The Difference between Post-Frame and Pre-Engineered Steel Buildings

Published July 15, 2016 by Whirlwind Team

pre-engineered steel building

The big difference between post-frame and pre-engineered metal buildings is that a post frame steel building uses wood in its make-up. Yes, wood framed buildings can still be considered steel buildings.

This post compares a post frame to a pre-engineered steel frame building to highlight their different construction and uses.

Post-frame buildings

Sometimes erroneously called a “pole barn,” a post frame building is built from an engineered wood frame building system that meets UBC and IBC standards. Pole barn is not quite right because a post barn has, you guessed it, wooden posts for load-bearing in place of a round pole.

Post frame structures were developed in the 1930s and were refines through the 1970s and 1980s. The typical post sizes are:

  • 4X4
  • 4X6
  • 5X5
  • 6X6

Larger sized treated lumber or laminated columns can also be used.

One advantage a post frame building has over a pole barn is that a post frame is easily permitted in areas where pole barns cannot.

The wood frame transfers loads to the ground or a surface mounted concrete pier or masonry foundation. If the wood or concrete needs additional protection, plastic barrier systems are available.

Foundation and materials

A post barn doesn’t need much more than a level building site. The used of concrete footer pads can help disperse the weight at the bottom of the post holes. You can pour a full concrete foundation, but it isn't necessary.

The skirt (splash) boards and posts are treated wood as well as any lumber that comes within 18 inches of the ground.

Posts are treated, solid-sawed posts of glue or nail laminated columns set at least four feet into the ground, or one-quarter of their length.

One or more rows of treated 2X6 or 2X8 skirt boards, sometimes known as grade board, is placed into a bottom wall girt, either center match or composition lumber rated for ground contact.

Exteriors

Here is where we come to the steel content. Pre-painted steel roof and wall panels are mounted on the wooden frame. If you require room for insulation, a post barn gives you plenty: eight-foot wide wall cavities for six-foot batt insulation. The attic can allow for 14 inches of insulation installed, and the roof pitch can be 3/12 to 8/12 depending on design.

Easy to build

Post frame buildings are extremely easy to construct of minimal materials that yield an inexpensive covered clear span. Post frame will handle larger loads than typical stud wall construction, using larger posts and an interlocking frame to save on materials. In addition, the posts are spaced farther apart than studs, leaving a larger wall cavity for insulating your metal building.

The exterior, like other steel buildings, can be made to look any way you want.

Post frame construction is appropriate for low-rise applications such as:

  • Commercial
  • Industrial
  • Municipal
  • Residential
  • Religious
  • Agricultural

Post-frame buildings can be sturdy but, due to the wood frame, you won’t get a discount on fire insurance. Also, expansion may not easy since the frame is nailed together and may not withstand being disassembled. In addition, wood frame construction is not always appropriate in earthquake prone regions.

On the upside, wood is a sustainable material, fairly flexible in its use. Recently, a post-frame firewall has recently been tested by the National Frame Building Association and theUnderwriter's Laboratory and achieved a three-hour fire rating.

Another innovation comes in the form of CLT, or Cross-Laminated Timber, into post-frame buildings. CLT is a stronger, more durable form of wood frame material created when multiple layers of wood are crossed perpendicularly to the previous layer and glued together (lamination).

Pre-engineered steel buildings

Pre-engineered steel buildings are also easy to construct. The major difference from post-frame buildings is that the frame in a pre-engineered steel building is made of steel. (Kind of obvious.)

Manufacturing

Pre-engineered metal buildings are professionally designed and engineered to meet local building codes and load requirements. Everything is created in a manufacturing environment to meet a high level of quality control. All engineering, architectural design, sketches, drawings, and plans are included in the price. You can usually get engineer stamped plans to use for permitting.

Because the building is designed, everything is easily integrated at the site and provides for better performance over a non-engineered building.

Each column, girt, purlin, and rafter is cut, welded, punched, and marked before it is bundled with the rest of the building kit to make it easy to match parts and erect the building quickly. The kit can be delivered about six to eight weeks after ordering.

High strength bolts and other fasteners are included in the kit.

Design and materials

Often, framing members are tapered and have flanges to save weight without sacrificing strength. Trusses and webs are manufactured in a variety of thicknesses to meet the needs of the various internal stressors.

Because a pre-engineered metal building is about 30% lighter than a conventional building, a simple, light foundation is all that is needed. Once the frame is up, metal wall and roof panels are installed that, like the post-frame building, come in any color or texture you need.

Pre-engineered metal buildings are easily repaired, expanded, or redesigned. Metal buildings also have a seismic advantage not seen in wood-frame construction. Steel buildings can be designed for earthquake resistance so that the steel members will bend but not fail and connections will not shear as easily.

The cost for a pre-engineered building may be higher than for a post-frame, due to materials, design, and off-site manufacture. However, labor costs and construction time are reduced with pre-engineering due to the ease of erection. Most connections are extremely similar from project to project, so skills learned from one pre-engineered building project are easily transferred to the next one.

Pre-engineered steel buildings and post-frame buildings are popular building methods using sustainable materials. The exterior finish for each is highly flexible, and metal panels can be used on a post-frame building, just as wood or wood-like exteriors are available for metal frame.

Each frame type is available for most projects. The design details will help you determine which material is best for yours.

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