Pre-Engineered Building Systems
I’ve helped build many pre-engineered buildings (often called metal building systems) over the years, from warehouses to air plane hangers to recreation centers. I like the efficiency of the pre-engineered systems, but have learned several lessons the hard way. To begin, let’s define a pre-engineered building. If it’s a steel frame building and the steel columns and beams get designed by the building manufacturer, such as Butler Manufacturing, American Buildings, Ceco Building Systems, or Metallic Buildings, then it’s a pre-engineered building. On the other hand, if a consulting structural engineer designs the steel beams and columns, using rolled steel sections, then it’s a conventional steel building.
The sketch below from the Navy Builder’s Training Manual shows a simple illustration of a pre-engineered steel building.
Pre-engineered buildings can be built for lower cost because of the system approach:
- Designed with complicated computer software that focuses on efficiency (individual Structural Engineers tend to add a “Fudge Factor” onto calculations to be more conservative and protect themselves).
- Factory produced with lower cost (less skilled) labor than steel job shops.
- Shipping weight carefully considered, even to trimming down unneeded steel on sections to reduce shipping costs.
- Roof and siding installed by the same trade, creating another efficiency on the job site.
- Single source responsibility and coordination.
- Quick erection in the field because of the system approach.
- For the Construction Supervisor, one of the advantages of pre-engineered systems can also be a big problem. Since the structural design is so efficient, the capacity for the beams and columns to carry extra loads is minimal. In a conventional steel building, a rooftop unit or a monorail crane gets added and the extra capacity in the steel design usually covers the extra load. You need to be much more careful with pre-engineered building changes. Don’t assume any added structural load will be acceptable. Make sure the pre-engineered building manufacturer knows about the proposed changes and approves them.
Along that same line of thinking, the Construction Supervisor should know the design loading used on a pre-engineered building. A knowledge of basic structural design helps prevent big and small problems. I also recommend a review of the selected building manufacturer’s website. It helps you get familiar with their products and procedures.
The steel purlins that get used on the roof and the steel girts that get used on the walls are made from cold formed steel framing (different from the hot rolled structural steel). The cold forming process passes steel sheets between large rollers to deform the steel, but at a lower temperature than hot rolling. The rolling process compresses and stretches the steel, hardening it in the process. Most cold rolled steel is either 33,000 psi or 50,000 psi yield strength.
The sizes and attributes for girts and purlins are shown at http://www.loseke.com/lgsi.html (which has all the technical information available about cold formed steel framing). At the site listed above, you can scroll down the left menu to find the properties of C sections and Z sections.
My general caution for Construction Supervisors working on pre-engineered buildings: understand that the cost efficiency comes with a cost. The pre-engineered buildings have less tolerance for changes or errors than conventional buildings. For example, the water-tightness of the building often has only one level of protection, if those details aren’t completed correctly, leaks occur. Conventional construction often provides flashings and counter flashings that catch water that leaks past a first layer. So you must understand how the pre-engineered system is designed to work and pay attention that it gets installed correctly.
