Building post house on surface pads

alex trent · October 21, 2011, 11:28:26 AM

I am zeroing in on this. I hope.

Bracing, to me seems to be the key. Couple of questions i cannot seem to find clear answers to on any site.

1. To me it logically seems that a brace from the top of the corner post to the very bottom of the next post...4 x 4...and lagged in provides the best bracing. seems almost impossible to rack the structure (posts) if the bolts hold.

2. On a 24 x 38 do additional braces like this or the one below away from the corner posts add much?

3. What about 3/4 inch plywood triangle on the post and connected to the beam. Say, in this case, two feet down the post and two feet along the beam...screwed in. I know the triangle is strong, but tying to the beam seems to somehow compromise this. Does it? if not, it seems like a nice simeple and clean way to do it.

MountainDon · October 21, 2011, 12:11:51 PM

One reason you have trouble finding recommended solutions to this is that none of the building codes cover this by prescription. They toss this into the "engineered alternative" bucket.

You need to look carefully for the triangles; connecting from the top of one post to the bottom of another could be creating a short 4th arm where the sloped brace connects to the post top. The two post ends in the ground do not count towards anything when looking at the bracing.

To my thinking it's better to keep the sloped side near a 45 degree angle. The fasteners are key as one can introduce point loads that could cause failure to start at a connection point.

The structure requires bracing in the lateral direction as well. This is frequently the less well braced direction but it is more often the direction that needs more bracing. Brace from posts to joists with diagonals.

Tying the beam to the posts with plywood gussets, plates, is good bracing. Nails though, not the common deck screws. Better plywood bracing would involve building a wall between posts. The space between the posts would be framed with vertical studs every 24" or less with a bottom plate. Plywood nailed to the post and beam side and this framework would be very good bracing. Double sided plywood like a box beam excellent, however cautions about creating an enclosed space for undesirable mold, etc to grow or termites to nest could be an issue. The single sided ply between posts amounts, more or less, to what you have with a permanent wood foundation, but above ground.

What Don_P said about not counting on the earth to hold the posts in place or as a part of the bracing, carries great weight. (not sure if he said that here or in another thread.)

Always consider the worst possible thing that could happen, and then how to counter that.

October 23, 2011, 07:36:19 PM

Well, this is still called "House on Surface Pads", but I have come a long way since then. Lots of great info and good references. I have read a lot of the posts so i can synthesize from them as well as the direct advice and gone to a lot of sites.

I figured the next step would be to sketch it all out the way i see it now. not quiet a plan, but I think EZ to figure out what i plan to do. Still needs a lot of finalizing as far as structural details, but i think this is a good start.

The wood i will use for framing is about 30% stronger in breaking and bending and hardness than SYP, #1 grade.

I welcome your comments.

There are 4 attachments..

https://public.me.com/alextrent

Don_P · October 23, 2011, 09:15:46 PM

No luck with the download at my end. Can you put these on photobucket or another photo hosting site and link to them?

alex trent · October 23, 2011, 09:37:56 PM

I will have to sign up.

I just sent these to another person and opened fine. my site is not password protected, so all you have to do is click on the site which takes you to my public folder at me.com and then click on the file and you should get a prompt to click.

Anyway, i will try to sign up on another in a bit.

In a couple of minutes i am meeting with a builder here..a gringo...who has done quite a bit of work.. This place was a hotbed of renovation 4-5 years ago. Most of it sucks, and I am not even a builder and can see it. i showed him the plans i have and he looked at me like they were plans for a trip to Mars. but i will meet with him anyway as want to here what he has to say. I will need some help but will be involved too.

October 23, 2011, 09:44:43 PM

https://public.me.com/alextrent

Squirl · October 24, 2011, 08:56:17 AM

Just typing short.
From conventional framing standards and sizing.
Floor joist to small for spacing.
Spacing off (18")
Ridge beam supported by foundation posts? Can't tell from pictures. Span is long for 4x6.
What are the purlins for?
Weight calculations sound a little light for all areas.
Load bearing walls require headers.
If the posts are extending to the roof and are the only load bearing element between them and the foundation, a beam is needed.
I thought you were going on 6 ft spacing, back to 8?

alex trent · October 24, 2011, 09:50:17 AM

Thanks for the feedback.

1. Joists should be 2x 10 on chart and I will do that.

2. Foundation post will support ridge beam at ends and in middle. I thought I showed end post support in end view...will have to check. So this is 20 foot span. The beam can be bigger...so I will say 6x8. In my current house in town it is a 4x5 on a 19 foot span and this will be the same wood, which i make to be 30 to 60 percent stronger than SYP from the charts. Before I do any of this i need to confirm the wood bearing specs. As you note in here, some of the dimensions are odd...like 5x5 for rafters..which I have never seen in USA. I thought i would follow that as the guy who will do my roof is familiar with that and want him to stay in his comfort zone. Rafters on my house here span 10 feet on about 7 x 12 pitch and are 4x4.

3. Roof purlins support the corregated metal roofing that lies below roof tiles. Local do not use it and put up with leaks. We put cane below the tin so looks nice. This is standard spacing.

4. Weight may be a bit light, but even if another 20,000 lbs, I am still OK overall. That may be the thing that drives the pilings back to 6 feet.

5. Headers...yep got to figure that out yet...the big opening..12 foot...likely be a 8x8, windows, 4x5.

6. Yep, need a beam....likely be 6x8 if I go to 6 foot piers or 8x8 if i stay at 8 foot..

I need to give a lot of consideration to bracing the house...not sure of the best way. is the corner knee or plywood brace as on the deck same-same for this?

Collar beams...I have one in my big roof and none in my bedroom..19 and 16 feet with span of 9 feet. I need one on every on, or spaced out? I think they look cool, so not a problem.

Squirl · October 24, 2011, 10:25:43 AM

Quote from: alextrent on October 24, 2011, 09:50:17 AM

2. Foundation post will support ridge beam at ends and in middle. I thought I showed end post support in end view...will have to check. So this is 20 foot span. The beam can be bigger...so I will say 6x8. In my current house in town it is a 4x5 on a 19 foot span and this will be the same wood, which i make to be 30 to 60 percent stronger than SYP from the charts. Before I do any of this i need to confirm the wood bearing specs. As you note in here, some of the dimensions are odd...like 5x5 for rafters..which I have never seen in USA. I thought i would follow that as the guy who will do my roof is familiar with that and want him to stay in his comfort zone. Rafters on my house here span 10 feet on about 7 x 12 pitch and are 4x4.
Collar beams...I have one in my big roof and none in my bedroom..19 and 16 feet with span of 9 feet. I need one on every on, or spaced out? I think they look cool, so not a problem.

There is no middle foundation post directly under the ridge support. So this post will fall to the center of a beam. Usually a big no no in most framing. A 20 ft span is still very, very long for that small of a beam. You would be resting 1/4 the weight of your entire roof on that center post.

http://courses.cit.cornell.edu/arch264/calculators/example8.1/index.html
http://www.forestryforum.com/members/donp/beamsizing.htm

From the calculator 6x16 or 8x18 minimum, with hemlock fir. Your size is might not be small enough for it to break, but would probably flex quite a bit and possibly cause leaks. Proceed with caution.

What are collar beams?

October 24, 2011, 10:44:17 AM

I can easily put a pier in the right place for that..just insert an extra one...as you say, just on a bean is not a good idea.

Ridge beam in my house is actually 6 x 6 not 4x 5 as I said before. But at 20 feet it is sound and while hard to tell if there has been any bend it does not appear to have any sag at all. i am using the same wood...or plan to and as i said, need to be careful there to get the right stuff. anyway will upsize from the 6 x 6...will check that out very carefully.

Collar beams go rafter to rafter high up on the rafter..about 2 feet down from the ridge.

at

Squirl · October 24, 2011, 11:00:51 AM

I just usually know them as collar ties. They are simply for uplift.

I have no idea about the design of your current house, and how or why it has a 6x6 ridge beam. There are many factors.
I'm just trying to show the how and why things are done, rather than pointing to something someone did without myself understanding why they did it. I do know how beam sizing is calculated. The forestry forums one is better. You can click through the various links on it, and it walks you through how to do the math and where to find the information.

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

I never noticed, that is posted by a DonP member they have

Squirl · October 24, 2011, 11:07:24 AM

Also note, that pier will have to be more substantial. Edit. I'm sorry 1/4 or 7,500 lbs.

alex trent · October 24, 2011, 12:38:24 PM

Calculation question.

If the entire house weighs 40K...the roof being about 15K of that, I am not sure I follow. This pier supports 1/4 of the total of the roof...I can see that, so it is 3,750 lbs. for the 1/4 of the roof and its share of the rest of the house..about 1,000 lbs for a total of 4,750.

If need be, I can not tie the center beam to this pier or put in a separate one to cut the non-roof load.

Does this mean that in what you say, the ridge supports 3/4 of the roof weight and the bearing walls 1/4?

About only commenting on what you know from direct observation or tables, I understand and that is a good reference point for me. As I said, main variable here is the wood and so I need to compare and scrutinize the relative values. Some empirical observations do make sense though...there are thousands of homes, shops and big hotels built to those specs and they been standing for 100 years. so something is different and the obvious thing is wood strength. This is not like a couple of people did it and got away with it.

Squirl · October 24, 2011, 01:13:34 PM

I estimated 20 psf because of the tile (vague recollection of estimates for roofing with tile) and a 10 psf live load (wind/rain/margin of error) for a 30 p.s.f. load

The ridge supports half of the weight (50% of each side) of the roof and 1/4 to each wall.

Since there are 3 posts to the ridge. The center post bears half the weight of the ridge (50% of each side) and the wall posts 1/4 each.

So for the center post 1/2 x 1/2 = 1/4 the weight of the roof.

I have no idea about the hundreds of houses you have seen, or why or how they were designed. You can plug the values of strength into the equations and see how carpenters size ridge beams with that type of wood.

October 24, 2011, 06:14:46 PM

Quote from: Squirl on October 24, 2011, 11:00:51 AM
"Just because something has been done and has not failed, doesn't mean it is good design."

I never noticed, that is posted by a DonP member they have

The phrase is not copyrighted and we both owe it to a mutual friend.

alex trent · October 24, 2011, 06:50:37 PM

I appreciate the very good advice and the thought (design) provoking comments. ,If you follow the thread, you will clearly see them.

To this good quote, to which i would subscribe...

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

i would add, that not everything is contained in "tables' and to disregard local expertise and empirical knowledge is not a good way to live on the frontier (which this is).

I learned early on that 'this is the way we do it (or don't do it) is not a reason but an excuse, so I look at it all with that kind of eye. But, you gotta believe what you see. The hundreds of houses you allude to are not the ones I have seen but representative of 100's of thousands built that way. I am not trying to be philosophical, just practical...and curious...i like to know why.

Don_P · October 24, 2011, 10:58:22 PM

I haven't seen the plans so just some general comments as to the why's in your last post.
The numbers you are looking at come from the wood handbook's ultimate strength values for small clear specimens, the best of the best. We, US and Canada, establish allowable design values, roughly, by using a 5% exclusion limit for the grade. The 5% limit means that 95% of the wood breaks at 2.1x the allowable design value (a factor of safety) and the remaining 5% can break below that but above the allowable design value. Grading is also conservative and sorts the wood according to levels of strength reducing characteristics. When you add it all up this should mean less than 1 failure in a million. So quite conservative. We also use actual elasticity of the species and grade to determine deflection, sag, and this rather than bending strength is usually the limiting factor... a serviceability rather than a strength limit. I've seen plenty of folks wing it and it generally works to some level of performance. They are generally unwittingly eating into their safety margin some unknown amount and have no clue about the serviceability over time. Overloaded wood tends to creep over time at loads well short of ultimate. Tables and engineering come from real world experience, the process is known as rational design. If you use failure as the criteria for design there is no safety margin.

In a beam, depth creates strength to the square of width, "deeper is cheaper". For the same volume if you can make a beam deeper it will make it stronger than if you make it wider, within reasonable limits. In the beam sizing link notice how section modulus is derived.

Squirl, those are calcs I wrote using well established engineering formulas from the Nat'l Design Spec for Wood Construction. Jeff at the FF gave me some space to post them for their members' use and although I doubt he would really mind, I don't link to them from other forums as it would use his bandwidth. I did have them on my own website as well and linked to them previously from here, unfortunately that site got hacked. Not admonishing, just explaining how they got there and why. (Never use a LL of less than 20psf)

Squirl · October 25, 2011, 08:38:48 AM

Thank you for the greater explanation. I used 20 psf for a dead load because I had recalled that from a roofing book I had read. Tile is one of the heaviest types of roofing design. In my reading averages for tile roofing fall from 7 psf to 11 psf for just the tiles. I figured 10 psf for just that, plus 10 psf for the everything else, which may be a little low. Thanks for the tip about the live load calculation. The footnote in the ICC code guideline said to calculate a minimum of 30 p.s.f. load even in no snow areas. I went a little low on my figures because from the calculation in this design, there seems to be a greater willingness to cut down on safety margins. Due to the fact that this is being designed for a possible seismic area (volcano), I probably should have adjusted my calculations for a greater safety margin.

I hope Jeff isn't worried about bandwidth or traffic. He is consistently the #1 or #2 search result from most search engines for "beam sizing."

I always wondered how to calculate beams for different species of wood. It was never listed in the timber frame books I had read. Yours was the simplest and easiest explanation I had found.

alex trent · October 25, 2011, 09:49:55 AM

Don, thanks for the info...always good to have some in-depth info to know where the numbers come from. This is especially important when you will have to interpret them to another species that is (may) not be on the list. That will be my biggest challenge..and being sure that what they say it is is what it is. It is not necessarily gross misrepresentation, but there are lots of varieties and I believe they have different characteristics. The current cedro macho is replacing cedro real (latter is stronger the former), but in some instances the data you see (as well as the personal perception) by some here who use it has not changed. I am in the process of sorting that out...found a building engineer who is supposed to have experience with wood and a good mill which seems knowledgeable and reputable.

Tile here is 6 to 8 psf. We figure in another pound when wet. The cane, tin and the wood come to about another 12 or so, and that is about 20 psf or the roof. I am figuring that is about half the house deadload...so I don't think that cuts safety margins at all. The piers will support 81,000 at 2,000 psf soil bearing...not counting the extra pier for the center beam support. That is a 2x margin on soil bearing.

I think we are running into some misunderstanding about "safety" because of the wood that is used in current buildings which I have referenced. Be interesting to see how the comparison stacks up when I get comparative data on this vs. SYP #1 , which is my reference wood. In any case, once I get it figured out, I will make the adjustments and use what is necessary. I brought my visual references to what is used in various structural members up as a point of interest as much as anything. The joist chart for this says 2X10. If the wood I use is, in fact, double the strength, i will not cit back on that, as the cost of going to 2x8 or even 2x6 is not a big deal. On the other hand, I will not use a 8X18 ridge pole if a 8X10 will do.

As for seismic activity, we have lots of little tremblors all the time. Nothing falls. The house like the one I live in now have stood through 100's of them over 100 years. But when the big one comes and it is close by, there will be trouble. Remember Managua in 1972...I doubt any safety margin you could reasonably build into this type of house would do much good.

I will try to get my drawing up on some program other than the one it is on now...me.com... but having trouble as being outside the USA is not allowing it right now.