feet long)........it appears you are building a one-story house.........with rafters also spanning the short direction. However, considering the 20-foot span, some foundation element should be installed at midspan of the edge joist to support weight of the endwall. Such foundation element can serve both purposes.........lateral and vertical resistance.

[glenn kangiser]

Thanks for the clear explanation of the forces at work here, jfmann.  to the forum.

[Arky217]

Hi, was looking at your pics.
I am also building on concrete piers.

What is the purpose of what looks like aluminum foil around the top of the piers ?

Thanks,
Arky

[astidham]

Hi, was looking at your pics.
I am also building on concrete piers.

What is the purpose of what looks like aluminum foil around the top of the piers ?

Thanks,
Arky

Hey Arky, I used a self leveling grout on top of my piers so my 6x6 post would sit flush to the pier.
I didnt want to shim the differents.
the tape held the grout from runnig off the pier

[astidham]

Here is what I did, I sandwiched a 2x12 floor joist with 2-2x6s 5' long and nailed to the foundation beam.
I did this a all 4 corners.

[jfmann]

The four individual braces are not nearly adequate to resist the large wind force for wind against long wall. Actually wind causes both inward force against the windward (near side) wall and ......believe it or don't.........outward (suction) force on the leeward (far side) wall. However, for overall design of lateral bracing for this type of building, we simplify and consider that the wind acts on the windward wall only.

If you were to install numerous angled braces .........in addition to those already installed........between floor joists and girder supporting each long wall, you could consider such bracing to resist wind force delivered to the first floor diaphragm by the wall studs. However, wind distributed (by wall studs) to roof diaphragm (or ceiling diaphragm) must still be transferred to the ground.......which almost certainly requires bracing of each endwall by some adequate method. See my previous post on foundation wall as bracing.

[astidham]

jfmann, Thank you for the info.
Do you have any pictures of adequate bracing that will help me in my installation?

[jfmann]

As described in my initial post on this ...........the most straightforward method is to provide a simple block foundation wall under each endwall............with adequate connection between edge joist and top of foundation wall to resist wind (shear) force along length of block wall. Minimum length of foundation wall should be 8 feet, within center of the 20-foot long endwall.

Lateral capacity of a round concrete pier (based on soil resistance) depends on the following factors;

1-Diameter of pier
2-Depth below grade (ground)
3-Type of native soil (even if compacted backfill placed around pier is different)
4-Height of pier above grade

If the pier is deep enough (generally much more than 5 feet deep), lateral capacity might conceivably be limited by strength of the pier itself. However, for low-rise buildings, lateral capacity will almost certainly be governed by soil resistance.

In order to develop soil resistance..........some lateral movement must occur. Although magnitude of movement should be very small, it is important to realize that lateral movement can sometimes be enough to cause problems.........especially if "compacted" backfill is placed around the pier.

Of course the primary purpose of piers is to support vertical weight (load) of the building. If load is off-center ("eccentric"), the pier can lean sideways. This tendency must be carefully considered if relatively small diameter piers are used...........such as 12-inch piers.

[astidham]

jfmann, here is my foundation being started.
the footings are 18" wide 24" deep continuous ~32' long with re-bar stubbed out for the sono piers

[jfmann]

Great photos! Footings could literally support a tank......or two.

As for wind resistance.........as long as piers are connected to large concrete footing with vertical rebar, they will definitely resist very large wind force........the house will be blown off piers before the piers fail.

However, the two basic issues noted in previous comments.......relative to wind pressure against long wall...... remain;

(1) How wind force that is distributed to the roof / ceiling diaphragm (by vertical wall studs) will be directed down to foundations.

(2) Whether wind force distributed to first floor diaphragm will be resisted by piers under long wall or by some new foundation element under each endwall.

For (1)..........it is conceivable that wind force distributed (by roof / ceiling diaphragm) to each endwall could be resisted by the first floor diaphragm (although this might be considered as a "reverse load path" compared to the more usual load path considered in design).........and then by concrete piers under long walls. However, connections (always those dang connections!) must be properly designed along the reverse load path........which is problematic. Therefore, I continue to recommend new foundation elements (whether block wall or concrete piers) under each endwall.

For (2) .........to distribute wind force into existing piers, there must be adequate sloped braces between floor joists and top of each pier (all of them). Connections remain important, however numerous nails or screws at each brace should work.

[astidham]

Thanks jfmann, I will be adding a center support beam on screw jacks sitting on pad footings (above ground) in addition to the existing.

[jfmann]

Center beam........taken out to endwalls........will help with vertical support of each endwall. However, without sufficient foundation element under each endwall, such beam does not resolve the lateral force resistance issue.

Of course, for "ordinary" conditions........there will be no problem. In fact, there may never be any problem with lack of foundation element under each endwall if windspeed approaching design wind speed does not occur. However, this is always the case for structural design of any building element.

For "ordinary" conditions.......the most pressing practical issue is bracing to prevent twisting / tilting of the edge beams (on top of wood posts). There should be angled braces (up to floor joists) at each wood post (on concrete pier).

[TheWire]

The need to laterally post or pier foundations seems to be another reason to consider a permanent wood foundation.  I glued nailed the foundation rated PT plywood and studs of the PWF on my cabin.  http://countryplans.com/smf/index.php?topic=4640.0 With the diaphragm of each wall tied to the floor diaphragm, it doesn't seem like the foundation has any need for lateral bracing and no concrete is required.  Just something for those still in the planning stages to consider.

[Don_P]

That is the reason pier and beam is not a prescriptive foundation, it cannot handle lateral forces well.
This is one idea I think I've posted before. Stick J bolts out of the sonotubes, put vertical treated 2x4's on those, then top and bottom plates, stud as necessary to prevent a treated plywood facing (the bracing) from buckling under load. Basically it is a shearwall bolted between the piers.


Here's another way, relatively thin poured walls on rubble trenches between piers


Neither of these methods is prescriptive, but they are braced.

[dug]

When I first started building I didn't give much of a thought to lateral bracing on the piers, me being a complete greenhorn. I guess I thought just being buried in the earth and connected to a solid footer would be sufficient.

I put J- bolts in the piers so I could later attach skirting around the perimeter. I mostly intended to do this for the skirting but then also added some 45 degree bracing after I began to learn a bit more about lateral stress on the piers. Like this-



This gave me what I considered at least some sub marginal bracing, better than nothing I figured. I retrospect, I should have placed additional J-bolts as Don P suggested above and then I think I would have had some solid bracing.

My question is this- after I skirt the perimeter, probably with steel, I planned on getting some fill dirt to berm up against the skirting to aid in drainage away from the house. Like my skirting, I was wondering if this would have an added side benefit of stabilizing the piers some, especially if I berm up higher than I originally planned, say about 2 feet?

I know this would only add resistance to the pier in one direction, but if the floor and walls were locked together good it seems that to move one pier the other on the opposite side would have to move as well.

I fully realize this is cave man engineering but was thinking several tons of dirt around the perimeter might give the structure some added stability. Or am I all wet? 

[jrbutler]

Anyone have comments on dug's bracing?  Looks like a smart budget bracing solution to me.  I was considering doing the same all around my foundation only with a single horizontal strip of doubled 2x8's held in place with 2 - 1/2" bolts per pier, and doubled up 2x6's for the angled pieces.

I've just added a 2 story 20x24 addition (bigger than the main house), and in order to keep the floor level with the main house, the land grade resulted in 2-3 1/2 foot pier, which are worrying me.

[Don_P]

Dug did a good job, the piers are continuous from footing to beam and are well braced. It would be stronger if sheathed brace wall panels were installed between the piers and bolted to them. Then the bolts would be simply holding the panel in place rather than acting in shear, the sheathing would provide more lateral bracing.

[John Raabe]

I think Dug has done a very reasonable job of bracing and the soil skirting will soak up additional forces that might want to twist or slide the house.

[Don_P]

In order to develop soil resistance..........some lateral movement must occur. Although magnitude of movement should be very small, it is important to realize that lateral movement can sometimes be enough to cause problems.........especially if "compacted" backfill is placed around the pier.

Of course the primary purpose of piers is to support vertical weight (load) of the building. If load is off-center ("eccentric"), the pier can lean sideways. This tendency must be carefully considered if relatively small diameter piers are used...........such as 12-inch piers.

When you use a sonotube there is fill placed back around the pier. It should be placed in small lifts of 3-6 inches and tamped firmly around the pier. This is still not as firm as native soil, the lateral resistance has been reduced. Piling up uncompacted fill around a building provides so little resistance to sideways movement that I really wouldn't consider it to contribute anything. It won't hurt but it shouldn't be considered bracing.

Our opinions on what amounts to adequate bracing are just that, opinions. A pier foundation should be designed by someone who knows what they are doing. This past week I have been working on the third foundation to be installed under a house. Originally the house was built on stone piers. By the 70's they had failed and someone attempted to install a block foundation. They did not place the footings below frost depth and that foundation has also failed. The house now has floors going in several directions to the tune of about 3". There are problems with the walls and up to the roof. I'll be under there for several months. I've had 4 more folks inquire recently about fixing theirs, there is no shortage of inadequate foundations and the problems they lead to down the road. It is sure easier to get the foundation right before the building is overhead.

[John Raabe]

Interesting and I agree, pier foundations and bracing for sideward forces should be reviewed by a local engineer - most especially when building in poorly drained soil, on steep slopes or (even worse) on fill. But just advising a standard concrete foundation over a pier system may not help.

Understanding the character of the soil and the building site before committing to a build is key.  Frost heave in expansive soils can be very persuasive, no matter what the foundation system used. If there is any question about the stability of the soil, it is worth a visit from a soils (geotech) engineer. There have and will always be houses that are built in soil that should not have buildings on them. Building on such sites can be expensive and is best avoided. This is a battle nature usually wins.

What are the soil conditions at the third foundation house and what are the engineers suggesting for a solution?

[Don_P]

Let's turn that around. Advising a non engineered pier and beam foundation is never a good idea. A prescriptive foundation is the standard, in areas where that standard does not meet soil or other conditions engineering is adviseable and generally required. We are generally safe advising prescriptive solutions. We are almost never safe advising home brewed solutions. The pier, and post, and beam that has become standard fare for owner builders here is not an adviseable foundation. I'd like for you to reconsider promoting a sub standard foundation.

The project I'm working on has an architect. There is a basic foundation plan. He and the building inspector are comfortable with my non engineered solution, a prescriptive foundation. There is a structural engineer in training living on site. The firm he is doing his internship with is there if we need them. The soil is decomposed granite and drains well, it doesn't get much better foundationwise. As an added bonus there is gold in the immediate area  . Hey it helps with motivation.  Neither of the previous foundations were below frost depth and they had failed to grade the site to drain water away. They were on the B soil horizon, some roots, some life, not bad but the really tight good undisturbed stuff is around 3' down. The house has also never had gutters. We have a stone retaining wall going in which has allowed the site to be properly graded to keep the water moving away from the foundation. The EIT and I were under there yesterday evening, we're still trying to figure out exactly why the current foundation failed so badly. The section of footing I just went through is not bad but is a small section. We'll know more as the old work is removed. We're putting in a full basement this time around so it will be well below frost depth. Thus far I've made it by hand across the building and poured for 2 steel support columns. Those footings are 116" below floor bottom level and as with most old houses around here part of the sill is at grade... we're 108" below ground level presently, needless to say I hurt today! Next week we'll turn 90' and head for 2 more columns at which point we can knock out the gable end of the foundation and bring a bobcat in to remove the bulk of the soil.