Quick sanity check for compressive axial loads ...

[gjbuilder]

I have a few rotted poles in my pole barn ... and am considering pouring concrete piers and sinking some W6x15 steel beams to replace them.

Just to be sure, I am comparing the compressive axial load of the two and making sure my instinct (that the I-beam is *much* stronger than the 8x8 doug fir timber) is actually correct.

So, I see on this page:

http://www2.wwpa.org/portals/9/docs/pdf/tn9.pdf

... that a doug fir 8x8 with length of 16' has a compressive axial load of 33801 lbs. Then I look on this page:

http://www.webcivil.com/usaxialw.aspx

(and I select W6x15 from the designation pull-down, and then click "apply" button in the upper-right) ... and I see that a 6x6 I-beam of 1/4" steel has a compressive axial load of 147 kips, or 147,000 lbs.

Does that all sound right ? The 1/4 steel 6x6 I-beam is about 4-5x as strong, vertically, as the 8x8 ?

Are steel beams just so much stronger than (roughly equivalent) wooden beams that it was silly for me to compare these in the first place ?

(FWIW, I will dig the piers down 5 feet, pour a foot of concrete for a solid footer, place the I-beam in, and then pour concrete to surround it for the next four feet to ground level ... so replacing a 16' 8x8 timber with a 12' 6x6 I-beam. I think this is probably total overkill.)

Thanks.

 

[nehog]

A wood piece is subject to flaws and similar issues, meaning you have to have a greater allowance on load. A steel piece is manufactured, with controlled quality, and therefore needs a bit less of a safety factor.

And yes, steel is stronger in compression mode than wood due to wood's grain.

 

[gjbuilder]

Thanks. Yes, I figured a W6x15 steel beam was much stronger than the 8x8 timber.

Now, about the piers ...

Existing barn poles are 20' 8x8 timbers sunk 8' into the ground. They sit on a foot of concrete down at the bottom, but otherwise are just surrounded by drain rock. Pretty standard pole barn configuration. They key is, they are 8 feet below, and 12 feet above the ground.

I am proposing replacing those with 12' long steel beams ... so 12 feet above, but only 4 feet below the ground. BUT, they will be resting on a new foot-thick footing at the bottom and surrounded by a cylinder of concrete 3 feet in diameter.

I'm hoping this is overkill. Does it sound like overkill ? That's the goal...

Random: I am going to weld a 8x8 piece of 1/2" steel to the bottom of the I-beams so that their weight is spread over 64 square inches on top of the concrete footing ... it occurred to me that the 1/4" thick "I" of the beam could start digging into the concrete footing with all that weight concentrated on such a small surface area.

Thanks.

 

[rieferman]

Do you have free beams that you're looking to use? Otherwise seems like I'd just replace with a perma-column and call it a day. Not sure I understand the big push for steel.

Post replacement using perma-column

Also, existing posts are 8 feet underground? Where the heck do you live that required that depth?

 

[gjbuilder]

Also, existing posts are 8 feet underground? Where the heck do you live that required that depth?

Marin County, CA. Just across the golden gate from SF.

It only gets down to +15 F (and only briefly overnight) in the winter, so I don't think that depth was required at all. No idea why the original builder of this barn did it that way.

Very dense clay soil, though ... rots things out ...

 

[gjbuilder]

Do you have free beams that you're looking to use? Otherwise seems like I'd just replace with a perma-column and call it a day. Not sure I understand the big push for steel.

Yes, I found those first and was of course considering them ... the problem is that the perma-columns only go to 6x6 in size, and I have rough cut 8x8 ... so even another vendor I found that *did* offer 8x8 perma-column-style concrete columns still didn't fit, since they were (roughly) 1/2" too small.

So it ends up being a custom build anyway ... and W6x15 is $15/ft, give or take ... so those steel beams are roughly $300/ea, so I don't know that I would save that much vs. a custom-built perma-column with a custom-built wet-set bracket on the top ... it might even be *more* expensive...

 

[pmiranda]

Is there a seismic requirement for attaching the steel to the new footing and/or the fill underneath it?

 

[rieferman]

Yes, I found those first and was of course considering them ... the problem is that the perma-columns only go to 6x6 in size, and I have rough cut 8x8 ... so even another vendor I found that *did* offer 8x8 perma-column-style concrete columns still didn't fit, since they were (roughly) 1/2" too small.

So it ends up being a custom build anyway ... and W6x15 is $15/ft, give or take ... so those steel beams are roughly $300/ea, so I don't know that I would save that much vs. a custom-built perma-column with a custom-built wet-set bracket on the top ... it might even be *more* expensive...

The builder I previously worked for used perma-columns in sizes exceeding 6x6 regularly.

If 1/2" too small, that's just a series of plywood shims wherever a connection to your exterior girts is needed.

I'd fill in to 4' hole depth so that you can use standard post frame processes. Not sure about the fill details though, check with your building department.

IMO, the advantage of using posts designed to be posts in structures just like yours is that the engineering piece of it is already figured out. Seems to me like steel beams in a wood post building is a mismatch. I'm not sure of the ripple effect or possible concerns - but I do know I wouldn't try to figure those out since a very good solution already is available.

 

[WNYflyer]

A W6x15 made of A992 (FY=50 ksi) steel has a compressive capacity of approximately 38,000# if unbraced for its entire 16' height. If perhaps braced by a concrete floor slab say the 4' up the then maximum "unbraced length" would be 16-4=12' then the capacity goes up to approximately 66,000#. A key for axial loaded post/columns is the assumed unbraced length.

FWIW. a 1' high piece of the W6x15 would have a capacity of 133,000# thus to give you an idea on how critical the assumed unbraced length is. There could be other factors involved but this again gives you an idea of the affects of the assumed "unbraced length" on any column/post calculation.

 

[WNYflyer]

Marin County, CA. Just across the golden gate from SF.

It only gets down to +15 F (and only briefly overnight) in the winter, so I don't think that depth was required at all. No idea why the original builder of this barn did it that way.

Very dense clay soil, though ... rots things out ...

Reading through the thread more, the reason the builder went down 8' is most likely the post/columns are designed as "flag poles/telephone poles" type columns. The post/column is subjected to vertical loads but more importantly subjected to lateral wind/seismic loads. Those lateral loads are resisted by the pole/columns acting as "flag poles/telephone poles".

Take a 16' pole and put it the ground 4 feet and the remaining 12' above grade. Push on it laterally on the top of the column. Now put a 20' pole in the ground 8 feet with 12' remaining above grade and push laterally on the top of the column. It will take a lot more force to get the post buried 8' in the ground to come ripping out than a post only buried 4'. A majority of pole barns derive their lateral stability from the column/posts being buried a significant depth in the ground, this depth is often signficantly deeper than frost depth depending on location

 

[gjbuilder]

WNYflyer - thank you for your comments, they are very helpful.

An engineer I consulted with said the same thing about these poles and about pole barns in general and explained that that was the reason you can't just bolt some new columns into the top of the concrete floor - they can't resist the lateral loads just at the surface there.

I wonder if you would comment:

On the big 20 foot main pole that rotted (8 foot underground, 12 foot above) I am planning on sinking a 8 foot steel beam (4 underground and 4 above ground) in the big concrete pier. (and then just cutting the wood column 4' above ground to match).

Setting aside the depth I am sinking it to, is there a weak spot at the 4' mark above ground where the steel beam mates to the remaining wooden column ? Is it weak with regard to lateral load there, or do I just have to connect them properly and the entire thing becomes a single "flagpole" ?

Thanks.

edit: I think this is the same question I would ask about a "perma-column" ... they are half in-ground, half out, and then you connect them to the bottom of your old column ... how does that union (where they mate, 4' above ground) not a weak point for lateral load just like a pole would be if it were mounted to the concrete floor ?

edit2: ok, a small correction - the permacolumns are 5 ft long, and typically 12-18" are above ground ... not half and half like I alluded to ...

 

[PT Doc]

Such good information from knowledgeable folks. And presented cleanly and repextfully.

 

[WNYflyer]

WNYflyer - thank you for your comments, they are very helpful.

An engineer I consulted with said the same thing about these poles and about pole barns in general and explained that that was the reason you can't just bolt some new columns into the top of the concrete floor - they can't resist the lateral loads just at the surface there.

I wonder if you would comment:

On the big 20 foot main pole that rotted (8 foot underground, 12 foot above) I am planning on sinking a 8 foot steel beam (4 underground and 4 above ground) in the big concrete pier. (and then just cutting the wood column 4' above ground to match).

Setting aside the depth I am sinking it to, is there a weak spot at the 4' mark above ground where the steel beam mates to the remaining wooden column ? Is it weak with regard to lateral load there, or do I just have to connect them properly and the entire thing becomes a single "flagpole" ?

Thanks.

edit: I think this is the same question I would ask about a "perma-column" ... they are half in-ground, half out, and then you connect them to the bottom of your old column ... how does that union (where they mate, 4' above ground) not a weak point for lateral load just like a pole would be if it were mounted to the concrete floor ?

edit2: ok, a small correction - the permacolumns are 5 ft long, and typically 12-18" are above ground ... not half and half like I alluded to ...

gjbuilder,

Yes that splice joint is an inherent weak point but is fine if properly designed. Perhaps another way to reinforce the post is put 6" or 8" steel channels on both sides of the existing 8" timber post and through bolting into good wood material to establish an adequate splice. This may allow you to not have to temporarily shore/support the existing building to cut out the bad portion of the existing column and slide the steel W section in. You would also have to do something with a base plate on the end of the channels most likely.

Instead of steel you may even get away with slapping wood pieces 4"x8" ? on either side and through bolting. Will never know until numbers were run and stuff is properly designed.

For the Perma-Columns it is also a weak spot of course. Perma-Columns has test studies and design information on their web site that indicate structural capacities and proper details for those joints. These capacities and details ensure that the joint makes the permacolumn and timber post act as a single unit/piece for the published design capacities. Looks like the installation of permacolumns would require temporary shoring/support of the column or truss

 

[gjbuilder]

gjbuilder,
For the Perma-Columns it is also a weak spot of course. Perma-Columns has test studies and design information on their web site that indicate structural capacities and proper details for those joints. These capacities and details ensure that the joint makes the permacolumn and timber post act as a single unit/piece for the published design capacities. Looks like the installation of permacolumns would require temporary shoring/support of the column or truss

Ok, thanks - that was the difficult part for me - trying to decide if a proper joint does indeed turn the two parts (perma column on bottom and existing post on top) into one single column. I supposed that it did, otherwise why would people do it ... but it was hard to conceptualize.

Thank you.

 

[robertlynk]

Something to think about is the seismic load and liquifaction especially being that you are in that area. With the beam I would weld nelson studs on and/or base plates to resist pull out

 

[gjbuilder]

Something to think about is the seismic load and liquifaction especially being that you are in that area. With the beam I would weld nelson studs on and/or base plates to resist pull out

Yes, definitely. If I replace any of the poles with a w6x15 steel H beam, I will weld a 10x10 square of 3/8 steel to the bottom of them which, when encased in the footer, should be quite a bit of uplift resistance.

I am leaning toward perma-columns for some replacements, but for the poles that will be inside finished walls where people can't see them, and are rotted up 3-4 feet anyway, I think I will just fully replace with a steel beam - 7.5' above ground and 2.5' below ground ... surrounded by a 3' diameter concrete pier. These are poles along the wing of the barn, not the main 20 footers that really hold the thing up ...

 

[matt_i]

Tube Steel, round or square. Do not have the Ixx/Iyy strong/weak issues if that makes sense to you.

 

[gjbuilder]

Tube Steel, round or square. Do not have the Ixx/Iyy strong/weak issues if that makes sense to you.

Yes, that does make sense and square tube is something I am considering for the larger poles if for some reason the perma-columns won't work.

Larger poles I am hoping to get custom made 8x8 permacolumns longer than 5 feet with brackets that will accommodate rough cut 8x8s on top ... and if those are difficult or impossible, I think 8x8 square tube will work.

Boy it's expensive, though ... 8x8 square tube @ 3/8 thickness is not cheap.

edit: .... and I just got off the phone with perma-column. They don't do any business in California and don't know anyone who makes anything like perma-columns in california.