Concrete Recommendations for Tubular Steel Structures and Carports
Creating the Ideal Slab for a Standard Tubular Steel Building or Carport Structure.
At American Steel Structures, we are frequently asked how to properly pour foundations or slabs for open carports/rv covers, enclosed steel buildings, or even combo units that are partially both, including horse barns that are often a combination of concrete and dirt. Even though we do not currently offer concrete services any further than about 60 miles around our offices near Houston, we're happy to offer our recommendations to all our clients nationwide with the best practices that we’ve learned from our own experience during local installations of pads for our engineered, galvanized-tubing steel structures that we design and sell through our network of manufacturers and installers.
We first recommend that customers thoroughly research and hire a locally certified & experienced concrete contractor specializing in slabs or foundations. Do your homework and don’t be afraid to ask for referrals from trusted associates or neighbors. The information provided herein will help you in your communication with your contractor and specifically for clarifications of what’s required for the structure you’ve ordered.
Not all steel buildings have the same concrete requirements. Wood frame pole barns, red-iron commercial/industrial buildings, and tubular steel structures with base-rail, each require quite different specifications and styles of foundation. Home foundations are entirely different still. So just because your concrete contractor is experienced with house pads or commercial pads, doesn’t guarantee his/her knowledge base for the galvanized tubular steel style of pre-engineered building you're about to install.
If you are knowledgeable and confident and want to complete the slab for yourself, then the following information most certainly will come in handy. Bear in mind that this is not an instruction manual. This booklet will give you a few tips and basic recommendations and for DIY, would conclude that you already know the basics or if you have hired someone else to do the actual labor while serving as your own General Contractor.
Before you break ground or order any steel building or carport structure, you should check with your local authorities for their building codes and/or requirements. You may need to obtain a permit for the upcoming structure, including potentially a different permit for the concrete slab itself. If you do not need a permit, you can probably skip this section.
FOOTERS AND SUPPORT BEAMS
Usually, you can call your city or county courthouse, and they will direct you from there. They will tell you what their regulations are for a building and/or slabs and foundations. These codes may not be the same for everyone in one county or even for everyone in one city. Areas with major elevation differences or different flood zones for example, may set the codes based on your actual addresses. Northern building code officers will usually tell you that you will be required to add footers/perimeter support beams (the outermost part of a slab supporting the weight of a structure from underneath) and may need to be set below the frost depth (usually over 18″ deep; but code is set by your local municipality, and they all have independent regulations). Southern building codes may require that you build up the underlying dirt pad to certain levels above the flood zone prior to pouring concrete above that, as another example. And there are often many more unique requirements so be sure to find out what YOUR jurisdiction requires.
The codes will often also set a wind load, snow load, or a live load requirement. Wind loads are usually defined as a 3-second wind burst that a structure should be able to withstand without taking damage (90 mph used to be a standard, but many municipalities are increasing requirements). Coastal areas around the Gulf of Mexico in TX, LA, MS, AL, & Florida often require wind load certifications above 130 mph, sometimes as high as 170 mph in zones prone to hurricanes.
Snow loads are usually referred to as “ground snow loads” and usually measured in pounds per square foot (lb. per sq/ft). A rule of thumb is a minimum of 20 lb is required. If your area gets more snow than others, you’ll likely need to make sure your metal building and its corresponding foundation is built to withstand more weight, and furthermore is structured with only Vertical roof metal orientations to better relieve snow build up on the roof. Live loads describe the weight of a "live" person on the building. These are usually measured in the same way as snow loads.
While inquiring and/or obtaining permits is typically the customer’s responsibility, you can ask the friendly experts at American Steel Structures or their associated manufactures/installers for assistance in finding out who makes those decisions. Rural installations often require no permits or notices. Certain Cities however, subject to excessive flood, wind, or snow issues can often be complex and difficult to navigate, occasionally even requiring the engagement of outside planners, engineers, architects or consultants to navigate the process. We strongly recommend that you get these answers PRIOR to committing ANY money to order your carport or building, even before conducting ground prep work and/or pouring concrete.
Many of our manufacturing partners provide generic, but stamped and Certified Engineering Plans that encompass each level of wind, snow or live load comprehensively offered by that Company, as well as each comprehensive style of anchoring (dirt, gravel, asphalt, concrete). Most also offer the ability to order a set of “site-specific engineered drawings” that will include specifications on the materials in which the building will be made of, how they will be put together, how it will be attached to the foundation or ground, and how the foundation needs to be built, for an additional engineers fee (and time delay). These engineered drawings will have a state-licensed engineer's stamp of approval. This will tell the building inspector and the permitting office that your building is being built to code. Of course, the building needs to be built according to those drawings and specifications, and each manufacturer's fabrication and installation teams are experienced with engineered specs and requirements, so will build to those specifications.
PICKING A TYPE OF SLAB OR FOUNDATION
There are four most common foundation options from which to choose below. If your building is very large (like 40′ wide clear span), you may need to go by drawings that you have purchased from a local civil engineer. In rural areas where no permit is required, methods explained below will generally still work for commercial structures by adding thickness to standard recommendations. Permits in Northern states often require additional footers and/or support beams deep underground below the frost line, which may be in addition to the suggestions below.
1) Flush Mount (Our Preferred Method)
Pouring a Slab Foundation for a Carport, Garage, Barn or Building with Flush Mount Concrete Slab Foundation. Flush Mount = The slab size is exactly equal to the outside dimensions of the building frame.
The Flush Mount slab uses the least concrete; therefore, is slightly less expensive (both labor and material) and allows your metal siding to be installed so that it overhangs the foundation by 1-2” or more, thus preventing water from seeping under the walls (as will be the case in Option 4 below). The Notched Edge Offset (#2 below) is probably the most expensive, but it is both strong and allows the siding to install below (overlapping) the top of the slab. The Flush Mount style is our most recommended option here in Texas (and the majority of the Southern USA) where we get torrential rains.
Our standard recommendations for residential use are a 4” thick pad in the exact dimension of your building. Of course level and square. We recommend plastic sheeting moisture barrier, #3 rebar at 18’ on-center grid, and 3500 psi concrete. We also HIGHLY recommend thickened and reinforced perimeter edging as outlined below.
The Edge Offset method in #4 below is sometimes recommended by installers as they are concerned that anchoring base rails too close to the edge may “crack out” the edge upon anchor drilling and setting, and in many cases it can. In order to prevent anchoring “cracks” we recommended two very important additions to our FLUSH MOUNT preferred recommendation.
#1) We add a 4” x 10” perimeter beam underneath the primary pad around the full perimeter. No equipment needed. Just a simple “shovel width” trench 4” below grade. This creates a “thickened edge” whereby the perimeter is now at least 8” deep (x shovel width wide, approx 10”).
The building base rails are generally 2 ½” galvanized square tubing. Base rails are flush mounted at the edge of the concrete, and are drilled for anchoring on the inside edge, at approximately 2” from the edge.
#2) As such, we also strongly recommend a double-rebar perimeter be added, also around the entire perimeter of the building, with one strand of rebar placed 1” from the outer concrete edge, and another at 3” from the outer edge.
Adding 4” of additional depth to the concrete edge, creating a thickened edge of 8” deep, plus the addition of perimeter rebar at 1” and 3” from the edge respectively, ensures that anchor holes drilled through the base rail and into the concrete at approx 2” from the edge are held secure, and reinforced to not crack out! Anchor bolts now inserted BETWEEN the double rebar strands and into 8” or more depth, thereby avoiding crack out and providing substantially better structural integrity to the concrete anchoring.
Keep in mind wind loads are not just about bracing inside the building, but the stability of the concrete anchoring, so thickened and reinforced perimeters on your foundation help to ensure structural integrity of the foundation during extreme wind loads!
Flush mount, 4” thick pad shown (no ledges or lips here) for sheet metal extension past concrete to ground level. Not shown are additional 4” deep x 10” wide perimeter support beam (footer) and double rebar perimeter reinforcements at 1” and 3” from the edge respectively.
No visible concrete on edges as sheeting expands past base rail & concrete to ground 1-2”.
2) Notched Edge Offset
Pouring a Slab Foundation for a Carport or Garage with Notched Edge Offset Concrete Slab Foundation. Notched Edge Offset: Similar to Edge Offset but the extra 2”-6″ is dropped to only 1.5″-2” lower or "Notched" or otherwise known as a “Lip” edge.
If a ledge is absolutely desired around the perimeter, OR if there is any kind of CENTER wall that crosses the pad between an open area and an enclosed area, dropping the depth of that ledge or lip by 1 ½-2” down at the exact outside measurement of the buildings base rail frame, will allow installers to “lap” the sheet metal down that 1 ½” – 2” BELOW the bottom of the base rail, preventing water from being able to enter or seep under the anchored base rail. Also known as adding a LIP.
Perimeter Edge Lip. Base rail mounted on upper ledge.
Sheet extends down notch/lip 1 ½-2”.
Inside Wall Lip <open to the left, enclosed to the right>.
3) Sloped Edge
Pouring a Slab Foundation for a Carport or Garage Sloped Edge Concrete Slab Foundation. Sloped Edge: Similar to the Edge Offset, but much smaller and sloped away or rounded off to prevent water retention or seepage.
The slab size is equal to the building frame dimensions PLUS 1”-2” max (0.5″-1” all the way around ) 0.5″-1” should be done with a ¼-1/2”″ slope away from your building or rounded and sloped like you might smooth a wooden table edge. This will ensure that water coming from the side of your building will flow away from the inside of your building. We have not used this method ourselves in Texas, but one of our manufacturing partners up north highly recommends it. Utilizing this method, we would still highly recommend adding a minimum of 4” additional depth on the edges (8” total) and double rebar perimeter, now space approx 2” and 4” from the edge.
Rounded Perimeter Edge no more than 0.5”-1”.
Sloped Perimeter Edge no more than 0.5”-1”.
4) Edge Offset
Pouring a Slab Foundation for a Carport or Garage Edge Offset Concrete Slab Foundation. Edge Offset: The slab size is equal to the outside dimensions of the FRAME of the building PLUS a 4”-6″ ledge in each direction.
We see several manufacturers that recommend this style, but we advise against this style and here’s why. Each tubular frame building utilizes a 2 ½ galvanized tubing as a “base rail” perimeter, generally with upright legs every 4-5’. The framing is then sheeted outside the frame. If the base rail is anchored 4”-6” (or more) in from the concrete edge in this style, the sheet metal will bottom out at the level of the concrete when applied. Water gathered off the 4’-6” ledge all around the perimeter (which catches roof run-off) can and WILL seep under the sheet metal ends and base rail into the interior of your building . Contrary to the recommendation of some larger manufacturers, this is a decision you will regret making, and you’ll be spending much time, effort and expense trying to figure out how to seal the inside edge of that anchored base rail to keep water seepage OUT of your enclosed building! This style is absolutely find for OPEN air structures where water seepage isn’t an issue. But its never appropriate for ENCLOSED structures. Illustrations below of what we do NOT recommend!
Edge Offset Foundation WILL seap water under the base rail.
General Slab Details & Recommendations
All four types of slabs described above will need a slab of at least 4″ thickness. Most pre-engineered tubing structures sold are light-duty residential use. As such, American Steel Structures generally recommends a 4” thick slab of 3500 psi concrete with a #3 rebar grid (16-18” on-center) and a double-rebar perimeter edge reinforcement over an 8” thickened edge. Standard grid spacing on the rebar is 24” but we generally recommend a 16”-18” on center spacing (depending on underlying ground stability), again with DOUBLE rebar perimeter (at 1” and 3” from edge respectively for flush mount) to reinforce the edges where the base rails will be anchored at approx. 2” from the edge.
Some customers prefer (or require) a little thicker 5” pad if they have larger equipment, and some cities demand a 6” or greater thickness on the pad, even for carports (which is ridiculous overkill in our opinion). Each inch of thickness adds to the cost. Tighter on-center spacing of rebar adds slightly to the cost. Upgrading to #4 or #5 rebar will add to the cost. Each of these upgrades will however increase strength and longevity of your pad, and are often required when underlying soil conditions are not ideal.
When commercial or industrial applications are intended, pads are frequently utilizing 6” or even more thickness, heavier gauge rebar, tighter rebar grid spacings, occasionally even higher PSI concrete to start. These pads can become very expensive quickly, and are only recommended for heavier intended uses and commercial/industrial applications. Commercial grade applications will likely require additional footers (aka support beams), particularly around the perimeter and in some instances crossing internally as well. Unlike red-iron i-beam applications, tubing steel structures do NOT need any type of pillars, as weight distribution is evenly spaced around the perimeter, not on 20’ centers with i-beam as is commonly found in red oxide construction.
The footer's (aka support beams or thickened edge) depth generally needs to be the frost line's depth or as per local code, but virtually anywhere, no less than 8″ deep. The width of the footer needs to generally be at least 8” to 12″ wide but deeper and wider footers are not uncommon for very large buildings, or in very cold climates to get below the frost line. Footer/support beams are sometimes recommended for smaller residential style garages, barns and/or carports/RV cover pads. We generally do NOT recommend adding more than 4” additional (8” total perimeter depth) or that added expense unless your underlying soil is unstable. No reason to bury money in the ground with unneeded expenses for support not required!
For houses and large structural steel buildings (and even Wood Frame Pole Barns), the weight of the building itself is substantial and requires significant underground reinforcement, so it's common to add 12”x24” support beam footers under the wall perimeter of those structures. But factor in snow load on the roof, or wind load torque, and you may definitely want to add additional or larger footers or support beams, as advised by your local concrete contractor professional.
Prepared for Footing/Support Beam Pour.
Tubular Building Structure Requirements
Pre-engineered tubular style structures like we most commonly offer from our nationwide network of manufacturers, are built using fairly light-weight 2 ½” galvanized steel tubing and 29 or 26-gauge sheet metal. They are NOT in and of themselves “heavy” structures like common red-iron I-beam. They can however be additionally stressed by snow or wind loads. So, keep that in mind when planning your pad specifications in your specific area. We don’t believe the added expense of heavy footers beyond an additional 4” deep is needed for the average smaller residential style steel structure building or carport. But areas with significant snow or wind load issues, as well as unstable earth areas may prefer to stay safe and add 12” x 12” or more sized footings.
Unlike nearly any other recommendations we’ve ever seen, we do STRONGLY recommend that there’s no misunderstanding with the concrete contractor that DOUBLE REBAR perimeters need to be added at the very edge of the pad all around. When installing a tubing frame building, the 2 ½” galvanized tubing BASE RAIL runs that perimeter except where doors are framed out. That base rail is then “anchored” into the edge of the concrete pad by drilling through the rail and deep into the concrete, where an anchor bolt is pounded in (think sledge hammer) and secured with a nut on top of the rail. The pounding of the anchor bolt spreads the bottom of it, locking it into place in the concrete itself.
Double Rebar Perimeter
The drilling of the hole and the pounding of the anchor bolt into concrete puts substantial stress right on the concrete edge (approx 2" from the edge in an Flush Mount pad) so needs the extra reinforcement. Even when using Sloped Edge or Notched Edge design, we highly recommend the double rebar perimeter reinforcement. Even pads with support beam footers should be reinforced near the location of anchor placements in the base rail, generally placed every 8’-10’ around the perimeter of the building!
Common Concrete Anchor Bolts ---------------- (Anchoring Process)
Finished Product – Installed generally every 8’-10’
Combinations with Dirt or Gravel and Concrete.
Combo buildings like Carport or RV Covers with end storage, side storage units, or even horse barns with concrete center isle and dirt stalls or gravel parking areas, it is required that the dirt/gravel areas be filled, leveled and packed to the same height as the concrete pad. All surfaces, including any combination of surfaces, need to create a level pad across the entire building at the time of installation. No step downs. Not even 4”. Installers can generally make leveling adjustments from 1” - 3” or so, but the pad needs to be level within that range across ALL surfaces.
American Steel Structures
For questions or concerns, please contact our experienced design & sales consultants at American Steel Structures, Inc. – Beasley, TX (Just SW of Houston) – Phone 979-387-3662. Email sales@AmericanSteelStructures.com or visit our website at www.AmericanSteelStructures.com