Tag Archives: concrete foundations

Ignorance is Bliss and Sometimes Architects are Happy

Ignorance is Bliss and Sometimes Architects are Happy

Portions of this article (in italics) are from “County explores options for new Highway building” April 29, 2019 by Nathan Bowe at www.dl-online.com

A city plow truck goes by the main shop building at the Becker County Highway Department complex in Detroit Lakes. www.dl-online.com File photo

Dear Architect friends ~ I didn’t learn much in architecture school, however one nugget was, “It is all about presentation”. Before you need to give a presentation including a possible post frame building, please discuss it with me, or at least read a few of my pertaining articles. I want you to come across as being as knowledgeable as possible.

“Hoping to save money on a new Becker County Highway Department facility, made of precast concrete and estimated to cost about $8 million, commissioners are exploring other types of buildings.

They are considering options including precast concrete, steel, and pole barn, and will tour facilities in the area made of those materials.

The firm working on the project, Oertel Architects of St. Paul, said in a report that any type of material could essentially be made to work, but a pole barn-type building would have to include steel in places to support a 5-ton crane in the maintenance area, for example.”

Post frame (pole barn) buildings can easily be designed to support a 5-ton crane: https://www.hansenpolebuildings.com/2013/07/overhead-crane/

“A less-expensive pole barn building also comes with a much shorter projected lifespan, and generally brings more problems with leaks and maintenance, unless a better grade of roof is used.”

Post frame buildings are permanent structures easily capable of generations of useful lifespan. Properly installed steel roofing will last decades without leaks or needs for maintenance.

“A pole barn is considered an agricultural type building in the industry, and is also referred to as timber frame. This is essentially like building a structure like an old-fashioned barn, with large timber columns and frames. It is typically made without a perimeter foundation. The wood frame structure is typically covered with a metal skin and the low-gable roof type is typically of metal. Its lifespan is projected at 15-30 years, depending on maintenance and other factors.”

Post frame and timber frame buildings are totally different animals. Post frame buildings have been used commercially longer than I have been in this industry (nearly 40 years). Very few buildings provided by Hansen Pole Buildings would be termed as being purely agricultural – nearly all are residential or commercial.  Isolated columns embedded below frost depth preclude needs for expensive and inefficient continuous concrete foundations. (Check out foundation costs here: https://www.hansenpolebuildings.com/2011/10/buildings-why-not-stick-frame-construction/). Most typically post frame buildings have 4/12 roof slopes (rather than “low” as in all steel buildings).

Amazingly, it appears my now 15 year-old million-dollar post frame home is due to expire any time now (like Windows 7)! In reality a properly engineer designed and constructed post frame building will outlive any of us who are reading this article.

“One way to meet the highway department needs and still meet code using pole barn construction would be to build three or four separate buildings, or build one building at different heights for vehicle maintenance, vehicle washing/storage, and office space, Oertel reported.”

Post frame buildings can be easily designed with a multitude of different wall/ceiling heights.

“Pole barns tend to be less energy efficient over time.”

As post frame buildings use exact same insulations as other similar construction types, if this is true it would be applicable across all construction spectrums. Post frame lends itself well to creation of deep insulation cavities and is far easier to insulate than all steel or precast concrete.

“Structural steel works better in a public works facility, with more salt and moisture in the air than usual, since these are made of heavy steel, just like a steel bridge. It is the less substantive metal materials that are a concern. A pole barn uses thin steel gusset plates and there is not much material to last over time if corrosion is present. Metal panels commonly used in pole barn buildings are also easily marred or dented by heavy duty operations.”

In highly corrosive atmospheres, steel can be isolated from corrosion (as in galvanized steel “gusset plates” used to connect roof truss members). Any type of siding – or even precast concrete or masonry, can be damaged by careless operations. Use of strategically placed bollards (https://www.hansenpolebuildings.com/2017/05/lifesaving-bollard/) can eliminate possibilities of significant damages.

“However it’s constructed, the new public works building will need the same mechanical, plumbing and electrical systems, floor loading, earthwork and mechanical systems, Oertel said. Costs can vary, but all of that might add up to perhaps 60 percent of construction costs, with the actual building structural shell 20 to 25 percent of the total project cost. So cost savings from a cheaper type of building might not be all that commissioners might hope for, compared to the long-term drawbacks.

“More could be said about the differences between pole barn construction and a more heavy duty construction using precast concrete,” the report sums up. “It mostly comes down to a lower front-end cost with a pole barn, at the sacrifice of longevity…””

Post frame construction is going to provide a greater value, without being “cheap”. Post frame buildings will have a usable lifespan as great as any other permanent building.

And – have you ever tried to remodel a precast concrete building?

Concrete: Why Not Bag It?

Rachel, one of the Hansen Pole Buildings Designers recently took a call from a client who was disappointed because we did not provide the concrete for their new pole building kit package.

Most people are familiar with Sakcrete® readi-mix concrete. There is probably not a lumberyard in America which does not have it available for sale.

Sakrete® offers a variety of concrete mixes for most any application including creating countertops, setting fence posts, anchoring poles, pouring slabs, walkways or footings.

In addition to the general purpose High Strength Concrete Mix, Sakrete® offers a number of specialty concrete mixes including 5000 Plus Concrete Mix. 5000 Plus is ideal for structural applications which demand high compressive strengths such as slabs which will be subjected to heavy loads, commercial traffic or warehouses subjected to forklift traffic..

In a hurry? Choose Sakrete® Fast Setting Concrete Mix. This product combines the convenience of high strength concrete mix with a set time of just 30 minutes. Simply pour the product in the hole, add water and install posts. Since this is a fast setting concrete mix, a pole building can be framed the same day posts are installed.

Sakrete® offers Maximizer, a high-yield concrete ideal for reducing the number of bags needed to complete a job. An 80 lb. bag yields 60 percent more volume than any other standard 80 lb. concrete mix. A job which would require 40 bags of regular concrete would only require 24 bags of Maximizer.

Most familiar, as well as most available is the Sakrete® general purpose High Strength Concrete Mix. When mixed per the manufacturer’s instructions, this mix affords a compressive strength of 4,000 psi (pounds per square inch) at 28 days.

The instructions are: Empty the contents into a mortar box, wheelbarrow, or mechanical mixer. When mixing by hand, form a crater for adding water.  Add water a little at a time.  Avoid a soupy mix.  Excess water reduces strength and durability and can cause cracking. A 60 lb. bag should be mixed with three quarts of water, an 80 lb. bag four quarts.

Now the realities of using bagged concrete for post frame building footings….

It is not unusual to have concrete encasements of 24 inches or larger in diameter and 18 inches or more in depth, in order to prevent building settling and uplift issues. One hole this size would take 4.71 cubic feet, or about 700 lbs. of concrete! Even a very small building with 18 inches of diameter and depth takes 2.65 cubic feet or about 400 lbs. of concrete.

With either 60 or 80 lb. bags, it is going to take a lot of bags! An average building could easily have 20 posts, and if looking at 700 lbs. of concrete per post, we are talking about 7 TONS of concrete (3-1/2 yards).

Ignoring the huge number of bags involved, there are some other realities.

Ever looked at the pallets of readi-mix bags at the lumberyard? Take a peak, next time. Notice how many of them are broken or leaking.

Due to weight, it may very well mean another delivery and another delivery charge. Trucks do not run for free.

Bags can (and will) break when being handled during delivery, unloading and being moved around the jobsite. It is going to happen, just plan on it.

From experience, lots of projects are not begun immediately after delivery. It is not unusual for delays of weeks, or even months before actual construction begins. Improperly stored, bags can get wet or absorb moisture and become solid before time for use. This equals a total waste of money, other than the chunks of concrete make for solid backfill.

Then there are the builders who insist upon throwing the entire bag (usually including the bag) into the hole. Their idea is ground water will cause the readi-mix to harden. Why does this not resemble the manufacturer’s instructions?

Readi-mix must be mixed thoroughly and evenly. How does mixing over 200- 60 lb. bags of Sackrete® by hand sound? Add too much water (three quarts exactly per 60 lb. sack) and the strength is reduced.

Use too much? As holes are always perfectly round (not), it is going to happen.

Save time, effort and money. Often all three can be saved by having the local pre-mix concrete company deliver concrete for holes (even if a “short load” fee is charged), as opposed to mixing on site.

And to think, the client sounded frustrated and disappointed we did not provide the concrete.

Concrete Brackets

Am sure yesterday’s blog posting on moments left a few folks scratching their heads. There actually was a method to my madness, as it leads into today’s topic.

Many may have seen various column bases at their local lumberyard or big box store. Manufactured by either Simpson or USP (similar to Simpson CB66), these products have some serious, and perhaps fatal consequences if used to support posts in a pole building.

Why?

These bases are not designed to resist overturning (moment) loads. Now this poses a challenge – how to connect post frame building columns, to a concrete foundation?

There is a solution…..

building bracketsSturdi-Wall concrete brackets are a heavy duty anchor system designed to connect post frame structures to traditional concrete foundations such as: monolithic slabs, formed walls, and existing concrete pads.

There are two types of Sturdi-Wall concrete brackets. When drill setting is preferred, the standard Sturdi-Wall is used. When setting into wet concrete, the Sturdi-Wall Plus is used.

One challenge, not directly mentioned by the product manufacturer, is the standard Sturdi-Wall concrete bracket is not designed for moment loads. Post frame buildings want to overturn; they do induce moment loads, which leads me to discourage people from using the standard bracket for this application.

Wet set installation with Sturdi-Wall Plus brackets provides the highest ultimate strength connection to a foundation, but requires being installed while the concrete is still wet. This technique avoids time consuming drilling with a masonry bit and expensive concrete anchors. Sturdi-Wall Plus concrete brackets require less concrete coverage than normal Sturdi-Walls, allowing them to work well in pier foundations.

A pier foundation would be one where isolated holes are augered or dug, then poured full of concrete.

For cases where, for whatever reason, it is desired to NOT place pressure preservative columns into the ground to support a pole building, the Sturdi-Wall Plus concrete bracket provides an solution which is capable of resisting uplift, shear and moment forces.

See, it was worth struggling through yesterdays’ blog on “bending moment” to make you “in the know” for today!

Concrete Collars Keep Columns in the Ground

Wind forces acting on relatively lightweight buildings can apply significant uplift forces to the foundation system. Post-frame (pole) buildings are relatively light buildings which typically feature several large wall openings. When I ask folks what they think the greatest pressure on their building is, invariably they answer with either “snow” or “wind trying to push the building over”.  But the true biggest factor in design is what I refer to as “sucking pressure”.  Stand outside holding an umbrella and imagine each of 3 different forces: 1. Wind trying to push the umbrella over; 2. Snow or raining pushing down; or 3. Wind trying to suck the umbrella right out of your hands making you fly like Mary Poppins.  Yes, Mary wins! Ensuring embedded post foundations can adequately handle uplift forces is fundamental to post-frame building design.

The resistance of an embedded post is typically increased by attaching something (referred to as an anchor) to the post at or near its base. Common anchor materials include wood, precast concrete and cast-in-place concrete. The method of anchor attachment to the post depends on the type of anchor.

Although several different anchoring arrangements exist for increasing the uplift resistance of embedded post foundations, performance data has only been published on a few of them.

This makes it difficult to assess the relative strengths of the many different designs which are used by practicing engineers.

A common practice over the past quarter century has been to increase uplift resistance by placing a reinforcing rod or rods (commonly referred to as rebar) or nails near the post base, and encasing the post base with dry concrete mix. This has been done under the assumption the dry mix will absorb moisture, after placed in the hole, producing a solid concrete collar.

Research, under a controlled situation, was done by a noted industry research professor to determine the relative uplift resistance of a number of commonly used post foundation anchorage systems. A secondary objective was to determine to what extent, if any, dry concrete mix will absorb moisture, when in place, to form a concrete collar.

In the testing a total of 45 post foundations were loaded to failure in withdrawal, and the following conclusions were found.

Inexpensive wood anchor systems can significantly increase uplift resistance. Uplift resistance increased 4200 lbf (pounds per foot) with the addition of two 2 x 4 blocks 10 inches long. This increase far exceeded the published design capacity of the wood block-to-post connections (nails). In several cases, these connections limited foundation uplift resistance. Not part of the report was the safety factor included in the published nail values, which would explain the test results approaching four times the numbers which could be calculated.

Prehydrated concrete collars (160 pounds or roughly a cubic foot of wet concrete poured around the base of the columns) with 19-inch diameters provided uplift resistances in excess of 22,000 lbf when embedded to a depth of 50 inches (over 5 times the resistance of the wood anchor blocks). A cubic foot of concrete in a 19-inch diameter hole will be about six inches thick. The recognized bond strength of concrete to wood is 30 pounds per square inch (Wood Technology in the Design of Structures by Hoyle and Woeste), as such, the predicted strength of the connecting bond between the concrete and the 4.5” x 5.5” wood column was only 3600 pounds. The Hansen Pole Buildings engineers prefer to use the more conservative recognized bond strength values in their designs.

Collar-to-post connection strength was enhanced when curing time for prehydrated collars was extended from 2 to 30 weeks. This enhancement was attributed to soil consolidation resulting from freeze-thaw cycles and/or snow piling which significantly increased uplift resistance. Our engineers have determined for post hole compaction above the concrete collar you must place compactible granular fill, free of clays or organic material in maximum six inch lifts. Each lift should achieve a 2000 pounds per square foot (psf) compaction, using a hand operated 4”x4” x8” post. The post is to be raised four feet or more above the compacting surface and dropped four or more times on each four inch square. The proof of compaction is when the butt end of a 2×4 will not penetrate the compacted material over 1/8” under 170 pounds of weight.

Significant self-hydration of dry-mix concrete collars occurred during the embedment periods. However the test results did not point out the core samples removed from these concrete collars had an average compressive strength of less than one-half of the pre-hydrated collars!

Using several small fasteners (in this case eight 60d ring-shank nails) in place of one or two larger fasteners (12” long #4 rebar) is more beneficial in attaching collars to posts where lower-strength concrete is used to form collars.

While wood anchors or dry-mix concrete collars may be a design solution (depending upon particular building characteristics and wind loads), wet-poured concrete collars afford a significant increase in resistance as compared to wood anchors. Wet-poured collars also exhibit far greater compressive strengths than dry-mix.  Bottom line is: concrete poured around the bottom of a post gives you more resistance against uplift than any other method.