What About the Purlins?

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Metal building roof system design has evolved since the 1980s. Design wind and snow loads for buildings with gable roofs are updated every few years. Also, consistent research investment by the Metal Building Manufacturer Association (MBMA) and the American Iron and Steel Institute (AISI) has led to improvements in the strength prediction of thin-walled cold-formed steel purlins with partial bracing from the roof panels. But what about the purlins?

Many existing metal building roofs show significant strength deficits when evaluated with modern building standards which makes it challenging when a roof is slated for a retrofit. And industry practice allows additional dead load without engineering and calculation that can lead to more roof performance uncertainty, e.g. the International Existing Building Code (IEBC) Section 502.4 “5 % rule” which states:

Any existing gravity load-carrying structural element for which an addition and its related alterations cause an increase in design dead, live or snow load, including snow drift effects, of more than 5 percent shall be replaced or altered as needed to carry the gravity loads required by the International Building Code for new structures.

A pre-engineered metal building built in 1989 in Delaware, Ohio, is shown in Figure 1. The overall building dimensions are 144 feet long by 120 feet wide, with varying frame spacings. The roof slope is 2 on 12. The building was designed according to the 1987 Ohio Basic Building Code which references the 1984 Building Officials and Code Administrators National Building Code.

The purlins are 12 inches deep with 2.5 inch wide flanges and 1-inch stiffening lips oriented 45 degrees from the horizontal flange. There are two purlin base metal thicknesses, 0.067 inch (interior spans) and 0.075 inch (end spans). The purlins are spaced at 5 feet on center and they are assumed to lap 1.5 feet on either side of a frame. (Fig. 2). The roof deck is assumed to be a 24 gauge R panel. The purlins are rolled from ASTM A570 steel with a nominal yield stress of 55 ksi. It is assumed that the building was designed for 25 psf live load plus 5 psf defined in the general notes, and this load is applied everywhere on the roof.

It is challenging to know exactly how purlin line strength was determined for the original building. Distortional buckling, i.e., partially restrained flexural-torsional buckling of the purlin compressed flange, had not been incorporated into the AISI standard in 1989, and combined action checks considering flexure, torsion, and shear were also not fully developed yet. Metal building manufacturers typically relied on simply-supported single span pressure box tests (base tests) and extrapolated these results to continuous spans which could miss strength limit states near the frame line purlin laps–for example the common failure mode of bottom flange purlin buckling near a lap (Fig. 3). Now, the AISI S100-16 North American Specification for the Design of Cold-Formed Steel Structural Members provides approaches for considering all the potential strength limit states including combined actions and even partial bracing from attached roof panels, see AISI S100-16 Section I6.1.

In this study the building purlin strength for gravity loads is calculated with the open-source software PurlinLine.jl (https://www.runtosolve.com/purlinline) utilizing AISI S100-16 Allowable Stress Design (ASD) and structural analysis (https://github.com/runtosolve/ThinWalledBeam.jl) to predict the roof pressure where the first strength limit state develops. This software considers the continuous spans, varying purlin sizes (end spans versus interior spans), and the partial lateral bracing provided by the R panel roof deck.

For gravity loads on this building’s roof, the typical 6 span purlin line allowable pressure is 19.6 psf. (A detailed structural report is available https://runtosolve.com/purlinline/structures_magazine]). The failure develops at the end of the lap in the 30 feet end span, consistent with Figure 3. The strength limit state is predicted by AISI S100-16 to be shear + flexure interaction, which makes sense for this 12-inch-deep purlin.

The original demand design load for the roof is 30.0 psf, and the predicted ASD purlin line strength is 19.6 psf, resulting in a demand-to-allowable strength ratio of 1.5 which means the purlins are probably overstressed at ultimate loads (Table 1). This is an example where industry practice of applying the IEBC “5% rule” should be questioned.

What would happen if this building was designed with modern building codes? Consider the unbalanced snow load described in ASCE 7-22 Section 7.6.1 since it will create the most severe conditions for the purlins near the ridge. The ground snow load in Delaware, Ohio, for Risk Category II is 32.0 psf, see ASCE 7-22 Figure 7.2-1B. The unbalanced snow depth is calculated as 2.7 ft., resulting in an ultimate-level unbalanced snow load on the leeward side of the roof of 42.0 psf extending 18 ft. down from the ridge. The ASD snow load factor is 0.7, resulting in a demand pressure of 29.4 psf and a demand-to-capacity ratio of 1.5. Both the original design and a design check following modern codes shows that the roof purlins are overloaded by about 10 psf.

When a metal building roof system like this one in Ohio is found to be overstressed, common engineered roof retrofit solutions considered are: adding additional purlin lines or reinforcing the existing purlins.

Roof retrofits where additional purlin lines are placed between the existing lines (Fig. 4) are effective and can bring a roof up to modern structural performance standards. This solution is not usually appealing to a building owner, however, because it requires either opening up the roof or disruptive work inside the building, either of which can be time-consuming and costly.
An exterior purlin line reinforcement solution like that shown in Figure 5 can be implemented while the building is still in service. A roll-formed pre-punched hat subframe is fastened to the existing purlin lines, adding 10 psf or more capacity to the roof system. A new roof (typically a standing-seam roof) is then attached to the hat subframe top flanges.

The roof purlins in this Ohio metal building were originally designed at or beyond their calculated structural limits. This does not mean that all metal building roofs are about to fail in a snowstorm. The conclusions herein are intended to highlight that: (1) there can be concerning mismatches between existing metal building roof strength and code-specified demand loads; (2) use of the IBEC “5 % rule” should be supported by engineering calculation; and (3) roof retrofit solutions are available to mitigate existing metal building roof system failure risk . ■

About the Authors

With over 45 years of experience in the metal construction industry, Tim Lane is the owner and president of TopHat Framing Systems. TopHat offers patented subframe systems as solutions for metal roof retrofit projects.

Cristopher D. Moen, Ph.D., PE, F.SEI is an engineer, software developer, researcher, and educator. His company RunToSolve LLC creates fast open-source computational tools for predicting structural system performance.