Tag Archives: concrete bottom collar

You Have Questions

You Have Questions, We Have Answers

Long time readers are aware I will answer any questions – even when they become highly technical.

Loyal reader CORY in NEBRASKA will be a fantastic client, because he wants to know how everything works, in advance – leaving nothing to chance.

About Hansen BuildingsHe writes (in italics):

“I would really like to have clear space of 12’ 6” from top of concrete to bottom of bottom truss chord. According to Zach’s calculations (and mine) with the way I had planned only there is only 12’ 3”.   With the 18 foot columns , if the frost line is 39” and I now set posts 42” down in concrete  (instead of 48 “), pour 5” concrete floor and truss has a heel height of 16”+/- that would yield a clear distance of 12’ 7”. Check my work: 18’ x 12 = 216 “, 216 – 16″ heel = 200 – 5″ concrete = 195 – 42″ sitting depth = 153” of 12’ 7” clear distance. Do you or your engineers think the 42” depth will be a detriment re loading?   If 48” buried is preferred, what is actual length of 18’  3 ply post from bottom to notch? If that measurement is less than 18’ ( meaning a post of 18’ is measured from bottom to top of outside plies, can one safely add a 4″ or  6” block into notch with adhesive and secure with either screws or nails through all 3 plies  and then remove outside ply to set double truss?  (Mike the Pole Barn Guru says: Good try but would not get engineer’s seal probably! )”

Mike the Pole Barn Guru answers:

Typically with a 39″ frost line, holes would be dug to 40″ and you would hold posts up 8″ from bottom of hole – to create a mono-poured bottom collar. This puts 32″ of column at below grade. Top of slab (regardless of thickness) will be 3-1/2″ above grade. 12’6″ from top of slab to bottom of truss means we have used up only 15′ 5-1/2″ of columns. Throw in a 16″ heel and you have used  16′ 9-1/2″ of an 18′ column.

“With a double truss on a 3 ply column, cutting away 2 plies to set truss into notch would increase bending moment to notch side, would it not? Also only one ply to hold truss vertical under side stress. I realize that purloins add stability once in place. On 10’ purloins  isn’t blocking (or doubling …..expensive) a good idea to eliminate flex out of purloin and keep all straight for steel installation?”

Our engineers check every column for eccentricity (truss loaded to notch in from one side) and only with a tremendously huge snow load and say a 3 ply truss notched into a four ply column, would this even begin to become an area of possible structural concern. Your roof acts as a long deep beam, prohibiting lateral movement of truss heels away from columns. Easiest method to keep purlins straight is to pre-drill roof panels and then adjust purlins towards or away from ridge so purlins fall in line with holes. It is quite simple to do and avoids having to add other members or blocking.

“I sent two plans to Zach , same building but roof peak at 90 degrees to each other. Zach has sent a sketch of the one ( east / west roof peak ) with the gable end facing the West , (  prevailing westerly wind as well as storms ) I believe that 40 foot width frontal height would be just under 21 feet with 14’ sidewalls and 4:12 pitch roof with 12” overhang. I thought that the North / South roof peak with longer 50 foot eave side with 24” overhangs and 14 ‘ sidewall and might have less wind load. Am I wrong? Is the 24” overhang a greater uplift hazard?”

Assuming eave heights, roof slopes and footprints are the same, 50′ span roof does add 20″ of overall roof height, however you only have 40′ of overall length to deal with. Endwall shear on the 40′ truss version is 3287.36#, 50′ truss version is 2962.8#. Overhang lengths are factored into the truss/column connections and frankly are such a small proportion of your total roof area so as to be near inconsequential.

“I used to work in the power line industry and to overcome forces from tensioning lines or angles, etc., the glulam poles we set had 4″ angle iron “ears” bolted vertically the bottom 6 or 8 feet to maintain plumb. Would adding 2” x 2” x 8” pieces of angle iron horizontally about halfway down the buried portion void your warranty? I feel it would counter uplift better than the pieces you sell.”

Our uplift plates are approved and have published validated numbers for uplift resistance. Should you desire to add angle irons ears, it would not void any warranty provided appropriate measures are taken to seal any column penetrations, although there is really no structural need for them.

“One-Pour Post Frame Concrete Footings

“One-pour” Post Frame Concrete Footings and Bottom Collars

As originally engineered Hansen Pole Buildings’ column encasement design, had pressure preservative columns placed to the bottom of an augured hole. Pre-mix concrete was then poured around each column’s lower 16-18 inches to form a bottom collar. Concrete to wood’s bond strength was sufficient to enable this assembly to resist both gravitational forces (settling) as well as uplift.

For further reading on concrete to wood bond strength: https://www.hansenpolebuildings.com/blog/2013/04/pole-barn-post-in-concrete/

There were, however, a few Building Officials who just could not wrap their heads around this as a design solution – they wanted to see concrete underneath columns. Prescriptive Building Codes do mandate for a minimum six inch thick concrete footing below bearing walls and load supporting columns, contributing to this effect.
Reader DENNIS in SALT LAKE CITY triggered this article as he writes:

“I see that you are a proponent of monolithic concrete pours around the vertical posts for your buildings. You have suggested a basket as one way to raise the post 8″ for the footing space. Since I don’t wish to purchase the baskets, how do you recommend suspending the posts at the correct level so all the post tops are level with each other and a monolithic pour can be accomplished?”

Mike the Pole Barn Guru writes:

In response to our friendly Building Officials, Plans’ Examiners and Field Inspectors, we had previously flicked switches on our creative light bulbs to arrive at an engineered solution – we changed our design so column bases “float” eight inches above the bottom of holes, prior to concrete being poured.

Unlike my caricature, levitation is not involved in this process what-so-ever. By nailing a “to be used later in construction” framing member temporarily across each column, at appropriate depth, it makes for a relatively easy design solution.
Once building columns are set in place, it allows for premix concrete to be poured in one simple step both under the column base as well as up sides to create a bottom collar.

This, and all other construction tips and procedures are fully outlined in Hansen Pole Buildings’ roughly 500 hundred page Construction Manual, furnished both electronically and as a hard copy with every new building.