steelbuilding foundations information

Does your steel building require a foundation?

Some agricultural steel buildings are designed to have dirt or gravel floors and consequently use concrete piers instead of a full slab; however, a concrete slab is best for most situations, such as commercial, industrial, and institutional steel buildings. A building needs a foundation to shoulder its considerable weight, as well as provide a flat and level base for construction.

Any steel building over 120 square feet will require an engineered foundation plan, drawn up by a local, professional foundation engineer. This will insure proper design, make the actual erection of the building go a lot smoother and reduce costs. This engineer can also recommend excavation procedures, drainage practices, form work, reinforcing steel requirements and concrete proportioning.

Proven construction techniques and adherence to OSHA and other local codes are highly recommended. When in doubt, it’s always wise to check with your local building department.

Many building departments will require engineered stamped foundation plans. Having these plans designed to the latest building codes and building department requirements will save you time, labor and money right from the initial stages of the steel building project.

The purpose of the foundation is to anchor the columns, provide a level base on which to erect the structure, and hold the whole construction together. Proper foundation design is critical to ensure the integrity of your pre-engineered metal structure.

Beware of firms which over-engineer the foundation in an attempt to make the structure safer and charge you more. This practice can cost you valuable time and money, and will not provide any additional strength or safety.

To avoid this, make sure you hire a foundation engineer who is familiar and experienced with the type of steel building you are erecting, and can provide references. Foundation designs should match your specific needs and requirements, eliminating the waste of over-engineering.

Otherwise, the cost advantage of purchasing a pre-engineered steel building over other types of construction can be nullified by an over engineered, overpriced foundation!

Here are some other common mistakes made by inexperienced foundation engineers-

• Pouring the foundation in inappropriate weather- either too hot or cold.
• Improper Vapor Barrier Placement
• Slab Edge at Framed Openings Not Sloped
• Foundation Placed Out of Square or Out Of Level
• Wrong Type of Anchor Bolts Used
• Misplaced Reinforcement
• Misplaced Anchor Bolts

Generally, there are two types of foundations used with metal building systems:

1. Floating slabs: Floating slabs consist of a concrete slab, monolithically poured with a continuous grade beam. The grade beam is either spread directly under the column or reinforced along the bottom to carry the vertical column loads.

2. Pier, Footing, and Grade Beam: Pier, Footing, and Grade Beam consist of a square, round or rectangular footing and a grade beam wall. A drilled pier may be utilized in lieu of the square or rectangular footing. Piers and footings carry most of the vertical loads. This type of foundation costs less, but may not be allowed due to local codes.

The type of foundation depends upon various things:

• the geographical location of the pre-engineered steel building
• the topography of the land
• the frame loads imposed on the foundation
• local building code restrictions
• architectural considerations

Your erection drawings will include an "anchor-bolt setting plan with reactions," which will give a qualified engineer the necessary data to design a suitable foundation for your building.

The importance of accurate foundation construction and anchor bolt settings cannot be overemphasized! The anchor bolts must be in the exact locations as specified in the Anchor Bolt drawing provided by your steel building manufacturer. There is an extremely small tolerance for the placing of the anchor bolts, only +-1/16" to +-1/18".

Foundation errors and improper location of anchor bolts are the most frequent and troublesome errors made in metal building construction. Errors can wind up costing you a lot of money!

NOTE: DO NOT start the erection process on “green” or “uncured” concrete. Anchor bolts may pull loose, concrete may spall (chip out along edges) and equipment may crash or crack the slab!

Normal “Portland” cement should cure in around seven days and high-early-strength concrete in around three days. Special circumstances may require even longer curing periods.

Soil Conditions

An experienced foundation engineer will thoroughly examine the soil at the proposed building site. Soil conditions are of paramount importance to the success of any building project and should not be ignored to save either time or money.

One potential problem arises if clay soils are found at your building site. Clay soils have been found to expand 23 cm or more if subjected to long cycles of drying or wetting, creating conditions that might shear the foundation or even lift lightweight buildings.

Soils with a high organic content may, over time, compress under the building to a fraction of their original volume, causing the structure to settle. Other types of soils might slide under the heavy loads of a building.

Sometimes the soil at a proposed building site varies so greatly over the entire site that a building cannot be constructed either safely or economically. Because of this, soil and geological analyses are a necessity in determining whether a proposed building can be safely supported and what type and specifications of foundation are required.

Did You Know?

POURING THE SLAB

Your slab must be at least 4" thick. It may need to be thicker than that or have a higher concrete strength depending on the size and use of your building. A storage building or garage would only need a 4" thick 2,500 psi foundation.

The floor of all foundation excavations should be level and smooth, and care should be taken to prevent cave-ins when utilizing the walls of the excavations for concrete forms. Strict adherence to OSHA and other local codes or laws governing shoring of excavation, to prevent accidental “cave-ins”, is critical.

• Fill areas should be properly compacted to prevent settling cracks.
• Care should be taken to obtain a good finish on the floor slab and to maintain the correct elevation throughout the slab.
• Shrinkage cracks can be minimized by pouring the slab in alternate sections, “checker board fashion”.
• The outer corners of the foundation walls and piers should be sharply formed with straight sides and level tops. This will allow neat seating and good alignment of the base angle.

1. To determine that the foundation is square, measure diagonal dimensions to be sure they are of equal length.
2. To determine that the foundation is level, set up a transit or level and use a level rod to obtain the elevation at all columns and posts.
3. Carefully check the location of all anchor bolts against the “Anchor BoIt” drawing furnished by your manufacturer. All dimensions must be identical to assure proper start-up.

ANCHOR BOLT SETTINGS

We have mentioned the anchor bolts several times already. It is extremely important that anchor bolts be placed accurately in accordance with the anchor bolt setting plan. All anchor bolts should be held in place with a template or similar means, so that they will remain plumb and in the correct location during placing of the concrete. Check the concrete forms and anchor bolt locations prior to the pouring of the concrete.

A final check should be made after the completion of the concrete work and prior to the steel erection. This will allow any necessary corrections to be made before the costly erection labor and equipment arrives.

THE LOW-DOWN ON CONCRETE

Concrete is a mixture of cement with sand, stones and water. The water is added to the dry components and it initiates actual chemical changes leading to hardening. After full hardening, or curing, the strength and durability of the material is comparable to some of the hardest known rocks.

However, rocks take millennia to form, whereas concrete can be mixed in a few minutes, and will approach final hardness in a few weeks - or even a few hours if chemical "accelerators" are added during the mixing stage.

The most important component of concrete is the cement, which is made of certain fine mineral powders. The basic cement is Portland cement. When mixed with water, it undergoes an actual chemical change, and binds the sand and stones into an astonishingly strong, amalgamated material. This chemical change is known as “hydration.”

In concrete production, the sand and stones are known as "aggregate". Stones are the "coarse aggregate" and sand the "fine aggregate". The stones are usually between about 10 and 20mm in size. Though less important than the quantity of cement involved, the sizes and proportions of the aggregate components are amongst the factors that determine the final properties of the concrete. Both types of aggregate should include particles with widely-varying sizes.

Extenders and fillers are sometimes added to cement for various reasons:

• Cost saving – extenders are generally cheaper than Portland cement.
• Technical benefits – extenders improve impermeability and durability of the hardened concrete.

Hydration is an exothermic reaction, i.e., it provides heat. The more extender in the mix, the slower the strength development, and the hardened concrete will only reach its full potential strength if it is properly cured.

CURING

As concrete sets, the water in the mixture forms a chemical reaction with the cement, and the paste gradually changes from a plastic state into a strong rigid solid. This is different from the drying of mud, wherein the water simply evaporates and leaves the remaining solid material, without having changed it chemically.

That is why it''s called "curing" (or "setting") of concrete rather than drying. It''s important to keep concrete wet during the early stages of curing, both to allow it to reach its full strength and hardness, as well as to minimize the tendency to crack. This is normally done by spraying it regularly for the first week or so, and keeping it covered with sacks, leaves or any other convenient materials.

There are various ways concrete can be cured:

• Covering the surface with a material which holds water such as sand, earth, straw or hessian (a loosely woven, strong fiber material similar to burlap) and keeping it continuously damp
• Sprinkling or spraying with water often enough to keep the concrete continuously moist
• "Ponding" water on the surface
• Covering with plastic sheeting or waterproof paper. This can be tricky, as the covering must be held in place in a way that does not damage the still soft concrete, and be sufficiently overlapped at joins to prevent evaporation
• Using a manufactured spray-on membrane (curing compound/agent)

In cold weather, protect newly placed concrete from frost by covering it with an insulating material such as sacking or straw.

There is conflicting information about how long concrete requires to thoroughly cure. It cures quicker in hot weather, slower in cold. Chemical accelerators can be added to speed the process. As always, the best way to proceed safely is by hiring a thoroughly experienced professional for your job.

Delays in constructing your building due to waiting for the foundation to cure can be avoided by hiring the foundation professional and scheduling the pouring of the foundation as soon as you know the building’s delivery date.

“HIDDEN COSTS” IN PURCHASING A STEEL BUILDING

The foundation is one of the largest hidden costs in steel building construction, running anywhere from $4 to $10 per square foot!

Sometimes, without realizing it, a customer will blow their whole budget on the building, not realizing how many other costs need to be factored in, and are unable to complete their building project. The quote you receive from a metal building manufacturer is for the basic building and delivery, only.

(Note that the price of a steel building will change as the price of steel fluctuates. To insure purchasing the building you have chosen at the price you were quoted initially, you must secure it with a deposit. This amount varies from manufacturer to manufacturer.)

Here are some potential additional expenses to be considered:

• Accessories such as windows, doors, insulation, gutters, vents, skylights.
• Outer architectural finishes such as stucco, brick, etc.
• Most parts of the country require building permits for any new construction. The costs of obtaining these permits vary. Be sure the building will be approved for your site before purchasing it.
• Hiring a professional to erect the building is a substantial expense, versus doing the job yourself.

COMMON TERMS:

Base Condition- a steel member that runs the entire edge of the building at or near the foundation. It is the connection point for the bottom of the wall sheeting, and prevents pests, water, and outside elements from entering the base of the building.

Rebar – (reinforcing bar or reinforcement bar), is a type of steel bar, commonly used in reinforced concrete and reinforced masonry structures. It is usually formed from carbon steel, and is given ridges for better frictional adhesion to the concrete.

Hairpin - Rebar bent on angle placed around the anchor bolts with a thrust angle. Used to counteract the horizontal thrust of the building.

Dobies - Concrete or plastic resting blocks or chairs for rebars. They are typically placed on 6'' grid holding the rebar at is exact location while the concrete is being placed and finished. These blocks remain in and become part of the foundation.

Slab-on-grade - foundation slab that is supported solely by the ground.

REFERENCES:

http://en.wikipedia.org/wiki/Rebar

Premier Steel Buildings, http://www.premiersteel.org/FAQquestions.htm

Steel Building Projects, http://www.steel-building-projects.com/about%20concrete%20working.htm

http://www.steel-building-projects.com/Alittlegemaboutfoundations.htm

Salsa Steel Corp., www.metalbuildingfoundations.com/mbfoundations.htmm

http://www.unitedsteelbuilding.com/building-design-specifications

Rigid Building Systems, http://www.rigidbuilding.com/PDFS/07-ErectionAndSafetyManual.pdf

Urban Dictionary, http://www.urbandictionary.com/define.php?term=hessian&page=2

http://www.cnci.org.za/inf/leaflets_html/cement.html