Embedded Wood Post Detail 2

[T_Bat]

Hey everyone,

I've got another wood building I'm looking at with a large covered porch/patio. The arch wants to avoid knee braces so I'm looking into doing an embedded wood post. Obviously some people say "no way - you are asking for trouble" but it seems this type of detail has been used with success in the past. I'm proposing to embed the posts (or as many as is economical and works for design). I've attached a sketch and an overall. Normal wind speed here and SDC C. My main questions are:

1. Is there a special preservative treatment I need to specify or does the typical stuff work here?
2. Any benefit of coating with a mastic coating? maybe extend up the post a bit (they will be wrapped)?
3. Seems I need to make sure the end that's embedded is treated (no cut ends down there).

I know there have been other threads on this topic but they seems to be for utility type buildings or smaller structures (not that this is huge or anything). Any comments on the detail are appreciated.

Thanks!

 

[Shotzie]

What about using a Simpson Moment Post Base? Link.

I've never embedded wood posts for these types of structures, maybe someone else can comment on the performance. I know moisture/rot is a big concern. I typically only embed wood for temporary signs.

 

[jayrod12]

There's no way you'd get enough capacity out of the simpson product. Trust me I've looked.

I've done a few pole barns, it can be made to work, however I've never seen it for a hip roof. Only gable. Not sure if/what the differences would work out to be.

 

[T_Bat]

Thanks to both of you - I already spot checked the Simpson base and yes... not even close to working for this application. There shouldn't be much difference with the hip roof right? It's all trusses so no weird thrust to resist. Just a matter of tying the diaphragm to the beams and then beams to the columns. My questions are more about detailing this so there aren't issues with the posts in the future (rot, decay, etc). Last thing I want is for the ends of these to be rotted to nothing in 10 years.

 

[SlideRuleEra]

1. Is there a special preservative treatment I need to specify or does the typical stuff work here?
2. Any benefit of coating with a mastic coating? maybe extend up the post a bit (they will be wrapped)?
3. Seems I need to make sure the end that's embedded is treated (no cut ends down there).

Those are good questions. If you will be doing this type work, learn the details about wood preservatives... it is a subject in itself:

My advice, forget Questions 2 & 3. Instead spend the "savings" by specifying a level of preservation that will work without (wasting) time and money on (useless) details.

Start downloading American Wood Preservers Association (AWPA) Standard U1 Excerpt from the AWPA website.

Go with treatment for at least "Use Category UC4C". There are several preservatives (chemicals) and levels of treatment that will meet this requirement. Don't tell me this will "cost too much" or other nonsense. If this is not done, you are wasting money. Actually, the cost for a higher level of treatment is usually reasonable.

Two other comments:
Omit all the rebar, the post is wood, don't need reinforcement for that. (Save some more money.)
Sit the post on the gravel, not suspended in "mid-air" (until concrete is placed). The proper preservative does not need that.

Edit:

Last thing I want is for the ends of these to be rotted to nothing in 10 years.

I did exactly as described above on my own house (CCA treatment for Marine Use)... 1986. When I dug down to check, in 2006, the post were "like new". Extra cost for the higher level of treatment was about $150 for enough posts to elevate the entire house.

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[T_Bat]

That's exactly what I was looking for thanks SRE! Can they paint them (oil base I assume)?

 

[kmart30]

UC4A or UC4B ground contact treatment should be fine. Has anyone had or heard of issues of long term damage if it was installed correctly with treated wood?

 

[SlideRuleEra]

Can they paint them (oil base I assume)?

Now that is a really good question. Usually, yes... but they need to be exposed to weather for 3 months, or so, before painting. A stain, some of which are very similar to paint, is probably a better choice.

UC4C Ground Contact Extreme Duty. Your posts are embedded in concrete, not soil... possibly poorer drainage than soil.
If you omit the concrete and direct bury, I'll agree with UC4B.

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[CANPRO]

Preservative issues aside, I’d also have concerns about placement. I’m lucky if I can get the contractor to get half the anchor bolts in the right place, what happens if this post is misplaced or out of plumb and cast in concrete?

 

[dik]

You cannot beat PWF treated lumber for longevity... won't outlast the aluminum beer can... but pretty close. There are a hundred, maybe a thousand, pole barns that have used posts embedded in soil/concrete.

Dik

 

[T_Bat]

So here is the detail I'm going to send to the architect for discussion.

A few final questions:

1. Does it make sense to stop the pier below the slab and then pour the SOG up to the column? This eliminates the small extension above the slab. How would an isolation joint work... diamond or round around column then cut on grid in the slab to intersect the columns?
2. What area is used for soil bearing calcs? I've downloaded some post frame literature to look into this but it seems I'll maybe need the granular fill at the bottom to be a small concrete pad instead.

 

[T_Bat]

And an alternate depending on how you answer #1 & #2...

 

[dik]

Either will work... slope the concrete slightly away from the post. The second one is a little more dependent on the granular backfill... and loads on the post will be significantly different, the one being restrained at ground level.

Dik

 

[kmart30]

I think the first detail looks better from a design perspective and easier to construct. I'm sure they both work but the first one just looks cleaner with consistency of materials. They can pour everything at the same time and not have to worry about the gravel in between the turn down and the bearing footing. Just my opinion tho....

 

[T_Bat]

Wouldn't both be restrained by the slab? I'm still not sure on the bearing area of the first one. Am supposes to count on load transfer between the post and concrete mass so that the entire diameter can be used? Or do I just assume a 1:1 (or sim) load distribution under the post through the granular fill?

 

[dik]

T_Bat... yes, my error.

Dik

 

[SlideRuleEra]

T Bat - Both are improvements.

1) IMHO, if the soils are "good" and water table is deep, put "properly compacted granular fill" at the bottom of the hole. If not, use the concrete pad. Don't skimp on the thickness of granular fill or concrete under the post, I would go with 12" thick for this detail. Then you can use the full area for bearing calcs. There are times to "save money" on permanent materials and times to forget about that. Underground is not the place to go cheap. You want to Contractor to complete underground work promptly. One way is to keep things simple (which by the way... "saves money").

2) Place the post and backfill with "properly compacted granular fill" (a lot quicker than placing concrete).

This combination of steps allows the Contractor to install each post one-at-time. A big advantage compared to having the entire area dug up... and then having heavy rains move in.

Specify slope of the floor. One per cent is the minimum that can reasonably be expected to drain.
"Slope away from column" is of little value... where is the "low spot" that the slab slopes to? What happens to water when it gets there?
Say, the Contractor drops the slope 1/16" in 10'... meets that requirement and is worthless. Besides, you are in a better position to handle elevation difference "problems" that a meaningful slope will create.

I can offer some advice on how to minimize the problem CANPRO mentioned... if you are interested.

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[T_Bat]

Great info everyone - I'll have to get with the arch for drainage. As of now this is outside of my scope so I'd need to coordinate with the arch or civil for slopes. I'm definitely interested ensuring this thing is built right. Drawings are useless if it can't get built. I've dealt with too many misplaced anchors in my short career so I can only imagine what could happen here.

 

[charliealphabravo]

I think the architect won't like the extension of the pier above the slab even though it is a good drainage detail. I think the pier would need to be extended more like 24 or 30 inches above grade to prevent it from being a tripping hazard. The option to form the slab over the pier and then cut the slab around it is probably the way to go.

 

[SlideRuleEra]

As mentioned, getting posts is the correct location AND plumb is very difficult. The drawing appears to lend itself to the compromise solution that I have used. The following procedure is for the Contractor, but perhaps you can have a (simplified) proposed "order of construction" on the drawing / spec to "guide" the Contractor.

1) In this case, establish four corners. That means locating the two outlying corner posts as accurately as possible. Completely install these two post, plumb, (in all directions).

2) Using the tops of these two posts, establish the two wall connection locations (with the line perpendicular to the building, I assume).

3) Using the tops of the two outlying corner posts, establish a line between the two. There are not a total of 3 lines.

4) With a plumb bob from the overhead line, lay out the desired location (on the ground) of each intermediate post.

5) Excavate with the intent of keeping the post location on the line, but with a reasonable tolerance to position along the line. I have proposed a location tolerance of +3" along the line.

6) Install the posts, plumb (parallel to the line) but with whatever slope is needed to put the post top at the correct position (perpendicular to the line).

This means you will have to allow the length of the wood members that cap the posts of vary a little.

The advantage of this is than anyone looking an elevation view of the house will see plumb posts. Any slope (perpendicular to the lines) will not be noticed by a casual observer. Also, with do care by the Contractor, the slope should be very slight, say an inch or two (horizontal) compared to the length of the post (vertical).

Ok, this is not "perfect", but it is a reasonable compromise... and it works.

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