Tag Archives: Uplift

Uplift Concerns, Retro-fitting Insulation, and High Water Tables

This week the Pole Barn Guru answers questions about uplift concerns, retro-fitting insulation, and setting posts in high water tables.

DEAR POLE BARN GURU: I am adding an open overhang to the gable end of my 40×80 pole barn. I bought 36’ trusses and will lower them to fit under the existing gable going out 16’. Since the underside is unfinished should I worry about wind uplift? SCOTT in SHERIDAN

DEAR SCOTT: Regardless of whether your addition will be open, enclosed, or somewhere in between uplift should always be a consideration. Ideally the original EOR (Engineer of Record) who designed your 40′ x 80′ building would be consulted, not only because of a potential uplift issue, but also due to what affect open carport will have upon existing building. You may be placing wind shear issues upon endwall closest to addition and these will need to be addressed, as well as if endwall column footings will be adequate to support added weight (not just dead load, but also potential weight of a snowfall). An issue of drifting needs to be reviewed also, as snow could build up upon carport roof against existing endwall.

If original EOR proves unavailable (or nonexistent), you should enlist services of a competent RDP (Registered Engineer or Architect) qualified to review your existing building, as well as your intended work.

DEAR POLE BARN GURU: Need your suggestions on any improvements I can make. Below is a sketch of my current building roof insulation. Basically I tried to seal off the cathedral ceiling using the bottom of the purlins to hang 4” Dow board sealed with aluminum tape. I did purchase the radiant reflective barrier that was installed per instructions between the roof purlins and the steel roof panels. I live in Michigan. 

I love my two pole buildings I purchased from you guys. Just need some help on insulating the roof on my last building. Nothing is wrong. Just concerned about the un-vented dead air space. JOHN in SAGINAW

 

DEAR JOHN: Always glad to hear back from happy building owners! As long as the cavity was dry when it was sealed up, and the Dow board is sufficiently well sealed there is a chance you will not have problems. The only guaranteed solutions involve having to take down the insulation board and make provision for airflow from eave intake to ridge exhaust by drilling holes through all of the purlins.

 

 

DEAR POLE BARN GURU: Mike can you please tell me what is the proper way to set posts in holes that have water. High water table. Thanks much I enjoy your blog. PATRICK

DEAR PATRICK: Back when I was a contractor we would run into this situation occasionally. Our solution then was to stand columns in holes, brace them and then backfill with pre-mix concrete with very little water in it. Concrete weight would displace water in hole. It did take a significant amount of concrete, however it was only about $30 a yard then.

How I would do it now – I would use sonotubes equal to or larger in diameter than what was specified by engineer. Cut tubes to depth of hole length, then cover one end of tube with six ml black visqueen (read about visqueen history here: https://www.hansenpolebuildings.com/2013/07/moisture-barrier/ ) sealed tightly around sides of tube. Place tube in hole with covered end down (this will take some work, as tube will want to float like a boat). Once sonotube has been placed, backfill around outside with compactable material – compacting no more than every six inches. Then stand column in tube, brace it and backfill with pre-mix concrete as engineer specified.

 

What Size Posts Should I Use?

What Size Posts Does My Building Need and How Deep Should They Be?

Reader ANONYMOUS in BENTON writes:

“1. If my building has 16 posts and posts are 12 feet apart do I need 4×6’s or 6×6’s?
2. If the plan shows 16 feet above grade how much do I need underground?
3. If the posts are set 12 feet apart will two 2x12s glued together support a truss system with a total length of 36 feet?”

Mike the Pole Barn Guru responds:

While I appreciate your questions, we as a company and me as an individual do not provide free engineering services. In answer to your questions:

1) Without knowing the full dimensions of your building, including roof slope and overhangs (if any), if the columns will be adequately tied into a concrete floor, as well as your site’s snow load, design wind speed and exposure, seismic zone as well as the dead loads which will be carried by your building there is no possible way for me or any RDP (Registered Design Professional – engineer or architect) to be able to answer this.

2) The depth of the columns into the ground should be shown on your engineered building plans (you do mention you have plans). At a minimum the holes should be no less than 40″ deep and must extend below the frost line. Ultimately the depth and diameter will need to be determined by the RDP in consideration of the factors listed in (1) above, as well as designing for the ability to adequately prevent uplift.

3) Since I would use double trusses which bear directly upon the columns, there would be no need to use any other type of dimensional lumber to provide headers for a truss system. Again, this is where your RDP can design to adequately provide an engineered system to support the trusses.

My best advice, since I am guessing you are somewhat floundering in this, is to invest in a fully engineered post frame building kit package which includes plans sealed by a RDP and designed specifically for your building, at your site. It just isn’t worth trying to avoid the small expense into a proper design – especially when the lives of the occupants depend upon it.

Here are some other articles which pertain directly to this subject and should be read: https://www.hansenpolebuildings.com/2014/12/free-pole-barn-plans/ and https://www.hansenpolebuildings.com/2017/11/dont-engineering-fool/

 

 

 

 

Soil Bearing Pressures Challenge

Soil Bearing Pressures Challenge

Fast and Loose With Numbers and Terms
DAN from MOUNDS VIEW is the first reader who admits they are an addict to my blogs – for this, I love you man!! Luckily there is no 12 step program to cure you, so you are just going to have to keep feeding the addiction.
Dan called me on the carpet, so I will bring out the Roomba® to clean things up.
Dan writes:
“This message is about an answer you gave to a guy 1-25-18, who asked for an analysis of the post load of his building. And I agree with your answer that he should get a pro to review the design but, why are you playing fast and loose with the numbers to scare him??

From the data given you came up with 8860 lbs/post, which is OK, but then you didn’t tell him that it results in just 292.9 PSI at the bottom of each center post, which is well under the 2100 psi given soil load. Instead you converted the 292 psi to 42,177 lbs/ft, which technically is correct, just to scare him. However, the post is only 0.21 ft^2, so the max load is still the 8860 lbs.

The real concern is ”is the 2100 psi” soil rating correct and what is he going to do about the uplift forces? And I agree he needs a footer, but your ”raise the post and pour” can easily do that.

I sent this message this way to keep it off line.

You have so much good stuff in the blog that you don’t need ”tricks” to get the message across.

Dan who’s addicted to reading your blog.”

Mike the Pole Barn Guru responds:
Dan, Dan, Dan – I’ll post just about anything which isn’t profane online, so no problems, we are still friends!

Here is the challenge – soil bearing pressures are to be expressed as pounds per square FOOT (psf), not pounds per square inch (psi) the difference being a factor of 144.
As an example – from the 2015 IBC (International Building Code)

https://codes.iccsafe.org/public/document/IBC2015/chapter-18-soils-and-foundations TABLE 1806.2 the vertical bearing pressure (capacity) of Crystalline bedrock is 12,000 psf or 83.33 psi. The probability of anyone having 2100 psi (302,400 psf) soils would be just below the chances of the Cleveland Browns winning the Super Bowl this year.

Fall Up, Go Boom

Fall Up, Go Boom

What? Sir Isaac Newton pretty much confirmed things do not fall up.

Well, this building did not actually “fall” up – it was sucked out of the ground. How would I know this? Look at the ends of the columns which are lying on the ground. There is no concrete attached to the bottom of the columns, nor is any method for preventing uplift even obvious to the more than casual observer.

 

In review of the NFBA (National Frame Building Association) Post-Frame Building Design Manual (January 2015) the issue of column uplift is all but ignored. Beginning with the end of Page 5-37, it is concluded two pages later. Options for preventing uplift are really not addressed.

For decades we, if not many other post frame designers and builders, have relied upon the bond strength between concrete and wood in designing column embedment to prevent uplift issues. More can be read about concrete to wood bond strength here: https://www.hansenpolebuildings.com/2013/04/pole-barn-post-in-concrete/.

I’ve expounded previously upon the use of nail on truss plates for assisting in uplift construction (https://www.hansenpolebuildings.com/2013/04/truss-plates-for-column-uplift/).

There truly is very little information available. Of all places, I did find some relevant information on the City of Hendersonville, Tennessee website (www.hvilletn.org):

Column uplift protection: Columns shall have uplift protection by one of the following methods:

1. Two 2x6x12 inch column uplift protection blocks attached to each side of the base of the column. The column uplift protection blocks must be placed horizontally, attached per Table 5 and comply with Section R317.

2. 12 inch high, concrete collar poured on top of footing around the post, with 2- #5×9 inch rebar placed through the post at 3 inches and 9 inches from bottom of post in opposite directions. The rebar ends shall be installed in accordance with ACI 332 for the specified distance in inches from contact with the soil.”

Table 5 mentioned above happens to be five 16d hot dipped galvanized nails into each block.

While I was researching for this article, I happened upon an example for preventing uplift in an all steel building. The building in this case was a 60 foot span and steel frames every 25 feet. In this case the design footing was eight feet square by 3’8” in depth!!

The all steel building is going to have footings which take nearly nine yards of concrete per bearing location!! This is near the capacity of a pre-mix concrete truck, per one end of each frame!

Getting back to the post frame building design solution, our engineers have determined reliance upon the concrete to wood bond strength only is not quite as conservative as they might like.

The solution – Hansen Pole Buildings, LLC engineered post frame buildings now have added the nail on uplift plate tot the roof supporting columns to tie into the concrete column encasement.

The investment is minimal and it does afford some added insurance of success in preventing uplift.