What architects get wrong when specifying pre-engineered metal buildings

A pre-engineered metal building is specified correctly when the architect defines the loads, geometry, serviceability limits and interface conditions, and lets the manufacturer's engineer design the frame to meet them. Most specification problems on metal building projects are not drafting errors. They are gaps, and a gap in a metal building specification is not a neutral event: it becomes an engineering assumption made by somebody who has never seen the site.

That single mechanic explains most of the change orders, field modifications and unhappy punch lists that architects associate with pre-engineered metal buildings. The Metal Building Manufacturers Association sets out the responsibility split plainly in its Common Industry Practices document: the manufacturer designs the system it supplies to the criteria stated in the order documents. If a load or condition is not stated, the manufacturer is entitled to design as though the load does not exist. The frame that arrives on site is a faithful answer to the question that was actually asked.

Collateral load is the omission that costs the most

Collateral load is the permanent weight a metal building carries that the metal building manufacturer does not supply: sprinkler mains, HVAC ducts and rooftop units, lighting, cable tray, suspended ceilings, catwalks, and any fixed service equipment hung from the structure. Collateral load is the single most frequently underspecified item in architectural specifications for metal buildings, and it is the one that cannot be fixed cheaply after fabrication.

The typical failure is a specification that either leaves the collateral load blank or copies a generic allowance from a previous project. Representative values sit in a narrow band, roughly 1 psf for a suspended acoustical tile ceiling, 1 to 3 psf for suspended gypsum board, about 1 psf for HVAC ducts, and around 3 psf for a wet sprinkler system. Those numbers are useful for sanity checking, not for specifying. A building that later takes a 6,000 lb rooftop unit and a wet system on a 2 psf blanket allowance has a problem that only steel can solve.

Give the manufacturer more than a number. State the magnitude, where the load acts, the tributary area, and whether it is uniform, concentrated, suspended from purlins or supported from primary framing. Concentrated hanging loads at mid-purlin are a different engineering problem from a uniform allowance, and the design is different too.

Deflection and drift are the architect’s call, not the manufacturer’s

Deflection and drift limits protect the finishes and adjacent construction that the metal building manufacturer does not supply, which is exactly why the architect has to set them. Deflection is measured over a member span as a fraction of L. Drift is measured over building or story height as a fraction of H. Confusing the two in a specification produces an argument during submittal review and a frame designed to whichever interpretation is cheapest to defend.

Common industry serviceability values give a starting point. Roof secondary members supporting a plaster ceiling are usually held to L/360, non-plaster ceilings to L/240, and roof members with no ceiling to L/180. Wall girts supporting metal siding commonly run to L/90, with metal panels themselves at L/60. Frame drift for an all-metal-panel building is often accepted at H/60 to H/100 under a service-level wind, which is a long way from the H/500 that a stick-built curtain wall or a rigid interior partition expects. Specify a masonry wainscot, a glazed storefront or full-height gypsum partitions without tightening the drift limit, and the building will perform as designed while the finishes crack.

In-house engineering and who seals the drawings

The metal building manufacturer's engineer is not the project Engineer of Record. The manufacturer's engineer designs and seals the building system supplied under the order documents, while the architect and the project structural engineer own the design criteria, the foundations, the non-metal-building components and the code submission. Foundation design depends on reactions and an anchor bolt plan that only the manufacturer can produce, which puts a real sequencing dependency into the schedule that many specifications never acknowledge.

Because of that dependency, the question of where the frame engineering actually happens matters more than it looks on paper. Some companies selling metal buildings are resellers who route the structural design to a third party, which turns every architect's question into a relay. Others present themselves as manufacturers with engineering under their own roof: Universal Steel of America, for one, describes its pre-engineered metal building systems as engineered in-house and supplied with PE-stamped drawings. Ask any candidate to confirm that arrangement in writing during prequalification, because when the engineer who seals the package is the one answering the phone, load questions, frame layout changes and bracing conflicts get resolved in days rather than through a sales desk.

Two verification steps are worth writing into the specification itself. Require sealed calculations and a design certification letter stating the code edition, the load standard edition and the design loads used. Ask whether the manufacturer's plant holds accreditation under IAS AC472, the inspection program that audits a metal building manufacturer's engineering and fabrication quality system, because several jurisdictions accept it in place of special inspection of the fabricator.

Code editions, wind speed basis and enclosure classification

Stating the building code without stating the edition is a small omission with structural consequences. Wind provisions have changed materially across editions of ASCE 7, including the move to risk-category-specific wind speed maps, and a manufacturer designing to a different edition than the authority having jurisdiction adopted will produce a compliant building for the wrong jurisdiction. Name the adopted code and edition, the ASCE 7 edition, the risk category, the wind speed and its basis as a 3-second gust, the exposure category, and the ground snow load with its exposure, thermal and importance factors.

Enclosure classification deserves its own line. A building specified as enclosed and then built with large overhead doors that stand open all day is, in load terms, a partially enclosed building, and internal pressure coefficients change accordingly. Aircraft hangars, fire stations, equipment shops and recycling facilities all tend to operate with openings the specification treated as closed. Snow drifting against a taller adjacent structure and sliding snow from an upper roof are the other two conditions that regularly go unmentioned and cannot be inferred from a site plan.

Openings, envelope and the interfaces nobody owns

Openings are cheap in the drawing set and expensive in the field. Every door, window, louver, skylight and mechanical penetration changes secondary framing, and framed openings added after fabrication mean cutting, reinforcing and field welding on galvanized secondary members. Locate every opening before the order documents are released, and mark which ones may move.

Envelope decisions carry the same logic. Insulated metal panels, standing seam roofs and screw-down roofs behave differently at thermal movement, at penetrations and at the transition to masonry or glazing, and the condensation control strategy for a conditioned building is an architectural decision the frame design has to accommodate. Corrosion exposure is worth stating explicitly for coastal, agricultural and food processing projects, because coating selection is not a default.

The cure for most of this is a short, disciplined design criteria sheet issued with the specification: code and edition, all loads including collateral with locations, serviceability limits by element, enclosure classification, opening schedule, required submittals and seals, and a clear statement of who designs the foundation off whose reactions. A metal building system will do almost anything the criteria ask of it. It will do nothing the criteria leave out.