Use of Treated Wood for Stacked Concrete Block piers under Manuf. Home

[TexasPE]

Is anyone familiar with the interpretations of the PART 3285—MODEL MANUFACTURED HOME INSTALLATION STANDARDS (with respect to use of 1" treated (yellow pine) wood for the pier caps. I have a full slab under the home and two stacked block piers. But instead of 2" hardwood caps or 1" painted steel, they went with 1 x 8 x 16 pine to place the shims on. Now these boards are all warping and in my reading of the installation standard specification should never have been used. I am trying to get my home releveled and do not want to repeat the mistake of the past, but I am getting a lot of pushbacks from all the local contractors. They say that FHA inspectors do not require it??? My interpretation of Figure A to Sec 3285.306 and Section 3285.304(b)(2) is that although they open the door to use of other listed materials in 3285.4(f) it does not list Section 3285.304(b)(2)(requirements for Pier Caps) as an approved application for either of the treated wood standards listed. Thus, I interpret the standard which allows treated wood to be used in the pier itself and the footer for the pier, it does not extend it to the flat cap of the concrete block which must spread its load across the perimeter of the blocks.

Am I correct in my interpretation of this standard which has been adopted by the State of Texas.

 

[TexasPE]

I am a bit surprised nobody in this forum wants to wade in on this issue. I am a licensed chemical engineer but have both experience and education in certain aspects of civil construction both in concrete and with wood. And yes, even Chem Engr have to take Statics, Dynamics and Strength of Materials in our basic engineering coursework.

It is not surprising the 1" boards that are really only supposed to be put on top of a full concrete cap block and thus would only be used under compressive load conditions have failed. When 1" treated wood boards are used as a cap for the concrete block pier and then subjected to almost point loading conditions with the shims they were put in both compressive and tensile load conditions. As the board over time have sagged they are creating an even worse loading situation as they put the concrete block in a tensile loading condition due to changing the degree of loading to an inside out loading of the concrete block from the hole out. As we all know concrete and wood are much weaker in tensile strength than compressive strength. Based on the approximate average strengths of yellow pine of 7,080 lbf/in2 in compression and only 470 lbf/in2 in in tension this is to be expected. Probably a calculation of the loading of the pier that is supposed to be able to at a minimum support 4,000 to 5,000 lbs of load (assuming a 6 foot pier spacing) and has a loading on a 4" wide shim from a 2" wide flange on the frame (8 in2) for a loading of 625 lbf/in2. Easy to see that the tensile strength of the 1" could be exceeded depending how much the point load is offset from the perimeter and center of the concrete block. Maybe a 2" thick board can distribute that load enough, but a 1" treated board. I think not. Also remember that freshly treated wood has a moisture content up around 19% and it will reduce those average strengths listed by about half from their strength at equilibrium moisture content of about 12-15%. (My source for the wood properties is from my EIT reference manual in the Engineering Materials Section 22.

So are there still no comments or discussions on this issue?

 

[jayrod12]

From my perspective, I didn't respond because where I practice these are all prescriptive foundation elements and so engineers are rarely involved.

If it were a situation where I need to provide real comment, I would be running the same bearing capacity checks you proposed and if it doesn't work, change the material. If it does work, then I'm not sure why it's an issue.

 

[TexasPE]

The issue was the only licensed contractors I could locate claimed they cannot find hardwood and they want to use 1" treated wood. They claim they were told by their trainers that it was acceptable. The problem is in the interpretation of the code which is not explicit enough for the average person to understand when it comes to "or other listed materials". Like you said this area is one where engineers do not get involved much probably because of the codes like the one I added to my original post.

https://files.engineering.com/getfile.aspx?folder=6c4f42f8-f443-47a2-9e18-

 

[SWComposites]

Well, if its your house, and you are paying for it, then tell the contractor to do what you want. Or find the hardwood yourself and ask the contractor to use it.

 

[TexasPE]

They refuse to even come and look at the job. It seems since the State made these contractors get licensed, they are few and far between. Sort of a take it or leave it attitude by the contractors.

 

[SWComposites]

Offer to pay them cash.