Metal buildings are referred to by many names including: pre engineered metal buildings, engineered metal buildings, pre fabricated metal buildings, and PEMBs. Whatever you call them, these buildings have several common features including large steel moment frames, light gauge steel wall girts and roof purlins, and metal roofing and siding panels. The photo at right shows the large steel moment frames, girts, and purlins of a metal building under construction.
Usually, a metal building is purchased as a kit from a supplier. The supplier will shop-fabricate the components of the metal building, and ship them to the job site for assembly (erection). The metal building supplier will also provide the structural engineering calculations and drawings for the superstructure. The superstructure is the portion of the building that is above the ground level. The basic components of a metal building superstructure are shown in the video below.
The metal building supplier typically doesn’t provide calculations or drawings for the foundation (the part of the building that is below ground level). The owner (or contractor) hires a local structural engineer to design the foundation for the building and produce foundation plans. So metal building engineering involves two main parts: superstructure engineering and foundation engineering.
Metal building foundation engineering involves the design of the concrete elements that connect the pre engineered metal building structure to the soil. The foundation might be considered the most crucial component of a metal building, as it provides the structural support for the entire building.
Metal building foundation engineering is a complex process that involves several main parts:
The most common types of steel building foundation systems (in Colorado) are isolated column footings and slab with haunch footings. Pier foundations are also sometimes used in poor soils. Isolated column footings are constructed as a series of individual footings, each of which includes a large concrete pad (typically between 4’ and 6’ below grade) and a concrete pedestal extending up to above the ground surface. The pedestal (also referred to as a pilaster) provides an attachment point for the anchor bolts, which connect to the metal building frame above. This type of foundation is shown in the image below. The photograph shows a pilaster that has been formed up, and is now ready for concrete placement. The deformed steel bars (rebar) have been positioned within the forms. The concrete footing in the photo has already hardened and cured; it was formed and placed before the pilaster. When this method is used, vertical steel bars (called dowels) are left protruding up from the footing. These dowels connect the footing and pilaster when the pilaster is poured.
Metal buildings are designed with large steel moment frames, which are typically spaced every 15’-25’. The isolated column footing spacing matches the spacing of the moment frames, so that each moment frame is supported by a pair of footings. The footings are usually connected around the perimeter of the building by a concrete grade beam. The grade beam provides an attachment point and bearing surface for the continuous walls around the building. The interior of the building usually contains a concrete slab on grade, but other floor types are possible. The last major foundation element is the hairpins, tie rods, or grade beams which are used to extinguish "kick-out" forces from the moment frames. The hairpins or tie rods are a crucial element of the foundation, and should be coordinated with the design of the floor slab.
An Isolated column footing system is well suited for Colorado, since the foundation elements extend down below typical frost line depths. Additionally, this foundation type easily accommodates tall metal buildings (18’-30’ wall heights) because the foundation elements may be extended deeper to counteract the effects of wind uplift loads.
This video shows the basic components of a pre-engineered metal building. The video shows the superstructure (the components that are above grade). The large steel moment frames connect this superstructure to the foundation.
A slab with haunch footing includes a slab on grade throughout the interior of the building, which ‘turns down’ at the building perimeter. The turned down perimeter will extends several feet below grade, to just below the frost line depth. The turned down portion is reinforced and thickened to provide adequate resistance against downward loads and uplift loads. Steel reinforcing bars are typically extended from the haunch back into the slab, to resist ‘kick out’ forces at the columns. A slab with haunch system often allows for somewhat shallower excavations than an isolated footing system. However, a larger total volume of concrete may be required, because the haunch system relies solely on the weight of the concrete to resist uplift loads (rather than relying on the soil above the isolated column footing).
Some contractors prefer the slab with haunch footing system, because it simplifies the concrete formwork (as compared to the isolated column footing system). This must be counterbalanced against the increased use of concrete in this system. An experienced concrete foundation contractor will be able to determine which system will provide the best value, based on the cost of labor and the cost of materials.
The type of foundation used for a metal building will depend on a variety of factors, including the building's size, location, and intended use. For example, a larger buildings or buildings located in areas with poor soil may require a more robust foundation than a smaller buildings or those located in areas with good soil conditions. Additionally, wind uplift loads can have a significant impact on foundation sizing and design. Many building owners are surprised to learn that in areas with typical snow loads (e.g. 30 pounds per square foot), the wind uplift loading is driving the size of their foundation! The building height, and building location on the site, are two of the factors that influence wind loading. The wind uplift loads are provided by the metal building supplier, and are part of the standard package of calculations that they perform. The foundation designer uses this data when designing the foundation.
When JLA performs structural design services, we evaluate all of these factors in order to design an economical and effective foundation system for the building. When owners evaluate pre engineered metal building costs, they often look only at the cost of the superstructure and fail to consider the cost of the foundation. If you are researching the cost per square foot of your proposed custom designed steel building, it is important to look at the entire building system!
After gathering initial information about the soil types, building size and loads, building location on the site, and other factors, the owner is ready to get under contract with a metal building supplier. The metal building supplier will typically require a down-payment before they proceed with producing engineered drawings for the superstructure. The owner should be sure they are happy with the size and configuration of the building before giving the "green light" for the building supplier to produce engineered drawings. If the owner changes their mind about the building configuration after this point, it is likely the metal building supplier will charge an additional engineering fee.
After the engineered superstructure drawings are received from the metal building supplier, detailed metal building foundation engineering can begin. The design process involves sizing the concrete footing elements, determining the size and spacing of steel reinforcing bar (aka rebar), and detailing connections between elements. The connection of the metal building to the foundation is also detailed. The foundation design must be in perfect dimensional conformance with the approved building design, and must adequately transfer all loads to the soil. Additionally, long-term performance aspects must be evaluated. These include slab cracking, foundation settlement, and resilience of the foundation system during future building modifications.
A proper foundation design for a metal building also includes drainage systems to ensure that water does not accumulate around the building. This can be accomplished through the use of perimeter drains, interior drains, or a combination of both. Water barriers may be installed on one face of the grade beams, to direct water to the perimeter drain system. Strategic use of course granular soil material in the vicinity of the drain can increase effectiveness.
With the increasing use of metal buildings as homes, workshops, and commercial businesses, building insulation has become more important. Buildings which will be conditioned (heated) are required to be insulated in accordance with the energy code. Foundation systems also must be insulated, to prevent heat from escaping the building through the floor and through the perimeter foundation walls. Rigid foam board products, such as expanded polystyrene (XPS) are commonly used. Proper detailing of the foundation, insulation, and water barrier systems is essential.
Once the foundation design is complete, and construction is underway, the structural engineer will visit the site to inspect the foundation at various times, to ensure that it is being built according to the approved drawings, and meets all applicable codes and regulations.
In summary, metal building foundation engineering is a crucial aspect of constructing a metal building. It requires a thorough understanding of soil conditions, building codes, and construction practices to ensure that the foundation is strong, stable, and able to support the building for many years to come. A professional engineer with experience in metal building foundation engineering should be consulted to ensure the job is done right. If you are starting a metal building project in Elizabeth, Parker, Castle Rock, or anywhere in Colorado we would love to talk with you and see how we might help!