Tag Archives: truss spacing

Ag Exemptions, Truss Spacing, and Concrete Vapor Barriers

This week the Pole Barn Guru discusses ag exemptions for building permits, the effect of spacing trusses at 12′ or more, and concrete vapor barriers.

DEAR POLE BARN GURU: Do I have to have a permit to build one poll barn on Ag land? DANIEL in PIERSON

Building PermitDEAR DANIEL: Many jurisdictions nationwide exempt true agricultural buildings, on agriculturally zoned land from building permits. A practice I disagree with entirely – as it places these buildings at risk of failure due to under design of critical structural components.

Please read more here: https://www.hansenpolebuildings.com/2011/12/exempt-agricultural-buildings/

To find out if you would need a permit, or not, is going to take a phone call from you to your local Building Department and asking them.

 

DEAR POLE BARN GURU: Guess this is where to ask questions? We are planning a pole building 40X72 and would like to space the Trusses at 12ft or more? I see you say no problem but what would it take for this? Is it heavier trusses or heavier purlins? Just not sure the requirements for more spacing. Thanks! BRIAN in WARRENSBURG

DEAR BRIAN: You have come to the right place. Changes in truss and column spacing impact more than just having “heavier” trusses. Your entire building structure should be reviewed and sealed by a Registered Professional Engineer to properly incorporate all applicable loads for your site. Just a few possibly affected areas are column footings, column depth and diameter, amount of concrete around base of columns, uplift prevention, wall girts, roof purlins, truss bracing….just to begin with.

Each set of building dimensions and loading condition can have their own best design solution from both economic and functionality aspects. Hansen Pole Buildings’ Instant Pricing system™ allows for nearly instantaneous pricing of various truss spacings – down to fractions of inches!

Please read more about post frame (pole) building truss spacing here: https://www.hansenpolebuildings.com/2011/06/pole-barn-truss-spacing/.

 

DEAR POLE BARN GURU: Does the vapor barrier under the concrete slab of a pole barn need to cover the poles and splash boards at the perimeter of the concrete? Or do I just lay the vapor barrier on the ground and not up the sides? I am using 10 mil Stego. Thanks for any help! JARROD

DEAR JARROD: You should extend vapor barrier up columns and to top of splash planks.

Information on Stego™ vapor barriers can be found here: https://www.stegoindustries.com/stego-wrap-vapor-barriers

 

 

Is the Double Truss System Stable for the Midwest?

Is a Double Truss System Stable for the Midwest?

Reader SHARON in NORTH DAKOTA writes:

“Dear Pole Barn Guru,

I have attached some pictures of a 62×96 pole barn with 12ft sidewalls. I am rather ignorant about truss systems, but this one looks atypical to others I have seen. What type of truss system is this, and is it stable for the midwest? How are your truss systems different? 

Thank you for your time and knowledge.”

Mike the Pole Barn Guru responds:

What you are looking at is a “double truss” system, where two roof trusses are physically joined side-by-side with the use of mechanical connectors (most often nails). It is absolutely stable for anywhere in the world. The most common Hansen Pole Building design utilizes the double truss system, typically with sidewall columns spaced at 10, 12 or even wider column spacing depending upon applied wind and snow loads as well as door locations. In the case of the photos you have sent, the roof purlins were placed over the tops of the trusses and staggered every other bay, this precludes the ability to pre-drill the roof steel, which would have minimized (or eliminated) the possibility of a roof leak caused by a misplaced screw. The Hansen Pole Buildings’ double truss system utilizes engineered steel connectors to attach the roof purlins to the sides of the roof truss top chords, as opposed to merely attempting to adequately nail through purlins to the tops of the trusses. The superior holding power of this connection resists wind uplift forces which could otherwise tear a roof off and send it swirling off like Dorothy’s home in The Wizard of Oz.

The most popular article I have written is on truss spacing – you can read it here: https://www.hansenpolebuildings.com/2011/06/pole-barn-truss-spacing/.

A caution – in the photos you have supplied, the trusses have knee braces, which could lead to a collapse if not engineered and accounted for within the truss design. More on knee braces here: https://www.hansenpolebuildings.com/2012/01/post-frame-construction-knee-braces/

The benefits of double truss systems vs. single truss systems include:

1- Fewer holes to dig for truss bearings – with columns every 12 feet, it reduces holes by 1/3rd.

2- Fewer columns to have to set.


3- Reduces total number of boards and trusses having to be handled and installed by as much as 50%.

4- Eliminates possibility of the one single weakest truss failing and pulling the balance of the roof down behind it.

5- Reduces the need for lateral bracing – a properly connected together double truss is twice as stiff in resisting buckling in the weak direction.

 

Can 2×4 Roof Purlins Span 12 Feet?

Can 2×4 Roof Purlins Span 12 Feet?

Reader DAVE in MICHIGAN writes: Hi, I saw on your webpage the Pole Barn Guru stated the trusses could be spaced 12’ apart (I called and was told it is a double truss one on each side of a post that is on 12’ centers).  That is exactly what I have, double trusses on each side of a post on 12’ centers.

My question is can I use 2” X 4” on edge spaced 16” or 12” apart?  I intend to have a metal roof on top.

Thanks for your help!

nailing trussesDEAR DAVE: My preferred method of post frame construction actually places the two trusses face-to-face nailed together and notched into one side of the column. In this fashion they truly act as a two ply member. Spacing them on each side of the column causes the trusses to work independent from each other and takes away the advantages of the true double truss system (load sharing and minimization of truss bracing).

Could one use 2×4 roof purlins in this system?

Let’s do some math and find out. (You can brush up on bending moments here: https://www.hansenpolebuildings.com/2012/09/bending-moment/)

Here is the formula for calculation of a roof purlin:

[(COS of Live load + dead load) X (COS of the purlin spacing) X (purlin span squared)] / [8 X (Sm = Section modulus of the member) X (Duration of Load = 1.15 in areas with snowfall) X (Cr repetitive member factor of 1.15 where members are 24″ on center or less)] = Fb (Fiberstress) required

The COS of the Live Load is due to us only having to check for bending about the strong axis of the member as the purlin is restrained in bending in the weak axis direction by the roof steel.

The COS of the purlin spacing is because the load on the purlins is vertical and the purlin spacing is with the run of the roof.

For sake of discussion we will assume a minimal roof snow load of 20 psf (pounds per square foot) and a 4/12 roof slope.

Dead load will be the actual weight of the roof steel and the roof purlins.

[(20 psf X .949) + 1.5 psf) X (12″ X .949) X (11.625′)^2] / [8 X 3.0625 X 1.15 X 1.15] = 972.75 psi

HemFir has a base Fb of 850 psi multiplied by the size factor of 1.5 = 1275 psi

In bending, 2×4 #2 purlins would work at 12″ on center, however I personally would not want to walk on top of them. Without even running the calculations, I would say there is a good chance the 2×4 purlins will not make the deflection criteria required for a roof framing member supporting steel.

Rather than having 2×4 purlins every 12″ it would make far more sense economically to use 2×6 purlins every 24″. Less expensive (1/3rd less board footage of lumber), fewer pieces to handle, fewer joist hangers to have to attach and only one half as many screws to attach the roof steel.

Sharing the Pole Barn Blame

Sharing the Blame

Welcome to 2017!

As you may recall, 2016 ended with me sharing an email from a builder who is constructing a new Hansen Pole Building and may possibly be a legend in his own mind.

Our company policy, when a challenge arrives, has always been to begin by looking to see what, if anything did we do wrong. In this particular case, we (and yours truly) share in some of the blame.

For you, gentle reader, I will paint a picture of the building in question, so you may get a better feel for the entire process.

The building is a 40 foot clearspan in width, 100 feet long with an eave height of 16 feet and five inches. It is designed under the 8th edition of the Massachusetts State Building Code, with a 90 mph (mile per hour) design wind speed and a 50 psf (pounds per square foot) design flat roof snow load.

It features 12 inch enclosed overhangs on all four sides, as well as three 14 foot wide by 14 foot tall overhead doors on one sidewall.

The most practical design solution actually (which is a rare case) turned out to be based upon the traditional “East coast” style of post frame construction, with a single truss spaced every four feet on top of “truss carriers” (beams) spanning sidewall columns generally every eight feet (other than at the overhead door locations).

This building happens to be narrow in width in relationship to length (1 to 2.5 ratio) and is fairly tall. As such, the wind load was great enough to exceed the shear resisting capacity of the steel roofing in the eight feet closest to each endwall.

In order to carry the load, the building was designed so the trusses in the affected areas would have a traditional ¼ inch butt cut (educate yourself on what a butt cut is here: https://www.hansenpolebuildings.com/2015/05/truss-butt-cuts/), while the balance of the trusses would have 11/16 inch butt cuts. This would allow for the top of all truss carriers to be placed at the same height, and 7/16” OSB (Oriented Strand Board – https://www.hansenpolebuildings.com/2013/10/osb-versus-plywood/) to be installed on top of the lower heel height trusses.

Pretty darn skippy sounding ……. Until we get to tomorrow!!

Yep – yet another cliff hanger!!

Let’s Talk Snow Load

Even though it is now May – the heavy snowfalls in the New England states this past winter have left many with concerns. Here is an actual conversation, between a client who recently invested in a new Hansen Pole Building kit package, and our Technical Support Department:

Client: Good Afternoon,

Quick couple of questions.  The plans look like the wall girts for this project are the “commercial wall girt” design (Option 1)? 

I was initially under the impression that Option 1 was just for the design where the “Roof Purlins /joist are hung vertically from the side of the Trusses”.  I did not know that Option 1 also included a commercial wall girt design. 

Is it necessary to use the commercial wall girt design if I have Option 1:  “Roof Purlins /joist are hung vertically from the side of the Trusses” design? OR could the Wall Girts be just 2 x 4s nailed on the outside of the Support Posts?

Just curious-

After answering these couple of questions, I will log back in and approve the drawings.”

Building Code Snow LoadsNo snow yet, but we will get to it. Here is the response:

Thank you very much for your investment in a new Hansen Pole Building. Every building we provide has each member and connection structurally checked for adequacy under the most stringent Code provisions. Other than for very small column spacing, this means wall girts will need to be placed “book shelf” style, in order to be Code conforming.

Here is some reading which may prove helpful: https://www.hansenpolebuildings.com/blog/2012/03/girts/

Now we will get literally knee deep into the snow:

“The structural support poles on my drawing are at 14 ft.   Not 10 ft, and not 8 ft.  This is my concern.  (I read the guru blog).  14 feet between poles with double trusses, still doesn’t cut it when I could have 5 feet of snow on the roof.  This barn will be located in Northern Maine.”

Thank you for your concerns. Your building has been designed for the loading recommended for your area, 50 psf ground snow load – which you acknowledged as being verified by you as adequate with your Building Department, prior to your order being placed.

If you are planning upon having five feet of snow sit on top of your roof, then we would recommend increasing the snow load capacity of your roof to somewhere in the vicinity of 100 psf (this would equate to a ground snow load of approximately 173 psf). To increase the roof snow load by this 346% would add $xxxx to your investment.

Please advise accordingly.

“After some further research, I have found that the recommended ground snow load for my county in Maine is 90 PSF.  Please advise on new plan design and associated incremental cost to my project to accommodate.”

Just want to confirm you feel this will be adequate for your particular site, as a 90 psf ground snow load will support about 30-32″ of snow on the roof. If indeed you believe a greater depth may be placed upon it, it would behoove to design accordingly.

The liability I am putting on myself here is tremendous.  Does Hansen Pole Barns have any culpability for designing this building for snow load correctly?  Because I don’t know.  I am at a loss, I am not an engineer.  We have little if any Code Enforcement in this county.   But I know we get a boat load of heavy wet snow and the building will be in a sheltered area with not much winter sunlight. 

My builder originally said it was 50 PSF (As he thought that was the code).  But I don’t think knows.   When I looked at the design that I was sent, (and I am a believer in engineering, as I am a non-certified Mechanical Engineer), I know the design is not adequate.  So, I started to research on the internet.  The best I can come up with is what I found on the internet.  SEE ATTACHED

 I am located in New Vineyard ME, Franklin County 04956.  Every town that is near me shown on the attached “Ground Snow Loading” document is listed as “Case Study”, so I am guessing at the 90 PSF. (because everything around me is either at 90 or 100 PSF 

Looking for advice”

Although you may have read this previously, it may prove good background: https://www.hansenpolebuildings.com/blog/2012/02/snow-loads/

Based upon your information, we’d recommend a change in the Ce factor from 1.0 to 1.2. This effectively increases the design roof snow load by 20%

Along with this, here are some Pg options (in psf) to pick from (as well as the investment to increase) and the approximate depth of snow on the roof for each:

90   $ 2252       38″
100      2498       42″
110      2578       46″
120      3169       50″
130      3368       54″

Me personally, I tend to go for over design – I prefer to be the one who owns the last building standing when the storm of the century hits.

Dear Guru: Trusses at 12′ Work?

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:I am looking for more information on roof trusses. I would like to know how double roof trusses at spacing of 8′ or more is as good as or better structurally than 4′ or 2′ spacing. The long spans between joists is something I have not seen before. I don’t need an answer immediately, I’m still planning my pole barn.

You’re web site is great, and has answered many of my questions. Keep up the good work.

Thank you. ONLY IN ORANGE

 

DEAR ONLY: Thank you for your question and for planning things in advance so your new building ends up being ideal for you. There are several facets in the answer to your question.

Looking at the trusses themselves – a single truss spaced every four feet and ½ of a pair of trusses spaced every eight feet are going to be carrying the exact same amount of load.

Now let’s imagine loading the roof beyond the actual design capacity (it does happen when the once in 100 year storm decides to visit). In a single truss system, when the weakest truss fails due to extreme loading – there is a pretty fair chance the rest of the roof is coming down along with it, as each successive single truss becomes overloaded due to the failure of the previous truss.

In a double truss system, each truss is securely attached to its partner. The probability of both of these trusses having the exact same weak point is infinitesimally small. In the event of the storm of the century, chances are very good the double truss system will still be standing, after the single truss system has failed.

With a true double truss system (where the trusses are attached directly face-to-face and not spaced apart by blocking), the truss pair is also twice as stiff against buckling in the weak direction as a single truss. This allows for a significant reduction in the amount of lateral bracing required for bottom chords and web members – more cost efficient and quicker to construct.

The real beauty of the double truss system is when the trusses can bear directly upon the columns – as opposed to a “truss carrier” (aka header). Connections often become the failing point in buildings, so the more they can be reduced in number and complexity, the less chance of a catastrophic failure.

I’ve heard more than one engineer or building inspector complain about the challenge in trying to get the connections specified on the plans for carriers to columns, actually done correctly in the field!

With trusses directly aligned with the columns – there are no truss carriers to deal with. The columns are taking all of the downward load, making the truss to column connection one of merely overcoming uplift.

Double trusses on widely spaced columns (typically 10 to 14 feet apart) with roof purlins on edge between them, also allow for easier application of wider sidewall doors – frequently without the need for structural headers. This becomes especially convenient when doors are added after the initial building construction.

DEAR POLE BARN GURU: I’ve just received delivery of the materials for my new Hansen Pole Building. I notice your lumber provider sent 8×8 pressure preservative treated columns, instead of the 6×8 called out for on the plans. While I appreciate the ‘under promise, over deliver’ my question is this – do I connect the trusses to the columns by notching them in as the plans show, or should the notches be made deeper into the columns. VEXED IN VIRGINIA

DEAR VEXED: I can imagine it looks as if you have been sent the entire tree – certainly they will not being going anywhere in a wind storm!

There are times when our suppliers will provide free upgrades, sometimes without clueing us in even! Although it often does not make a difference structurally or cosmetically – it is always best to check, just in case.

With your particular set of circumstances, it turns out you can install the trusses either way, without any negative structurally effect, or compromising assembly.

Make sure to send photos as you go, and be extra careful with lifting – we don’t want to hear reports of any strained body parts!

Dear Pole Barn Guru: Can I Use Spray Foam Insulation in My Pole Barn?

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: In northern Idaho, just east of Coeur d’Alene, I had a pole barn put up without insulation figuring I would build 2×6 walls between the posts and insulate with batts and then plywood the walls later for a work shop. Can spray foam be used instead? Could I frame 2×4 walls instead of 2×6? Can it be sprayed onto the metal siding and roof without any negative effects showing up later? I will have a heater in there, but probably not on full time. QUIRYING IN COEUR d’ALENE

DEAR QUIRYING: Can you use spray foam? The answer is yes, however it is probably the most costly choice, and the Building Code requires any spray foam to be covered with non-combustible material (e.g. gypsum wallboard).

For the walls, you could frame a non-structural 2×4 studwall, holding it flush to the inside of the columns, and place batt insulation between the studs. The studs do not have to be the same depth as the insulation, and in doing so, you will eliminate a thermal bridge.

Beware, less costly (per inch of thickness, not R value) open cell foams are permeable to moisture – so condensation could become an issue. To obtain an R-19 rating from spray foam, be prepared to spend around $3 to $4 per square foot of insulated area.

While spray foam is relatively light weight, always check with the manufacturer of the roof trusses and the Registered Design Professional (RDP – engineer or architect) who designed your building to verify the weight of the insulation being added will not compromise the structural integrity of the building.

DEAR POLE BARN GURU: I sent in three pole barns that i am looking to get a quote for. my families barn just recently burned down a few days ago due to undetermined causes. We lost our animals in the fire which was devastating. Our pigs and chickens were our livestock and our food. We need to get a barn up and built soon to get our farm running again. I have a few questions about this, first off if i go ahead and purchase this kit how does it get delivered to my house. Second, if this is purchased is the supplies all in one kit that you ship out or is it a list a what we need and i have to get it? please get back asap thank you. NEEDY IN NEW YORK

DEAR NEEDY: I am deeply sorry for your losses. Fire is so devastating.

When you order from us, the materials are delivered to you via truck.

You are purchasing from us the design, the complete 24″ x 36″ blueprints which are specific to your building and show where every board is placed. Not only to we provide all of the materials for construction, but we give you detailed instructions as to how to assemble everything.

DEAR POLE BARN GURU:  What is your standard design practice to accommodate a wider O/C truss?  Double the truss? Or increase the truss member sizes?

VASCILATING IN VERSAILLES

DEAR VASCILATING:  Our most common practice design in general is a system using doubled trusses spaced most commonly every 12′ (although spacings 10′-16′ are also very common). The doubled (or more technically “ganged”) truss system affords some safety not found in single truss systems – as multiple trusses physically connected to each other all for true load sharing. The probability of two or more connected trusses having the exact same weak point is phenomenally low – reducing the risk of a catastrophic failure in an extreme loading situation. Ganged trusses also require less bracing than single trusses, adding to ease of installation, as well as lower costs and a “cleaner” finished appearance.  It also may mean no cumbersome (and expensive) truss carriers.  Lastly, if you put overheads in a sidewall, having the double trusses mean you could put as wide a door as 12’, and have plenty of room to put in a hoist system to lift a vehicle between the sets of trusses.

Vascilating then responded:

Thanks for the quick response. I assume, then, the practice is to utilize 12′ 2×6 purlins placed on edge on top of those trusses?

Is the spacing of 12′ the same for an attic truss? I recently got a quote from Hansen for a gambrel building with which I intend to have attic trusses. Is it common practice to finish a room using the knee walls of those trusses?

Dear Vacilating:

Every client gets my individual and undivided attention for as long as they need to get their questions answered.

The snow load will dictate purlin size, but they will be 2×6 or larger, joist hung between the trusses. Attic trusses will be the same, however may be more than just a 2 ply truss depending upon the span and load. Most typically people will finish those spaces with a knee wall.

Unless you are absolutely married to the gambrel look, the most efficient and cost effective design for multi-story space, is to just design a multi-story building. For about the same cost, you can get full room height from sidewall to sidewall

 

 

Dear Guru: Can My Pole Barn Trusses Handle a Ceiling Load?

DEAR POLE BARN GURU: I have a question regarding truss loads, specifically ceiling loads, for a pole barn.  I know you have touched on this before, but I was hoping for a little more detail.

I recently purchased a property that came with an existing pole barn, and other than a few material ratings I cannot find any data on the trusses (no manufacturer tags, etc).  The building is 32×48, and the trusses 4/12 pitch with 2×6 top cord, and 2×4 bottom cord and webbing, on 4′ centers.  I want to add a steel ceiling, plus insulation and lighting. By my math I am looking at a dead load of 2-3 lb/sq ft, including the weight of the bottom cord of the trusses.

I have done a lot of research on the subject, and it seems that it is common in pole buildings to have trusses spaced much wider than 4′ (8’+ seems common).  Is the tighter spacing a indication that these trusses should support a 2-3 lb/sq ft dead load for ceiling and insulation?  If you were specing trusses for my requirements, how would you design them?  If these trusses are not sufficient, what sort of reinforcement would be required?

One of the only bits of data I have been able to find is this page https://www.pole-barn.info/gable-roof-trusses.html, where toward the bottom it lists a 30′ span with the same lumber sizes as my trusses.  While it says “no ceiling” it also lists a bottom cord dead load of 5 lb/sq ft, which would be plenty for what I have planned. BAFFLED IN BOZEMAN

DEAR BAFFLED: The spacing of the roof trusses has no influence upon their load carrying capacity. In reality, trusses spaced 12 feet on center could easily have a greater ceiling load carrying capacity than ones placed even every two feet!

As a starting point, you should assume the trusses are NOT designed to support dead load weight of anything other than the trusses themselves, required bracing and minimal wiring and lighting.

The size and or grade of the truss chords as well as the webs and their quantity, and the size of the roof truss plates may not be adequate to carry the weight of the ceiling load. It’s not as easy as just knowing the size or spacing of each part, but rather it’s more of how all the parts function together in any configuration.  In other words, it’s a formula with many parts which change the final answer of “yes” or “no”.

The only safe way to make sure your new ceiling doesn’t end up on the floor with the rest of the roof following it, would be to hire an engineer to confirm the trusses are adequate to support the ceiling load, and to design a repair for them, if necessary.

DEAR POLE BARN GURU: We are builders, putting up a Hansen Pole Building currently. It is large enough so the main clearspan building and attached shed had to be shown on two different pages of the blueprints. Anyhow, we made a mistake on setting the poles. Where we are looking at the 4 posts going across up by the “matchline”  we have the left sidewall posts set correctly but the 2 main building posts and the right side shed post are set incorrectly. We ran a string line but the workers ended up setting those 3 posts on the other side of the string line.

I am wondering if there is an easier solution than pulling out the posts. We have a lot of concrete in the holes and it would be hard to get them out. Could we fir out the left sidewall posts and go out and buy longer purlins? ERRANT

DEAR ERRANT: While this doesn’t happen often, you are not the only person to have this problem. In this particular case, you are constructing an engineer certified post frame building, which means any deviation from the plans is going to have to be approved by the engineer. This is going to mean time delays and the expense of paying the engineer.

Even if it was not an engineered building, using purlins for a span which is now 5-1/2 inches greater could overstress the purlins in bending. While 5-1/2 inches may not sound like much, in the design calculations for the purlins, the span is squared.

I’ve had to pull out concreted in columns before, and it isn’t fun. Best method I found was to use a backhoe or loader, wrap a chain around the column and lift it out. Fairly fresh concrete can be chipped away from the column and the process of setting the three columns can begin again.

I’m sorry I don’t have an easier solution, but you will be much happier with the outcome if you do reset the posts. And all in all, it may end up being far less expensive as well, in both time and materials.  As I said, I’ve done this myself, so I am right there with you.  The good news is, once you reset the posts, just knowing you have things all “in order” will make the project run smoothly from here on out

 

 

 

Single Truss System: Purlins Laid Flat

Before any of my readers drop their teeth due to me saying something I don’t exactly extol… is easy….read on!

I’ve spent the last few days in Michigan – where I am being told no Building Official would EVER approve of a pole building designed with double trusses spaced 10 or 12 feet on center and 2×6 or 2×8 purlins on edge. To the contrary, I’ve been involved in the design of about 15,000 buildings designed exactly this way. Throughout the Western United States, the typical post frame construction utilizes the concept of pressure preservative treated columns, spaced 10, 12 or even 14 feet on center, with two ply trusses, aligned with the sidewall columns. Literally hundreds of thousands of buildings have withstood snow loads to upwards of 200 pounds per square foot (psf) and wind speeds far over 100 miles per hours (mph).

The originally patented post frame (pole building) design concept utilized columns spaced every 12 feet. Even the unimpeachable source of all knowledge (Wikipedia) references columns spaced every 12 feet (https://en.wikipedia.org/wiki/Pole_buildings).

In post frame construction, roof purlins are the members running the lengthwise direction of the building, either placed on top of, or between the roof trusses (or rafters), to attach the roof steel or other roof sheathing to.

To the good folks in Michigan, the only way to construct a pole building, is to place the columns every eight feet. Truss carriers (basically headers) are then run from column to column. The trusses are installed on top of the carriers, every four feet.

This four foot truss spacing, allows for 2×4 roof purlins to be placed flat on top of the trusses. These purlins are attached by driving nails through the wide face of the purlin, into the tops of the trusses. It does make for a fairly easy purlin install (other than dimensional lumber typical runs about ¾ of an inch over length, so every purlin must be trimmed), and it gives a big wide surface (3-1/2 inches) to run screws through the steel roofing, into the purlins.

Like any system, there are some downsides – and my opinion is this method has some significant ones, which outweigh the positives.

In comparing to my idea choice of construction, this method requires 50% more holes to be dug. It takes half again more trusses and involves handling about double the number of framing members. Granted, often the framing members are smaller, but handling is still handling.

Structurally, there do exist some issues. The great majority of building failures are from connections – either inadequate, or improperly installed. The fewer connection locations, the less places for a potential error.

The trusses every four feet, columns every eight, results in purlins having to be connected to trusses, trusses to truss carriers, truss carriers to columns. Lost is the direct truss to column connection provided by them being aligned with each other.

In a single truss system, if one truss fails due to extreme loads, the balance of the roof is sucked down behind it. The face-to-face connected double truss system, creates a redundancy where no two adjacent trusses share the identical weak link. This load sharing prevents many would be failures.

With the single truss system, as each truss is only 1-1/2 inches wide, requires double the lateral bracing against buckling of a double truss (three inch wide system). Again, more pieces, more connections, more possible locations for failure.

And one more thing – I mentioned nailing the steel to the purlins on the 3-1/2” face as being an easy target to hit.  However – what about nailing the 2×4 purlins where they come together over a single truss?  You now have two purlins butting into each other, sharing that 1-1/2” space.  The chances of missing the truss beneath, or splitting either the end of the 2×4 and/or the ¾” space on the truss are….pretty high.  Another connection failure.

And back to the Building Official issue – our company has dozens of clients all across the state of Michigan who have been granted Building Permits and are enjoying their buildings with double trusses and wall columns spaced every 12 feet. Never be able to obtain a permit? Never is a big word, and it just is not the case.

Pole Barn Truss Spacing Rerun

Happy 4th of July!

On holidays, I take a day to relax, and “re-run” some of my most highly read blogs.  From over a year ago, today’s subject has been viewed close to 8,000 times.  Yes, that’s 8 thousand. So here you go, for what I consider one of the hottest topics in pole building design: Pole Barn Truss Spacing

What do you mean they aren’t 2 feet apart?

Back in the day (early 1990’s) I was on the National Frame Builders Association (NFBA) Board of Directors. One of my fellow board members from the Midwest wanted to take a peek at how pole barns were constructed in the West, so I invited him out for a tour.

After spending a day looking at several of our building projects, his comment to me was, “The inspectors in our area would never let a pole building be constructed with roof trusses placed every 12 feet”.

Twenty years later, I beg to differ. Hansen Buildings has buildings in each of the 50 states and all of them have roof trusses on what my board member friend would describe as being “widely spaced”.

Framed Pole Barn

Modern truss design is highly computerized. Enter the span of the truss, bay spacing and load conditions and the engineering programs will design a truss spacing which will meet the design criteria. The lumber and steel plates the trusses are constructed from, have no idea how far apart they are going to be placed.  They are inanimate! Yet, somewhere in the deep, dark reaches of history, lies the theory wood trusses must be spaced no more than 24” on center, or maybe 48”, or perhaps even eight or ten feet? The reality is, there is no magic number.

While H. Howard Doane is credited with being the innovator of the modern pole barn, it was his Agricultural Service farm manager, Bernon Perkins, who is credited with refining the evolution of the modern pole building to a long-lasting structure.  It was Perkins who pioneered roof purlins being placed on edge. With this design change, roof trusses could be placed 12 feet apart, making it possible for roofs to support the loads to which they would be subjected.

I’ve had roof truss manufacturers try to convince me it is impossible to place wood trusses at spacings of over every 4 feet. Their defense is, “Our engineers will not allow us to”. The manufacturers of the steel roof truss plates (also referred to as gussets or Gang-nails), provide the engineering design for pre-fabricated wood trusses. Their programs will allow for trusses to be placed on 12 foot or even 16 foot centers, and their engineers will place their engineer’s seal on the drawings to verify.

The practicality, cost effectiveness and ease of construction of pole buildings is based upon efficient use of the fewest amount of materials, to do the most work, within safe engineering design. Hundreds of thousands of pole barns are in use today with truss spacing every 12 feet, or even more. They stand as a tribute to the ingenuity of modern pole building design.

 

Building Design: Best Client Line Ever

Sometimes a client will put forth a statement which says more than anything I could ever write.

Bob, one of the Hansen Pole Buildings Building Designers, was talking on the phone with one of his clients this morning. Bob shared this with me:

“Competitor was trying to tell my client that more posts and trusses are better than our system.  Client wasn’t buying it and told the guy “Listen Pal, I’m not looking for the best price per pound, I’m looking for the best design.””

For the most part, I have never looked upon myself as being a great innovator, when it comes to pole buildings. But, I have always felt I was blessed with the ability to look at how others do their structural building designs and do an impartial analysis of them.

The buildings Hansen Buildings provides today, barely resemble the ones I first designed and sold back in 1980 at Lucas Plywood and Lumber in Salem, OR.

By looking at what other people do which is good, then trying to make those things better (and incorporate those improvements), my firm belief is we have created the best possible value for the dollars invested by our clients.

In the case of the quote above – certainly we could design to place posts at any spacing desired. In most instances, spaced every 12 feet turns out to be the most efficient from engineering vs. cost standpoint. The side benefit is there will be fewer holes to dig. Unless one would happen to be part gopher, most are like me – we hate digging holes. With a passion. In many cases, the “more posts” are smaller in size or lower strength posts…..in which case, what was the point?

More trusses do not a stronger building make. Having spent what seems now like a past life either building, selling, designing or owning in the prefabricated roof truss industry, I do know just a little bit about it. Whether a truss is placed every 24 inches or a pair of them is placed every 12 feet, the trusses are designed for the given snow and wind loads – at the spacing they will be placed at. Connections are an issue, most building failures come from connection failures. The more individual trusses, the more individual truss to bearing support locations, the more the probability of one of those connections being under designed or improperly installed (either of which could result in a catastrophic failure).

At the end of the day, it is truly about the best building design, not the best price per pound.

Pole Barn Truss Spacing

What do you mean they aren’t 2 feet apart?

Back in the day (early 1990’s) I was on the National Frame Builders Association (NFBA) Board of Directors. One of my fellow board members from the Midwest wanted to take a peek at how pole barns were constructed in the West, so I invited him out for a tour.

After spending a day looking at several of our building projects, his comment to me was, “The inspectors in our area would never let a pole building be constructed with roof trusses placed every 12 feet”.

Twenty years later, I beg to differ. Hansen Buildings has buildings in each of the 50 states and all of them have roof trusses on what my board member friend would describe as being “widely spaced”.

Modern truss design is highly computerized. Enter the span of the truss, bay spacing and load conditions and the engineering programs will design a truss which will meet the design criteria. The lumber and steel plates the trusses are constructed from, have no idea how far apart they are going to be placed.  They are inanimate! Yet, somewhere in the deep, dark reaches of history, lies the theory wood trusses must be spaced no more than 24” on center, or maybe 48”, or perhaps even eight or ten feet? The reality is, there is no magic number.

Framed Pole Barn

36′ long garage with 12′ bays

While D. Howard Doane is credited with being the innovator of the modern pole barn, it was his Agricultural Service farm manager, Bernon Perkins, who is credited with refining the evolution of the modern pole building to a long-lasting structure.  It was Perkins who pioneered roof purlins being placed on edge. With this design change, roof trusses could be placed 12 feet apart, making it possible for roofs to support the loads to which they would be subjected.

I’ve had roof truss manufacturers try to convince me it is impossible to place wood trusses at spacings of over every 4 feet. Their defense is, “Our engineers will not allow us to”. The manufacturers of the steel roof truss plates (also referred to as gussets or Gang-nails), provide the engineering design for pre-fabricated wood trusses. Their programs will allow for trusses to be placed on 12 foot or even 16 foot centers, and their engineers will place their engineer’s seal on the drawings to verify.

The practicality, cost effectiveness and ease of construction of pole buildings is based upon efficient use of the fewest amount of materials, to do the most work, within safe engineering design. Hundreds of thousands of pole barns are in use today with trusses spaced every 12 feet, or even more. They stand as a tribute to the ingenuity of modern pole building design.