Tag Archives: geotechnical engineer

Planning a Post Frame Building Upon Sandstone

Planning a Post Frame Building Upon Sandstone

Reader GEORGE in FARMINGTON writes:

“Mike, in the early stages of planning a pole barn 32’x48’x12′ on a newly acquired piece of property. We’ve cleared the land and found the area we want to erect the building is 100% sandstone. Everywhere we’d have to drill for poles is sandstone as the ground in-between so the sandstone would be the base for the slab as well. Drills real easy but I’m getting conflicting stories from concrete contractors regarding the slab. One says we should “de-couple” the pad from the sandstone by spreading a layer of crushed stone between the sandstone and the slab while another says we should drill into the sandstone and epoxy rebar verticals into it then tie that rebar into the slab rebar. Really don’t know how to proceed as both of these guys appear to be authoritative in their opinions. The de-couple guy claims the sandstone could “heave” the slab up and crack it whereas the other guy says by epoxying vertical rebar to the horizontal rebar, we’ll avoid any issues. Help!” 

Mike the Pole Barn Guru responds:

Thank you for reaching out to me. I have never before been faced with building upon sandstone, so I will give you a logical answer and then advice as to what I would do personally.

In regards to our “de-couple” guy’s advice, if the sandstone would heave your slab up on its own, placing a layer of crushed stone between sandstone and slab seemingly would not prevent slab movement in a “heave event”. Rock types such as bedrock, limestone, sandstone, shale and hard chalk have high bearing capacities. These are very strong and good for supporting foundations because of their stability and depth. As long as sandstone is level your slab will be well supported. My question to him would be more of, “What could possibly cause such a heave event?”

From my research, it appears concrete adheres well to sandstone as long as lime is not added to concrete mixture. This seemingly precludes epoxying rebar verticals into it to tie them together.

Now – what I would do. I would search out a Geotechnical Engineer in your area and broach this situation to him or her. This way you get an expert opinion and it should be a reasonably priced solution.

Why Your New Barndominium Should Be Post Frame

Why Your New Barndominium Should Be Post Frame

For those who follow me – you know I am all about people loving their end results. As long as one has a fully engineered building they love, I couldn’t be more pleased, regardless of the structural system.

In My Humble Opinion – fully engineered post frame buildings are your best design solution in nearly every case. My exception would be if one desires a clearspan over 80′ or a very low slope roof – then I would recommend PEMB (red iron).

But, what about stick frame?

Perhaps stick built construction’s biggest advantage is builders and tradespeople are very comfortable working in and around stick framing. All registered architects and most building inspectors are very familiar with stick framing. International Residential Code (IRC) provides a prescriptive ‘cook book’ to follow for adequate structural assembly, within certain limitations. These limitations include, but are not limited to, no story height of greater than 11 feet 7 inches (R301.3), no hurricane prone areas with a design wind speed of 130 mph or greater located south of Virginia, or 140 mph elsewhere (R301.2(5)B), and no ground snow loads over 70 psf (R301.2.3).

IRC802.10.2.1 further limits truss spans to a maximum of 36 feet and building lengths to 60 feet (measured perpendicular to truss span). Trussed roof slopes must be at least 3:12 and no greater than 12:12.

Want a 40’ x 72’ x 14’ shop/house barndominium? Sorry, outside of what IRC allows.

And because IRC tables cover up to a worst case scenario (given maximum design parameters), it creates overkill, wastefulness and redundant framing members. Translated to – it takes more pieces than might be necessary in a fully engineered structural system.

Fully engineered post frame buildings include foundation plans, PEMBs require one to hire another engineer to provide foundation design. Plan upon a grand or more just for PEMB foundation engineering and this can quickly escalate should a Geotechnical Engineer need to perform soils testing.

Post frame does not require precision placed anchor bolts, and requires no use of cutting torches or welding.

Post frame requires minimal concrete to resist settling, overturning and uplift. Any slabs on grade do not need to be thickened or have continuous footings/foundations.

Post frame can be erected D-I-Y without a need for heavy equipment (although I do appreciate a skid steer with an auger to dig holes). Any physically able bodied person, who can and will read step-by-step instructions can successfully erect their own beautiful post frame building.

Post frame has no highly conductive steel frames to thermally isolate and attempt to conceal.

Post frame is easily adaptable to more complex rooflines, and can be done in any combination of alphabet letter shapes. Fully engineered post frame can easily be erected over crawl spaces as well as full, partial or walkout basements.

When properly designed (with bookshelf wall girts), exterior walls are ready for wiring, insulation and interior finishes – no need to frame a home inside of a building shell.

Post frame provides deep wall insulation cavities and with raised heel trusses, any depth of attic insulation can be blown in above a finished ceiling.

Post frame can readily be done multi-story, with up to 40′ sidewalls and three stories (or 50′ and four stories with fire suppression sprinklers).

Rebar for Post Frame Concrete Slabs on Grade

Building with concrete involves many steps to achieve best results, including grading, forming, placing and finishing. One crucial step is placing reinforcing bars (rebar) correctly.

An engineer should do technical design work and provide specific information regarding sizes, configuration and placement of rebar. Slabs-on-grade for post frame buildings do not usually carry building loads, these are usually carried from roof and walls, through building columns to ground. This makes for far less complicated applications, unlike PEMBs (Pre-engineered Metal Buildings) or “weld ups” either being far more convoluted and beyond the skill level of all but experienced professionals.

rebarFor slab reinforcement, necessary rebar can usually be obtained from a big box store (like The Home Depot®) or your local building supply. Should your specific application be more involved, it may behoove having a fabricator supply rebar. A fabrication supplier can review your building’s engineered drawings and produce a shop drawing with details and identifying tags for each type of rebar to be used in your building. For simpler projects, your building plans should provide spacing requirements and bar sizes. Use these documents to determine where and what rebar is needed in individual locations.

Most often rebar is tied with annealed steel wire, either purchased in four pound bulk rolls, or if using a bag tie spinner, in bundles of pre-cut wire pieces with loops formed on both ends. Bulk rolls are easier for novices to use, however are slightly more expensive.

Prior to placing any rebar, grade and properly compact the ground after all grading and any utility rough-ins are completed. Make sure all compaction testing has been completed and you have your geotechnical engineer’s sealed report in hand before moving forward. Any termite pre-treated should be completed, as well as a moisture barrier installed.

As post frame construction places columns and splash planks prior to pouring your slab, this gives you ready made forms for your slab perimeter.

Determine the size of bars to be used in each direction and mark several of them with layout measurements in each direction (front-to-back and side-to-side). Bars can be marked with a soapstone marker, a paint pen, lumber crayon or even spray paint.

This will be an ordinary slab mat concrete, the force interacting with rebar during placement is minimal. As mat movement is unlikely, a simple single twist of wire around each rebar intersection, twisted together tightly will be adequate. This tie can be done easily with a pair of nine inch lineman’s pliers.

To use your pliers to tie these efficiently, pull feeding end from wire reel with your non-dominant hand (for sake of this article, we will call this your left hand, with pardons to lefties). Grip wire end with pliers in your right hand. Push wire behind (under) rebar at an intersection. Angle end towards where you will be grabbing it, reach from this side, grip it again with pliers, pull towards next location pulling enough slack to complete tie. Hold resistance on wire with left hand, so wire bends snugly against bar being wrapped, at each stage. Release wire so pliers can be used to grip it. Pull end around bar and twist two ends together, pulling wire with pliers so tie is tight.

Once mat is assembled it must be held in place so concrete will cover it completely. Rebar chairs or concrete brick are often used for this. Place these positioners close enough together so rebar will not sag enough to reduce desired coverage of concrete – usually about 1/3rd distance from bottom of slab.

Watch rebar configuration as concrete is placed. If shifting occurs, support rebar with a shovel or alter direction of concrete flow so force is applied in the opposite direction.

Use caution when working around exposed rebar. Construction workers have suffered serious injuries or been killed when falling on projecting rebar.

Stilt Post Frame on Permafrost

I have written previously about post frame design involving concrete slabs on grade in areas of permafrost: https://www.hansenpolebuildings.com/2018/04/post-frame-permafrost/. Today we will venture into a land where “stilts” are a design solution.

Permafrost is loosely defined as soil and/or rock remaining frozen for more than two years. Big trees do not guarantee an absence of permafrost; it might just mean permanently frozen ground or ice is down far enough so soils in those spots can support a larger root system. Only way to be certain of what ground contains is to have a soils test drilling done.

With permafrost, a safe bet is to it avoid it altogether and move to another piece of land. This is easier said than done, particularly because of a scarcity of affordable buildable land. If you decide to build on permafrost, be as strategic as possible. Smaller and simpler structures will tend to fare better than larger, more complicated ones.

Minimal site disturbance is an accepted practice. Trees and ground cover are your best friend. They protect and insulate ground from summer’s heat. A great example is green moss you find on many shaded low-level areas. Moss has a high insulating value, and in many cases if you dig down a couple of feet, ground might still be frozen in middle of summer.

Strategies for construction on permafrost include:

  • As a general rule, organic layer of ground cover provides insulation and should not be removed, as this will increase risks of thawing any frozen ground underneath.
  • Elevate and properly insulate bottom of your post frame building to prevent floor system heat losses from reaching ground underneath, leading to thawing.
  • Use a thick gravel pad significantly wider than post frame building itself (also insulated if possible) to stabilize the ground and spread building loads.
  • Embed columns to a depth able to both support the structure and resist frost jacking from seasonal ground movement.
  • Cut trees sparingly to maximize site shading (while permitting for a fire break).
  • Build a wrap-around porch, which will help shade the ground around and underneath your post frame building.
  • Incorporate large roof overhangs to shed water away from building and provide shade.
  • Install gutters and manage site drainage well away from building.
  • Retain a geotechnical engineer familiar with local soil’s conditions to assist in designing a foundation system adequate to safely support your post frame building on soils specific to your site.
  • Septic systems also must be engineered to function on permafrost, and remember conventional systems might risk thawing the ground.    

More information on permafrost is available at these websites:

If you have a question, contact the Cold Climate Housing Research Center at info@cchrc.org or 1(907)457-3454.

Building Department Checklist 2019 Part II

BUILDING DEPARTMENT CHECKLIST 2019 PART II

Yesterday I covered seven of what I feel are 14 most important questions to ask your local building department.  This not only will smooth your way through permitting processes, but also  ensures a solid and safe building structure.

Let’s talk about these last seven….

#8 What is accepted Allowable Soil Bearing Capacity?

This will be a value in psf (pounds per square foot). If in doubt, err to side of caution. As a rough rule – easier soil to dig, weaker it will be in supporting a building. A new post frame building will only be as solid as it’s foundation, and it’s foundation will be only as strong as soil it rests upon.

Some jurisdictions (most noticeably in California and Colorado) will require a soils (geotechnical) engineer to provide an engineered soil report, spelling out actual tested soil strength.  Other states may have requirements as well, so be sure to ask ahead of time.

#9  Is an engineered soils test required?

If so, get it done ahead of time.  Don’t wait. It’s easy to do and there are plenty of soil (geotechnical) engineers for hire.

#10 What is your Seismic Category (such as A, B, C, D-1, D-2)?

While rarely do potential seismic forces dictate design of a post frame building, there are instances where they can.  A high seismic potential, with high flat roof snow load and low wind load will be one case. Other case will be when you are considering a multiple story structure.

#11 Are wet-stamped engineer signed and sealed structural plans required to acquire a permit?

Some Building Department Officials will say no to this, yet during plans review process they request structural engineering calculations to prove design, or (worse yet) they make wholesale changes to plans, based upon how they think a post frame should be constructed.

Engineer sealed pole barnMy recommendation – invest in engineered plans. It becomes an assurance a registered design professional has verified your building will meet Code mandated loading requirements. In some cases, insurance companies offer discounts for buildings designed by an engineer. It’s certainly worth asking your agent for one!

In some cases, Building Permits will be granted with only requiring engineer sealed truss drawings. We do not condone this practice, as it creates a false sense of security.

Are exterior finished (showing roofing and siding) elevations required with building plans? Will more than two sets of drawings be needed for permit submittal?

#12 Verify Building Risk Category.

Most buildings not frequently occupied by public (not a home, business or municipal building) represent a low hazard to human life in event of a failure and are ASCE (American Society of Civil Engineers) Category I. This information can be found by Building Officials in IBC Table 1604.5 (not to be confused with Use and Occupancy classifications from IBC Chapter 3).

#13 In areas with cold winters, is frost depth greater than 40”?

All building columns or foundations must extend below frost line to prevent heave. We don’t design for any depth less than 40”, and have designed for up to seven feet deep in some areas!

#14 Does the Building Department have any unusual Building Code interpretations, amendments or prescriptive requirements for non-engineered buildings which could affect this building?

If so, get a copy from your building department for us, or anyone else whom might be considered to be a provider for your building project.

Even though “the Code is The Code”, there are a plethora of local folks who think they have better ways or better ideas than world’s smartest structural minds, who have actually written the Code. And once again, I can’t stress enough: build only from plans sealed by a Registered Design Professional (architect or engineer). It will make life easier all around when it comes to getting your permit, even if you have been told seals are “not required”.

No one inside or outside of a permit office wants a construction process to be any more difficult or challenging than necessary.  Being armed with correct information (after doing homework of course) will be a solid step in the right direction.

 

Acquire a Building Permit First

You want a new pole barn, so you put together some plans, order up some materials, have them delivered and start building. In all of this excitement something was overlooked – acquiring a Building Permit!

Reader SHELBY in COLORADO writes:

“Hello,

My name is Shelby, my father and I are trying to build a pole barn and they are saying we need a ground inspection and trusses report to say that it can handle so much wind and snow! We have the building plan and started having all the materials delivered but we can’t get a building permit till we get these two inspections! I was wondering if that is something you could possibly help us with or if you know someone who could! So we could get this permit!”


It appears you have placed a proverbial cart before the horse. Before even contemplating any building project there are a pair of conversations you should have with proper authorities. First of these will be to your Planning Department: https://www.hansenpolebuildings.com/2013/01/planning-department-3/, second to your Building Department: https://www.hansenpolebuildings.com/2013/01/building-department-checklist/.

Building PermitYour Building Department wants an engineered soils report for your site, not an unusual request in much of Colorado, as there are some fairly unstable soils. You will need to contact a Registered Professional Engineer in your area who specializes in geotechnical work (Google – Geotechnical Engineers near me). They will visit your site and do an analysis to determine if it is even capable of being built upon. With this report in hand you can then take this soils report and your proposed building plans to yet another RDP (Registered Design Professional – architect or engineer) who can prepare a set of sealed drawings for your building. “Truss report” being asked for are engineer sealed drawings for prefabricated trusses you will be utilizing to support your building’s roof. You will need to provide your RDP sealed plans to your choice of truss providers, so they can design trusses adequate to support loads detailed within your sealed plans.

Sadly, you may have already invested in some materials you will be unable to use in construction of a properly designed building. All of these reasons are why I always encourage clients to invest in a complete post frame (pole barn) building package from a supplier who can provide engineer sealed plans specifically for your building, along with correct materials delivered to your site – it could have saved you a significant amount of heartache, as well as money.