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Fear of Concrete Slab Cracking at Post Corners

Fear of Concrete Slab Cracking at Post Corners

Nothing appears to add to the self-importance of a contractor more than instilling fear into the hearts and minds of their clients. If I had a dollar for every fear mongering story I have heard over the years, I would be a wealthy man!

Hansen Pole Buildings’ client PAUL writes:

“Hello, I’m Paul in McCall Idaho and I purchased a 14′ x 36′ pole building last fall from you and I had a quick question.  I am getting ready to pour the concrete floor and the concrete guy helping me was concerned that the floor will crack on all the post corners and wants to do some changes to prevent it. I have never heard that that was a concern and thought I would consult you guys as I know you’ll have the correct answer. SO, is that something to be concerned about??  Thanks for the help and I know my account is closed by now but figured you’d have a quick answer for me.  Thanks a lot.”  

Mike the Pole Barn Guru responds:

We’ve never heard this as a concern either, so perhaps you can get your concrete finisher to share research studies he has read which elaborate upon this supposed challenge. I personally have owned (or still own) several post frame buildings with concrete slabs and have never experienced issues with floor cracks other than where they are supposed to be – along control joints or saw cuts.

The most important things to avoid cracking where not desired is to have dedicated allowance locations for cracks (e.g. expansion joints or saw cuts should be located every eight to 12 feet for a four inch thick slab), have a properly prepared and well compacted site, eliminate sources of water which would or could flow under the slab, reinforce the slab (by use of one or more of the following – fiberglass strands, wire mesh, rebar) and to have the slab tied into the columns by use of rebar hairpins through the columns. The clean sand between the vapor barrier and the concrete should be moistened prior to the pour as well. Concrete mix with too much water in it will lead to future cracking. It is more work to pour with less water, but the end result will be far better. Keeping the slab well hydrated (water on top of the slab) for the first month after the pour will retard the speed of curing making the slab not only stronger, but also will reduce the cracking.

When I was building myself, one thing I would always guarantee with concrete slabs – they will, at some point, crack. It’s the nature of concrete.

 

 

This entry was posted in Concrete, Footings, Columns, Pole Barn Questions, Pole Barn Design, Constructing a Pole Building, Building Contractor and tagged , , , , on by .

Redwood Columns in Pole Buildings

Redwood for Post Frame Building Columns

Just this week we had a client in the California Bay area interested in a roof only post frame structure. His caveat, he really wanted to have Redwood columns, as opposed to properly pressure preservative treated timbers.

The characteristics of redwood make it a popular choice for outdoor applications. Redwood’s color and grain are attractive even in an unfinished state, but more important is the wood’s pronounced resistance to decay and insects. Yet even though redwood is more resistant to decay, it will eventually succumb to rot.

So what sort of lifespan would a redwood column, embedded in the ground, have?

Luckily, Oregon State University’s College of Forestry has done the research for me!

In 1927, Professor T.J. Starker of the College of Forestry at Oregon State University (OSU) established a “post farm” to develop data on the natural durability of native woods and the effectiveness of various preservative treatments for species used as fence posts. Since the first posts were set on January 7. 1928, OSU has placed 2,662 posts in the farm. Three introduced and 25 native species in untreated condition and 8 Oregon species receiving various preservative treatments have been, or are being, tested.

The post farm is located on College of Forestry land in the Peavy Arboretum about 7 miles north of Corvallis, Oregon, on the West side of Highway 99W. Soil in the test area, located on a well-drained south slope, is Olympic silty-clay loam. The top 8 inches of soil, slightly acid (pH 5.4), have 1/2 inch or less of humus. Its organic matter and nitrogen content are 4.71 and 0.14 percent, respectively. In the past, the test site has been sprayed with herbicides to control brush.

The area typically has dry summers and rainy winters, a generally mild climate which favors growth of wood-destroying organisms throughout the year. During the past 92 years through 1984, annual precipitation averaged 42 inches, 81 percent of which fell from October through March when average monthly temperatures ranged from 39° to 53°F. Only 3 percent fell during July and August when temperatures averaged 66°F. Occasionally the temperature falls below freezing or rises above 85°F. Afternoon breezes from the Pacific Ocean cool the area almost daily during summer months.

Since 1949, various causes of deterioration of the posts at the test site have been identified. Decay-producing fungi or fungi in combination with termites do the most damage. Discarded wings of damp-wood termites have been found at bases of some posts, and entry holes have been detected at or below ground line. However, termites alone have been the primary cause of failure in only a few instances. Carpenter ants and wood-boring beetles also contribute to the deterioration.

It was found the average service life of square redwood posts, was 21 years. This would not meet with even the lowest Risk Category of the IBC (International Building Code), in which the design for snow, wind and seismic events is based upon a once in 25 year occurrence.

These excerpts from the 2012 IBC, specifically address the issue in Chapter 18:

1807.3 Embedded posts and poles. 
Designs to resist both axial and lateral loads employing posts or poles as columns embedded in earth or in concrete footings in earth shall be in accordance with Sections 1807.3.1 through 1807.3.3.

1807.3.1 Limitations. 
The design procedures outlined in this section are subject to the following limitations:

  1. The frictional resistance for structural walls and slabs on silts and clays shall be limited to one-half of the normal force imposed on the soil by the weight of the footing or slab.
  2. Posts embedded in earth shall not be used to provide lateral support for structural or nonstructural materials such as plaster, masonry or concrete unless bracing is provided that develops the limited deflection required.Wood poles shall be treated in accordance with AWPA U1 for sawn timber posts (Commodity Specification A, Use Category 4B) and for round timber posts (Commodity Specification B, Use Category 4B).

As far as strength characteristics, Redwood posts and timbers have a Fb (some discussions about Fb are available here: https://www.hansenpolebuildings.com/2014/08/lumber-bending/) for #2 grade open grain of 750 psi (pounds per square inch), whereas Hem-Fir (the most commonly used western wood for post frame building pressure preservative treated timbers) is only 575 before downward adjustment because it must be incised (what is incising: https://www.hansenpolebuildings.com/2014/08/incising/).

In the end, the design solution for using redwood for building columns would be to mount them into brackets (https://www.hansenpolebuildings.com/2012/09/concrete-brackets-2/) which would prevent them from coming into contact with the ground.

So the ultimate answer is that yes, redwood columns can be used in post frame construction. However, they need to be mounted into brackets above ground rather than into holes in the ground as with most pole building construction, or they will rot.

This entry was posted in Columns, Pole Barn Questions, Building Department, Pole Barn Structure and tagged , , , , , on by .

Pole Barn Holes

The Hole Enchilada

Yesterday I began hacking away at my neighbor’s new pole building under construction. Today, I will dig even deeper (pun intended)!

Leroy and his building crew arrived on Thursday to begin building. My bride and I had to take a detour most of the day to go to Fargo to visit grandchildren and assist with cutting a set of stairs for a deck for our son, so we missed some of “the action”. Early action should have included setting up batter boards and stringlines so as to make sure the building holes ended up in the right locations, and that the building was square and columns were in straight lines.

Surprisingly, there was no evidence these steps were followed, read why this is important: https://www.hansenpolebuildings.com/2014/12/setting-pole-barn-posts/

In case you, dear reader, would like to know, I did venture over late in the day and introduced myself to Leroy and found he had been constructing pole barns for the past 43 years. We had some interesting discussion, mostly me asking polite questions and then quietly enjoying the answers while avoiding making snarky comments. After all, I was there to LEARN.

Pole FootingI found it strange – no posts were set on the front endwall of the building until after the balance of the building had been pretty much framed up. When those pole barn holes were dug, I watched as one of the crew members climbed into an overlarge hole to place a post. The hole could not possibly have been three feet deep! So much for frost issues. If you remember from yesterday – the frost depth here is 5 feet. They were only about 2’ short.

I had inquired of Leroy as to what sort of footing they used, thinking they would probably be using concrete cookies. https://www.hansenpolebuildings.com/2012/08/hurl-yourconcrete-cookies/

Well, good news and bad news in the footing department. No cookies (yaay), instead Leroy had his men throwing two bags of non-mixed Sakrete® in the bottom of each hole. Yes, you got that right – dry Sakrete in hopes ground moisture will eventually turn it solid. You can read why this does not work here: https://www.hansenpolebuildings.com/2012/11/concrete/

For those of you who are unfamiliar with the soils of this part of the United States – let me say here in SD is some of the blackest topsoil I have ever experienced. Right here along the lake, it appears to have been created from ancient lake bottom. Great for growing crops, like corn, but not so great for holding up a building. But being generous, we will assume it will support 2000 pounds per square foot. On the building across the street, this means footings of at least 18 inches in diameter. Perfectly formed and properly mixed, if those bags of premix were 80 pound bags, they would have adequately formed a six inch thick footing.  Six inches – 18 inches – it’s close!  (Not).

I asked Leroy what he was doing to keep the posts from lifting out of the ground. His solution is to drive a piece of 3/8 inch diameter rebar through the posts and call it good, although he did admit to having seen more than a few of posts treated this way pulled entirely out of the ground with the rebar bent right over!

Here are a few of my thoughts: https://www.hansenpolebuildings.com/2012/02/concrete-collars/

Come back tomorrow for another segment of the Lake Traverse pole building Saga….

This entry was posted in Concrete Cookie, Columns, Concrete and tagged , , , , on by .

Setting Pole Building Posts

We Don’t Always Do Things Perfectly, But We Do Listen

Last summer Hansen Pole Buildings Supplied a pole building kit package to a client who experienced a few challenges and I took the time to address each one of them. This is Part I of a four part response.

First – here is my initial response (same day as received from client) to his initial complaint:

“Mr. xxxxx ~

Thank you very much for taking the time to discuss your issues. Our building kit packages are not only the resulting product of the 16,000+ buildings we have been directly involved in, but also the 100,000+ buildings which have been constructed by builders we have done business with, and (most importantly) valuable feedback from clients just like you.

I will personally be reviewing each of your concerns and responding to them within the next several days. This may very well result in changes to some of our processes, both internally, in our Construction Manual, and with our vendors. We do take all input very seriously.

Please feel free to address any other technical or design issues or concerns directly to me at this email address.

Best regards ~ Mike Momb, Technical Director
Hansen Buildings Technical Support Department”

Here is the email I was responding to:
“On the design flaws, and other issues, here is what I have experienced.

1-The concrete footing on the pole building posts

Standard practice in this area is to have an 8-10 inch concrete pad and the post to be 48″ in the ground.
The concrete collar created some interesting problems.  It is very difficult to place the poles when the bottom of the post is not in contact with anything.
Typically a post is ordered over height and dropped into the hole.  The excess is then cut off.  Suspending the post above the ground was such a time consuming task.  It required each post to be shot with a transit to make sure that it was within the allowable height variation. This took some time.  I know that you recommend leveling the site before the operation gets under way but in my case I had an 18″ drop from front to back and it was not feasible to do the rock work before the posts went into the ground.  It also took a bunch of back breaking work to lift those posts out of the ground and get them suspended and then nailed into supports.
The pendulum effect is very noticeable and a small amount of movement in the bottom of the hole makes a huge difference at the top.  When the concrete gets pumped into the hole it comes in with some force and there was a heck of a time trying to make sure that the concrete did not displace the post at all. (Yes, the posts were staked into the ground.  3 2X6’s for each post, one on the ground, and 2 vertical supports).
Around here the ground water is fairly close to the surface and when digging a hole 36″+deep ground water is going to seep in. There is no way to tell encasement depth when there is water in the hole when the concrete is being pumped in.  I sprayed the posts with Orange paint at the collar height but it was useless.  Once the concrete started flowing you could see nothing as the 4-6 inches of water clouded.  We tried checking it with measuring sticks but could not be sure of the depth.  We ended up just over filling them to make sure there was enough concrete in the hole.
Overall, I think the concrete collar is an unneeded step that does not really aid in the construction process or stability of the building.  If you really believe that it is necessary you can achieve the same thing by putting a couple of sticks of rebar in when pad is poured and gain connection with the rest of the concrete this way.”

My response: I can see how the 18 inch grade change posed a great deal of challenge for you.

Setting the poles is a snapThe Hansen Pole Buildings Construction Manual does address this issue in Chapter 2: “Grade change is ideally checked before placing building order, however this is not often feasible as a practical matter. If grade has not been checked before order placement, do so within 24 hours. Longer posts are far more economical when provided with original lumber delivery.”

Longer pole building posts would have eliminated the delay caused by having to shoot each post in with a transit.

This would also have allowed the column depth to be set as per the installation instructions in Chapter 5, which would have entirely eliminated the “bunch of back breaking work to lift those posts out of the ground and get them suspended and then nailed into supports.”

I’ve personally built more than several buildings – with columns set of top of footings, placed to bottom of the holes and suspended. I frankly like the ease of moving columns into place afforded by them being suspended. When adequately braced, movement (in my experience) has not been an issue.  As recently as this past summer a new self-storage unit was constructed on the Hansen Buildings property by the two owners and myself. We set 125 poles as suspended, and experienced no problems with them shifting, with them adequately braced. Our ground, however, was graded to “level” prior to starting, which was key.

The thickness of the concrete collar is merely the minimum requirement. There is no structural issue with having more concrete in the holes than the minimum.

Structurally the concrete collar makes all of the difference in resisting uplift forces. You can read more here: https://www.hansenpolebuildings.com/blog/2012/02/concrete-collars/

Also read why concrete cookies are not the answer:

https://www.hansenpolebuildings.com/blog/2014/03/concrete-cookies/

Come back tomorrow for part II in my response to this client’s letter…regarding his carport attachment to the main building. Mike the Pole Barn Guru

This entry was posted in Columns, Pole Barn Questions, Pole Barn Structure, Concrete and tagged , , , on by .

Digging Holes for a Pole Barn

If you didn’t read my blog yesterday – it might help you to back up a day and read where the following blog got started. I nice young gentleman asked me to help him with his boy scout Eagle project – constructing an equestrian barn. So back up a day – then continue here after you get measurements taken and batter boards set up….

Digging Holes

Temporarily remove string lines. If building in an area requiring inspections, call your building inspector to schedule a hole inspection.

This is important! Get off on the right foot with building inspectors. Call for all required inspections!

post holeConfirm hole diameter from building plan. While usually 18-or 24-inch diameter, verify from building plans.

Building holes may be made larger in diameter or greater in depth (provided posts are long enough) without adversely affecting building structure. Digging holes which are too small in diameter, or not to depth shown on building plans, could cause a myriad of future structural issues – or even a building failure.

Why would smaller diameter holes be an issue? The building weight, including a “loaded to failure” roof load, must be adequately distributed to soil beneath the concrete around columns. Hole diameters specified on building plans include a sufficient area to resist settling, given stated soil strength. Avoid the temptation to use concrete “cookies” placed beneath columns, as they also do not offer enough surface area to resist settling.

To help prevent frost “heave”, dig holes so width at top is less than width at bottom. This can be done by “belling” out hole bottom with a shovel.

Augering HolesUsing an auger mounted on a skid steer, bore holes to depth required on building plans. Holes slightly larger in diameter than auger bit can be created by first digging a pilot hole then offsetting auger slightly from hole center and boring again.

NOTE: High water tables or water in holes will not cause premature decay of pressure preservative treated columns. Treatment is for structural in ground use, which includes being exposed to ground water.

Helpful hint – an auger will NOT remove any rocks larger than half the auger bit diameter.

Pole TrenchIn cases where two adjacent posts will be located in close proximity to each other, the two holes may resemble a short “trench”. This is acceptable.

Holes maybe dug larger in diameter than what is shown on building plans, as well as oblong or rectangular. Dimensions stated on plans are “minimum” requirements.

Do not “over dig” holes! If holes are too deep, extra concrete will be needed and concrete is expensive fill! A visible marker, placed on the auger bit at required depth, is often helpful. If large rocks are present, dig holes with a backhoe, mini- excavator or other similar equipment.

Extend hole depth below area frost line. If unsure about frost depth, ask the local building inspector.

After digging holes, clean any loose material from hole bottoms.

Setting building columns into “sonotubes” or other forms is not recommended. Use will lower the friction coefficient which is created by the concrete encasement cast against native soil. This may adversely affect building performance (or longevity).

Hansen Buildings’ engineers also do not recommend concrete “cookie” placement or pouring concrete “punch pads” at hole bottoms, beneath columns. With column holes properly backfilled with poured concrete, both “cookies” or “punch pads” are usually both structurally inadequate and a needless expense.

There you go – the first big step is done – and you are ready to position columns in the holes. Once again – this may be the time to call your building inspector for a hole inspection.

Good Luck and Happy Digging Holes!

This entry was posted in Pole Building How To Guides, Columns and tagged , , , on by .

Setting Pole Barn Posts

DEAR POLE BARN GURU: What is your method for spacing the poles on a pole barn, how do you line up the locations of each pole on a wall? This is for my boy scout Eagle project, a 32x36ft pole barn to be used for equestrian therapy. ANXIOUS IN AUSTIN

DEAR ANXIOUS: The best advice I can give you is found in the Hansen Pole Buildings’ Construction Manual, which I excerpt from on setting pole barn posts:

The building layout establishes exact reference lines and elevations. Care in layout makes construction easier and helps keep building square.

Building width and length are from corner post outside to corner post outside!

After all framing has been installed, finished framework will normally be 3” wider and longer than ordered or “call out” dimensions. Not paying attention to this will likely result in more effort during construction.

To start, stake out a “base” line string. This will become either the building front or side. If trying to align the new building with an existing structure, roadway or property lines, have the first wall line parallel to the reference point.

See Figure 1 below.

Figure 1

Figure 1

Locate and set front corner stake “A” along baseline. Drive a nail partially into stake top as a reference point. See Figure 2.

Figure 2

Figure 2

Hook a tape measure on nail at Stake A. Measure building length along base line from Stake A and set corner Stake B. See Figure 3.

Use a construction level (transit) and drive Stake B in so Stake A and B tops are level. Drive a nail partially into Stake B top at exact building length (as measured from column outside to column outside).

Figure 3

Squaring a building

Next make endwall perpendicular to sidewall. Measure 12 feet along base line from Stake A and set a temporary stake. The intersection point 20 feet from this temporary stake and 16’ from Stake A is perpendicular to base line. Set a second temporary stake at this point. (Figure 3)

Measure outside building width along this line and set Stake D. Drive Stake D into ground…level with Stake A and B tops. Drive a nail partially into Stake D top at exact outside building width. (Figure 3)

From nail in Stake D top, measure outside building length. From nail in Stake B, measure outside building width. At the two measurement intersection, drive last corner Stake C, with top level with earlier three corner stake tops. As before, partially drive a nail into Stake C top, at exact outside corner point. (Figure 3)

Before proceeding, make certain all four corner stakes tops are level. Then double check, in this order – baseline length (A to B), Width B-C and A-D and then length C-D. Adjust nails or stakes B, C, or D as needed.

Diagonals AC and BD are to be equal for a rectangular building. Adjust by shifting C and D along rear wall line.

Do NOT move A or B.

Keep widths B-C and A-D equal. Recheck any shifted stake levels.

Drive batter board stakes 8 to 12 feet from all corners. If batter board materials are not provided with your building kit, girts make excellent batter boards, as long as they are not cut or otherwise damaged. The batter boards provide a level reference plane for building layout. Place so as not to interfere with excavation, pre-mix deliveries or construction and remain undisturbed until columns are backfilled.

Level and fasten batter boards to stakes at same heights as corner stake tops.

Stretch building string lines between batter boards, barely touching nails on corner stake tops. Partially drive nails into batter board tops to line up string lines.

The temporary and corner stakes can now be removed. Corners will be located where lines cross.

Photo above shows corner column in hole with batter boards in place.

Measuring along building lines, use small temporary stakes or nails painted with fluorescent paint to mark each post location center. Remember to locate the post center, ½ post thickness inside string lines. (Example: 5-1/2” post, post center is 2-3/4” inside string lines.)

See Figure 4.

Setting a Post

Figure 4

Figure 4 shows post centers as compared to “outside” building line.

After post centers have been located, offset (move) building line strings 1-1/2” (skirt board width), from post face outsides.

Why offset string lines? While this may sound confusing, not offsetting string lines could result in finished walls which are not straight, due to posts inadvertently touching lines. We’ve seen professional builders make this error far too often, and in this case, an ounce of prevention, is worth a pound of cure.

Once offset, building string lines will now measure 3” greater in dimension than building width and length (post outside to post outside).

Measure in from building string line 1-1/2 inches to set each post. Rather than having to use a tape measure each time, a 2×4 or 2×6 scrap block (which is 1-1/2” in thickness) can be placed between post and string line.

Now that you have the holes marked –it’s time to start digging holes!

Come back tomorrow for pointers on how to dig single and “multi-column”holes.

This entry was posted in Columns, Pole Building How To Guides and tagged , , on by .

Are the Poles Close Enough?

Welcome to Ask the Pole Barn Guru – where you can ask questions about building topics, with answers posted on Mondays.  With many questions to answer, please be patient to watch for yours to come up on a future Monday segment.  If you want a quick answer, please be sure to answer with a “reply-able” email address.

Email all questions to: PoleBarnGuru@HansenPoleBuildings.com

DEAR POLE BARN GURU:How close is close enough for pole placement? After setting and leveling poles to the string, the poles on one side of the barn are 1/2″ off (lengthwise) from the other side. Is this close enough? KARMIC IN KANSAS CITY

DEAR KARMIC: There actually exists a document entitled, “Accepted Practices for Post-Frame Building Construction: Framing Tolerances”. In the document, in Section 6.4: “Wall length. In rectangular buildings, the overall length of opposing walls should not differ by more than 2.0 inches.”

In my humble opinion “only” two inches would be a HUGE difference. Variations such as this need to be hidden somewhere and two inches would be huge.

In your particular case, if the poles are merely placed in the holes and braced, I would recommend adjusting a corner column to get equal overall lengths.

If the columns have been set in concrete, it is best to then make the overall dimensions at the roofline correct. This will make squaring up the roof to install roofing far easier. In the event this circumstance is the choice, when it comes time to do the siding, plumb the corner(s) which are most likely to be noticed.

On the out-of-plumb corners, the edge of the corner trim will not align with the steel ribs (there will be a ½ inch variation from top to bottom). Most people will never see it – but putting it on the least viewed corner reduces the probability.

DEAR POLE BARN GURU: Hi: How do I install fiberglass batts of R 19 in my walls of pole barn without touching the metal walls? Thanks. ART IN ALBION

DEAR ART: The easiest way would be to install a quality housewrap over the outside of the wall girts and under the wall steel before siding it.

In the event your pole building has been sided, there really is not a negative effect in the event the fiberglass happens to be in contact with the wall steel. It IS essential to have a vapor barrier on the inside of the insulation which provides a total seal. If the vapor barrier is not completely sealed moisture will escape into the wall cavity, and be trapped by the steel siding. When the siding is cold enough, condensation will form, saturating the fiberglass and reducing its efficiency.

You may want to read more on climate controlled pole buildings at:

https://www.hansenpolebuildings.com/blog/2012/04/climate-controlled/

DEAR POLE BARN GURU:What about putting the concrete up to the slab level?

CONCRETING IN CANTON

DEAR CONCRETING: I will assume your question is in regards to backfilling the columns. If so, there is no documented negative reason (lots of old wives’ tales) to not fill the holes entirely with concrete – other than cost (concrete can become expensive backfill). It will make your building very resistant to uplift forces.

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Building Official Out on a Limb

Don’t get me wrong – in case I have not previously gotten my message across in earlier articles – I genuinely like Building Officials.

Most of them really do care, and go out of their way to help both do it yourselfers and building contractors. With very few exceptions, Building Officials, field inspectors and plans checkers are not registered design professionals (architects or engineers). It is when structural concerns arise, which can get them in over their heads. Going too far out on a limb without support can be dangerous.

I was contacted by one of our clients over the weekend, who is putting up his own building. The site is in the western United States, and in his particular case the design wind speed requirement is for 105 mph (miles per hour) with an Exposure C for wind (a site which is not protected from the wind). In the west, the lumber species of choice for treated timbers is generally Hem-Fir. It takes a pressure treatment well (albeit with the need to be incised) and is fairly plentiful.

Building Column OrientationThe building plans call out for the sidewall (double truss bearing) columns to be 6×8, with the six inch face oriented towards the sides of the building.

Columns in pole buildings must be able to support combined forces from bending (the wind) and compression (weight of the roof and any applicable snow loads). In most cases, the bending forces are going to be the majority of the issue.

The reason for the contact from the client was – he placed his posts with the wrong side towards the wind!

Now in construction, things happen…..it is solving the things which keep it interesting. And in pole buildings, I can’t think of a time a challenge didn’t have one or more solutions.

The client was told by his building official, “normally a 6×6 is sufficient” and he would sign off on the posts the wrong way, as long as we provided a letter stating so.

Our proprietary pole building design software gives the calculations for every component of the building. In this scenario, the columns needed to resist a ground line moment of 37,184 inch-pounds (for more on “moments” please read: https://www.hansenpolebuildings.com/blog/2012/09/bending-moment/).

Oriented properly, the 6×8 has a section modulus (Sm) of 51.563 inches. Sm is derived by squaring the depth of a member, multiplying by the width of the member, and then dividing by six. The calculations showed the columns, as designed, to be stressed to 98.4% of capacity.

Turn the column the wrong way, and the Sm is reduced to 37.82 inches and it is now overstressed by 34.2%!! Same timber. Wrong direction. Wrong answer.

BTW (by the way) – the Building Official’s 6×6 scenario would be overstressed by 83%!!

The client does have some options:

The building official could sign off on the columns as placed, making him now the designer of record (I would not recommend he do so).

In cases such as this, building officials could agree to allow the building to be designed for a lesser wind speed and exposure. At 90 mph and Exposure B, the columns would work in the orientation they were placed.

The columns could be pulled out of the ground and placed the correct direction.

Or, we could come up with a repair – which could prove to be difficult and costly. It will depend mostly upon how far along the construction process is.

Moral of the story, the direction any column is to be placed in a pole building is specified on the plans. It is not a suggestion. There are valid structural reasons it must be oriented exactly as designed. This is one place where a mistake is not so easily corrected, as the columns are literally set in stone (concrete).

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Pressure Treated Posts: When Future Building Owners Think They are Engineers

When I was just a little tyke, my Mother used to watch Art Linkletter’s “House Party”. A highlight of his show was the segment, “Kids Say the Darndest Things”.

I have my own version, “Potential Building Owners Say the Darndest Things”.

Our Building Designer Lauri seems to be a magnet for these lately. Here she shares another one….

“Speaking of the poles. I don’t want to sound like a picky little prig or an officious jerk but I do have a requirement about the bottoms of the poles. I don’t know what your standards are for the poles, but I do not want the bottom of the poles sitting in a concrete cup.

When the holes are dug for the poles I would like them to be dug about a foot deeper than needed. Then the bottom of the hole can be filled with at least 6 inches of cracked limestone or 2B river wash stone.

The poles are then set on the stone and an additional 3 or 4 inches of stone are added to the holes. Then set your cardboard casing and fill it with concrete. This method allows for moisture to drain out of the bottom of the pole thus reducing wood rot. If the poles are set deep enough I think you can get away with out using concrete, that is your call, but if you do use concrete the bottoms of the poles have to be able to drain. That is carved in stone.”

Ah, where to even begin?

Let’s talk about pressure treated posts. Rotting fungi need water to work. Other conditions necessary for wood rot to develop include a supply of oxygen and temperatures between 32 and 90 degrees Fahrenheit. (Decay stops below 35 degrees and above 100 degrees.) Wood becomes susceptible to rot if its moisture content exceeds 25 to 30 percent. Keep wood dry, it will literally last for centuries. On the other extreme, if the wood is 100 percent saturated with water, the decay fungi won’t get the oxygen they need. Decay won’t occur, and the wood can last for centuries.

Nowadays building columns are pressure preservative treated for structural in ground use. They are designed to last lifetimes, under the most adverse conditions. If the soil at the site drains well, there is no reason to place rock or gravel beneath or under a column to “drain water away”. If the soil at the site does retain water, any gravel in or around the base actually becomes a collection basin for water, rather than allowing water to drain “off”.  In a quick summary, this customer came up with a poor investment in rock and the labor to dig deeper holes. Plus, this person added chance of the building settling due to inadequate compaction of the stone.

Other previous blogs have covered topics such as the lifespan of pressure treated posts, the need for concrete column backfill to resist settlement, uplift and over turning, as well as why not to use “cardboard casing” (better known as sonotubes).

While I appreciate customers who are concerned about the longevity of their buildings, it is best to leave the structural design of buildings to the experts – registered design professionals (RDP’s).  A RDP (or P.E., i.e. Professional Engineer) has not only the complete educational preparation, but also the decades of experience needed to combine proper structural design and efficiency of materials.

This entry was posted in Constructing a Pole Building, Pole Barn Structure and tagged , , , , , on by .

Pole Barn Post Spacing Revisited

Pole Barn Post Spacing Revisited

By far, my most read blog has been on, “Pole Barn Truss Spacing”. With nearly 50% more reads than any other blog I have written, it clearly is a fan favorite. I’ve had it referenced by clients, building contractors and code officials.

So when one of our clients wrote: “After talking with the building inspector he does not like the idea of post spacing 12ft apart with double rafters. Can you quote a standard setup so I can compare apple to apples, post 8ft on center with double 2×12 header and trusses every 4ft, no joist hangers needed for perlins?”, I felt obligated to answer.

(For clarification of the above, “double rafters” are “double trusses”.)

My response: While the building inspector may not “like the idea” of using post spacing 12 feet on center, it is not only a tried and true method, but it is also one which our engineers recognize as being structurally superior and will engineer seal. We have thousands of buildings in all 50 states, done with the exact same design. It affords the benefits of fewer holes to dig, fewer pieces to handle and install, engineered connections and the reliability of doubled trusses.

Given the correct loading criteria and an engineered building, we will guarantee the ability to obtain a structural permit from our plans. The post spacing every 8′, single trusses every 4′ resting upon headers is a system our engineers are not interested in risking their careers on. In the event of a single truss failure, this system will result in a domino effect and the collapse of the entire roof system.

After the winter ice storm of ’96-’97, I spent quite a bit of time studying roofs that collapsed -some of which were on my own buildings!  At that time, we were using two trusses on a pole, so we had this part “right”.   But instead of putting the two trusses together to act as a single unit, we put one on each side of the column with blocking in between.  The trusses were notched in, so this was another part we were doing well – transferring loads into the ground.  But the one part we missed, was putting the two trusses together.  Lumber is after all, a tree…with inherent knots and defects.  With the huge loading of ice that year…the truss “weak spots” gave way – and if one truss fails, it pulls the rest of the roof down.  This is when I asked an engineer to evaluate the truss system I was using as well, and he concluded the “probability of a second truss adjacent to the first one having the same exact ‘weak spot’ – just could not be calculated.”

From then on, I started using double trusses, nailed together according to a specific nailing pattern (supplied on all our plans).  Since then we’ve had other winters with similar ice/snow loadings, and…no more failed roofs!  I had the engineer’s word, but even better – I had real proof from thousands of buildings which survived nature’s “test”.  We’ve been using the double interior trusses ever since.  Only one roof I’ve seen fail since then…and it was determined to have been due to the two trusses were not nailed together adequately – they had few nails holding them together.   Again – the trusses acted as “singles” and pulled part of a roof down.

We get the requests for abnormal (for us) truss spacings or post spacing every once in awhile. Instead, I prefer to turn it around and stress our advantages –

(1) Fewer holes to dig, digging is always the worst part, and the one which is outside of anyone’s control. If they hit a Smart Car sized rock on the next to last hole, are they going to move the whole building?

(2) Fewer posts to set, trusses to raise, purlins and girts to handle. The real advantage of pole building construction is having the least number of pieces, in order to do the job structurally. By using slightly larger pieces (generally 2×6 instead of 2×4, where it takes only 50% more wood, to be 246% stronger) we are being material efficient.

(3) Wider sidewall door openings without the need for structural headers. In the event someone wants to add a door or window at a later date, they have far more flexibility to do so.

(4) Does anyone REALLY want to stand on a 2×4 roof purlin 16, 20 or more feet up in the air? When the 2×4 purlin snaps, it is a long drop to the ground.

(5) Most building collapses come from connection failures. In our case the purlins connect to the trusses with engineered steel hangers (not just nails); the double trusses bear directly on the posts (not on the sides of the posts, or nailed onto a header). The load is transferred down into the ground and is not dependent upon nails to hold the entire weight of the roof.

(6) If you think about it, if you have a 48’ long building, with single trusses every 8’, you have a total of 5 interior trusses.  If you have double trusses every 12’, you have a total of six interior trusses, but only have to dig 3 holes instead of 5. Which would you rather do?

By the way – our client DID get his building permit issued, using our plans, by the very same inspector who originally “did not like the idea”.

This entry was posted in Pole Barn Structure, Pole Building Comparisons and tagged , , , on by .
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