Author Topic: Question of foundation piers  (Read 58664 times)

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alcowboy

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Question of foundation piers
« on: January 27, 2009, 10:18:08 AM »
I am considering a 24x32 home with an 8x12 porch along one 24' side giving me a 8x12 porch and 8x12 dining room.

What I am concerned with at this point is the placement of piers for the foundation. I thought maybe a 8x8 grid which would give me 20 piers total (4 for the 24' width x 5 rows for the 32' length) and using 6x6 posts as my foundation rim?? to which I would use 2x10 (again not sure here) for my floor joists on top of the 6x6 post and 2X10 encompassing the floor joists.

I have been studying ScottA's pictures and think it would be easier to use block for my piers instead of 6x6 PT piers (last MUCH longer too).

Will this work? Is is overkill? Need help please.
Scott Gleaton - Alabama

Offline MountainDon

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Re: Question of foundation piers
« Reply #1 on: January 27, 2009, 05:30:31 PM »
For something like a 24 foot wide all you really need is a beam down each long side and one down the center. Concrete block piers would be excellent. The size of the beam and the bearing capacity of the soil determine what the spacing would be. I do not have figures on that, but an engineer or architect could calculate it.

The floor joists would be placed across the top of the beams. 2x8's on 16" centers could be used but are getting close to the maximum span; 2x10 on 16" centers would definitely give a solid stiff floor as well as more room for insulation.

If I understand the porch and dining room thing correctly you plan adding these to the 32 foot length, with the porch on one side of the 24 foot end and the dining room on the other. Correct? How do you envision the roof; an extension of the house roof covering the porch as well?
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline ScottA

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Re: Question of foundation piers
« Reply #2 on: January 27, 2009, 06:32:38 PM »
I agree with Don on only making 3 rows of piers. On an 8' spacing you'll need to use a triple 2x8 for your beams asuming a single story. The spacing on my piers is only 6 1/2'. With the blocks running with the beams this leaves 4'10" clear between the piers for the beams to span. your's would be different. Need to check the span tables for your floor joist size.

Offline ScottA

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Re: Question of foundation piers
« Reply #3 on: January 27, 2009, 06:50:50 PM »
The way I self engineered my piers was to determine the weight of the house. To do this you'll need the square footage area of the floors walls and roof. I figured 40# per sq. foot for floor, 10# per sq. foot for walls and 40# per foot for roof. If you where in a higher snow load area you'd need to add the extra snow load. I figured mine fo a 30# snow load. Keep in mind that nearly all the wieght will be on the long outside beams and piers. The inner piers will carry half the floor and half the end walls plus any interior walls. My soil is suposed to carry 5500# per sq foot. Each 2x2' pier footing has a bearing area of 4 sq.ft so this is 22,000 pounds per pier. Your soil may not carry near this much weight. Your footings should sit on undisturbed soil below the frost line and be well drained.

alcowboy

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Re: Question of foundation piers
« Reply #4 on: January 28, 2009, 05:03:13 AM »



Here are images of both the floorplan and the completed project. I hope this helps you guys better than my messed up descriptions.

Offline Squirl

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Re: Question of foundation piers
« Reply #5 on: January 28, 2009, 06:10:23 AM »
This is the way I calculated mine. 

http://www.mcvicker.com/resguide/page013a.htm

According to this chart for a 12' span you could do a 4x8 girder with piers at less than 4' O/C  or you can go with a 6x8 girder with piers at 6' O/C.
I tend to like charts like these because they help me calculate the cost of different options, such as the cost of changing the piers vs. beam size.  More piers = smaller beams.  You can calculate what is most cost effective for you. 

I would check to see if your soil holds the normal load.  The building local building inspector will usually tell you what he wants. 

alcowboy

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Re: Question of foundation piers
« Reply #6 on: January 28, 2009, 06:46:38 AM »
squirl doesn't that put me right back at 8x8 spacing on the piers? I am so confused.

I don't want to have to special order anything (more $$). 6x6 PT posts are readily available but 6x8 would have to be special ordered.

Offline John Raabe

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Re: Question of foundation piers
« Reply #7 on: January 28, 2009, 07:07:56 AM »
The chart that Squirl is using is concerned ONLY with the floor loads
• 40 psf live loads = people, furniture, parties, etc.
• 10 psf dead loads = framing materials, floor coverings, interior framing, etc.

Those charts do not cover the loads (dead or live) of the roof, loft or anything above the floor. The biggest of these in your design will be the snow, wind and roof structure itself.

When you design a foundation you have to do a load trace. You start at the top of the roof and work your way downward accumulating the areas and what their design loads are and distributing them to the structural members (walls and beams) that will carry them to ground. The beams carry that accumulated load (in PLF = pounds per lineal foot) and deposit them at the footings where they turn into PSF = pounds per square foot based on the carrying capacity of the soil (as Don has described).

In stock plans this is normally done by the designer/architect for standard snow loads and average soil conditions. What is in your plans?
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Offline Squirl

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Re: Question of foundation piers
« Reply #8 on: January 28, 2009, 07:29:19 AM »
Are you talking posts or girders?  This chart is to explain how many posts/blocks/piers you need based on size and spacing.  Of course this could change based on your local soil conditions. Many people build their girders from 2 by lumber nailed and glued.
http://www.mcvicker.com/resguide/page008.htm#FOOTINGS
How deep the footings have to be is regional.

So, If you put three rows of piers as many have suggested.  One row of piers down each side of the house and one down the center you would have around 12' span for the floor joists.  You would run a girder beam under the floor joists over top the posts coming off the piers.  You would need to space the posts based upon the size that you use for the beams.  Also depending upon the size that you use for the posts you would.  

The general rule is to use pressure treated wood when it touches concrete.  This is also the code in some areas.  Some like using PT for any exposed wood because of the longevity.  You can use untreated wood with treated wood.  So what many people do is pour piers, use 4x4 or 6x6 PT posts, and use built up regular 2X lumber built up to 4x or 6x girders. When you eliminate the posts you are touching the concrete block directly to your girders.  In this case you need to use PT wood girders or some type of protection.  Concrete holds moisture.  Moisture and wood do not mix.   Many other people here would probably be better at explaining it than me, but I tried.

Sometimes a picture is worth a thousand words.
http://www.jshow.com/y2k/listings/7.html

Offline Squirl

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Re: Question of foundation piers
« Reply #9 on: January 28, 2009, 07:34:54 AM »
John and Don are right.  I just use this for a general calculation.  The local building inspector will usually tell you what is standard for your area.

Offline MountainDon

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Re: Question of foundation piers
« Reply #10 on: January 28, 2009, 07:42:29 AM »

Considering the lengths required it may be easier to use the built up beam method, staggering the joints over posts. My beams are larger than required but provide a very solid platform. 4 layers of 2x10 DF#2 per beam.

Just because something has been done and has not failed, doesn’t mean it is good design.

John_C

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Re: Question of foundation piers
« Reply #11 on: January 28, 2009, 07:45:42 AM »
Those tables are confusing and not very comprehensive, even for floor joists.

The beam/girder table are only for floor load and assume 40 psf   - quite common but some areas are higher or lower. They are limited to Doug Fir - Larch and appear to favor some sawn beams rather than laminated beams of 2x stock which is what most people will use.

The joist tables are limited to Doug Fir - Larch and only show 1/360 deflection.

The tables linked from this site with a very good explanation by our host, John, are far better. 
Explanation  http://www.planhelp.com/public/98.cfm
Tables   http://www.awc.org/calculators/span/reversecalc/reversecalc.asp?feet=13&inches=0&species=Douglas+Fir-Larch&member=Floor+Joist&deflectionlimit=L%2F360&spacing=16&liveload=40&snowload=-1&deadload=10&wet=No&incised=No&submit=Calculate+Span+Options

You can choose different woods and loads 1/480 is useful if you are going to put tile over the subfloor for example

Offline John Raabe

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Re: Question of foundation piers
« Reply #12 on: January 28, 2009, 08:01:03 AM »
The above charts do the best job I know of designing rafters and joists. But when the joists and walls sit on beams (or girders) the sizing and spans of these get more complicated as I tried to explain in the load trace discussion.

For my designs I use a computer program to analyze the load trace and pick the size and grade of the members based on the span and accumulated beam loads.

The best discussion of this process, along with a method of engineering that involves only a calculator comes from one of the great owner builder teachers - Charles Wing. See the books "From the Ground Up" and "From the Walls In". He taught me everything I needed to know about engineering. Lots more than I learned from the overly complex engineering course in architecture school!

(Note: these books are hard to find and out of print.)
None of us are as smart as all of us.

Offline MountainDon

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Re: Question of foundation piers
« Reply #13 on: January 28, 2009, 08:15:40 AM »
Now that I see the proposed plan things are a little clearer. When you stated a 24 x 32 building I automatically assumed the beams would run in the long direction. Assumptions nearly always lead to misunderstandings.  d* The illustrated plan is actually a 24 x 24 building with a room bumped out under a shed roof. Then the porch is placed in the inside corner formed by that. That's how I see that. Others may differ.

When thinking about this also remember that the exterior porch floor should be 1 to 2 inches below the finished floor level inside.

Viewing that floor plan on the screen, I now visualize it as three parts; the 24 x 24 main cabin, the 8 x 12 room and the porch.

The main cabin beams would run sideways across the screen, in the same direction of the roof peak. There would be a central beam between them. The roof loads would then transfer down to the two 'outside' beams.

The dining room would be built as an addition. That would require an additional beam placed under the 'bottom' wall as drawn on the plan. (12 ft wall with twin windows).

Lastly the porch would be added using PT wood from the porch deck down.

The above is not as easy as building a square or rectangle and adding a porch on one side or end. At least it's not so easy without detailed plans. Not impossible; simply more difficult. At least that is how I see it. If anyone has a better method please add your thoughts.
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline John Raabe

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Re: Question of foundation piers
« Reply #14 on: January 28, 2009, 08:27:38 AM »
Yes Don:

A agree with your analysis.



In the above diagram the red lines are girders carrying the floor and wall loads. The green lines are where the roof loads will land (assuming a truss roof for the main building and stick framed at the porch and dining areas).

This can show us where the load bearing walls will be and where headers will be a concern.
None of us are as smart as all of us.

Offline Squirl

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Re: Question of foundation piers
« Reply #15 on: January 28, 2009, 08:28:16 AM »
I use these as a general guide because they are similar to the many designs I have seen built here.
Like this one. http://www.countryplans.com/lemay.html

These seem to be 6' O/C 4x4 posts and 4x8 beams with an 8 ft span for the floor joints.  That is almost identical to the chart for #2 Douglas fir beams.  If this could survive lake Michigan snow loads, I wasn't as worried.  I always try and make sure I am scaled back by a few inches to be to be on the safe side.  

The American wood council site is better for calculating the span options for different kinds of wood.

Offline John Raabe

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Re: Question of foundation piers
« Reply #16 on: January 28, 2009, 08:40:20 AM »
Note that the LeMay project and the post and pier foundations for most of my small structures use a gable roof and full span floor joists landing on the same girder. That simplifies greatly the design of the girders and pier spacing. Those layouts can be followed as a rough guideline only for similar designs.

The stock plan discussed above (not mine) is quite different and has more complex loading especially at the foundation and with some non-obvious load bearing walls.
None of us are as smart as all of us.

alcowboy

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Re: Question of foundation piers
« Reply #17 on: January 28, 2009, 10:22:25 AM »
Thanks everyone for trying to explain this to a wooden-headed ding-dong (ME).

I have the plans in hand for this cabin. However, it only shows for a slab foundation. I want a crawlspace. The floor joists are drawn as front to back with cross member center of the 24x24 running from the left to right. Would I place my footings at ZERO, TWELVE and TWENTY-FOUR feet in the 24x24 area, giving me a total of 9 piers there and adding 3 for the bump-out room and porch on the front, then building my porch similarly to that room?  Is that what John and Don are so graciously trying to tell me. HIT ME IN THE HEAD IF NEED BE. I understand some of this stuff but not a contractor and have to make sure I get it right the first time.

John_C

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Re: Question of foundation piers
« Reply #18 on: January 28, 2009, 10:35:38 AM »
Quote
giving me a total of 9 piers there

You could do that but you would need a pretty stout beam to span that far.

You are going to have beams @ 0', 12' & 24'

Piers on 6' centers would allow a reasonable size beam.  That would be 15 piers under the main 24 x 24 structure.

There are other solutions but that one makes the most sense to me.

alcowboy

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Re: Question of foundation piers
« Reply #19 on: January 28, 2009, 10:46:11 AM »
so that I am understanding this:
15 piers (basic cement block with cement and rebar inside), 3 girders (6x6 PT posts)
on top of that I would use 2x10 or 2x12 for rim joists and floor joists. Maybe I am overthinking this but would you put double 2x's perpendicular to the girders in the "box" that you will be creating on the top of the girders and then put floor joists perpendicular to these doubled 2x's?

John_C

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Re: Question of foundation piers
« Reply #20 on: January 28, 2009, 11:01:46 AM »
The girders/beams would not be 6x6 posts.  They would most likely be built of several layer of 2x10  or 2x12. (back to the load trace)

The floor joists would run perpendicular to the beams.  They can be hung between the beams but on top is better.

I had another thought.  The wall between the living room and bedroom is not on the center line.  Do the plans show trusses for the roof?  It may well have been designed without interior load bearing walls, the roof load being supported by the front and back walls of the 24x24 box.  If that is the case and you want to go with trusses the loads on those beams will be substantially higher than on the center beam that would then only support 1/2 the floor load.

If that is the case what do the plans show for a header between the kitchen & dining area? It would need to be pretty stout.

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Re: Question of foundation piers
« Reply #21 on: January 28, 2009, 11:13:15 AM »
I'd like to take a step back before going forward.

Are any of the interior walls in your plans noted as being load bearing? If so, which ones?

What size is listed for the ceiling joists and what direction do they run. Both for the 24x24 and the 8x12 areas.
Just because something has been done and has not failed, doesn’t mean it is good design.

John_C

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Re: Question of foundation piers
« Reply #22 on: January 28, 2009, 11:19:51 AM »
I'd like to take a step back before going forward......

Killjoy! Full speed ahead... blindfold please.

alcowboy

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Re: Question of foundation piers
« Reply #23 on: January 28, 2009, 11:33:02 AM »
I'd like to take a step back before going forward.

Are any of the interior walls in your plans noted as being load bearing? If so, which ones?

What size is listed for the ceiling joists and what direction do they run. Both for the 24x24 and the 8x12 areas.

2x6 16" O.C. ceiling joists runing from front to back wall (both), 2X8 rafters 16" O.C. with 1x6 collar ties 32" O.C.
Floor joists are 2x10 16" O.C. running same direction as above. porch is 2X8 24" O.C.
As far as I can tell there are no load bearing walls ( I am guessing it would say LB or Load Bearing).
By the way, there is a built up beam of 3-2x10 between the kitchen and the front dining area. (5 1/2" wall) and the wall between the kitchen and bath is 5 1/2" also.

People I do apologize for not understanding this but the plan shows only a concrete foundation.

John_C

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Re: Question of foundation piers
« Reply #24 on: January 28, 2009, 11:43:09 AM »
I'm confused.
Quote
However, it only shows for a slab foundation.
I thought you said it had a slab.  Where are the floor joist coming from?

2x6 ceiling joists won't span 24'.  Either those are the bottom chord of trusses or there is a bearing wall somewhere. 

 

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