Testing Standards Required By The Building Codes

In addition to the above-mentioned ANSI/SPRI ES-1, the IBC also requires UL 580, ASTM E1592, FM 4474, and UL 1897.

Per the International Code Council, the following reference documents are connected to the required wind resistance tests: FM 4470, UL 1897, and ASTM E1592. For systems installed over solid and closely fitted deck, UL 1897 is required; for metal roofing installed over spaced sheathing or space supports, UL 580, ASTM E1592, or UL 1897 are required; and for non-structural standing seam over solid wood sheathing, UL 580 is required.

Per the Florida Building Code (FBC), the following wind resistance tests are enforced:

• UL 580, Class 90 with supplemental pressures applied per UL 1897 to meet code requirements
• ASTM E1592
• TAS 125 which includes TAS 100 Wind Driven Rain, ASTM E 1592, and UL 580
• FM 4450
• FM 4470
• UL 1897

Hayward explained, “For example, in the state of Florida, all exterior building envelope elements must either have a Florida Product Approval, Miami-Dade Notice of Acceptance (NOA) or an engineered third-party design and calculations. In fact, these documents must be submitted the Building Department or Authority Having Jurisdiction prior to ordering material and installation. Having the physical test data/Miami-Dade NOA is validation that the manufacturer’s design works. People don’t want to find out after a hurricane event that the engineering was flawed. The test standards identify the allowable wind resistance (including safety factors) of the metal panel system for use on particular structures.”

Building Design Considerations

In addition to ensuring that metal roof systems pass key wind uplift and other performance tests, architects need to ask a number of important questions when designing commercial roofing system.

Essentially, the main issue here is whether the roof can withstand wind loads, so specifiers must determine the strength of the roofing system under consideration and the size of the potential wind load.

The American Society of Civil Engineers ASCE 7 Minimum Design Loads for Buildings and Other Structures is used to determine wind pressure. Adjusted design wind pressure can be determined using allowances in AISI A5.1.3 for steel products only. Adjustments for air permeability are permitted per ASCE 7.

It should also be noted that the interpolation of test results is often necessary for the varied deck assemblies used today. This analysis should be performed by a qualified engineer.

Overall, key information used to inform the design of a roof for wind loads includes the applicable codes or standards, as noted, in addition to occupancy classification nature, physical building considerations, and building location characteristics.

Nature of Occupancy Classification are as follows:

• Classification 1 – Low hazard to human life including agricultural, minor storage, and certain temporary facilities
• Classification II – Most typical buildings.
• Classification III – Substantial hazard to human life including facilities for more than 300 people, i.e., schools, jails, healthcare, power generation, and toxic chemicals.
• Classification IV – Essential facilities including surgery, fire and rescue, communications, emergency shelters, and national defense.

For physical building considerations, the building height and parapet height are important variables. The wind speed and uplift load increases as the building height increases. Any hills or cliffs surrounding the area will also increase wind speed. Further, an increase in parapet height will result in reduced peak loads and will spread the load to the interior roof surfaces.

For building location characteristics, the architect must evaluate basic wind speed, the surrounding terrain, and exposure.

The following are key exposure categories:

• Exposure B covers urban and suburban areas and single family dwellings.
• Exposure C includes open terrain and scattered obstructions.
• Exposure D includes flat, unobstructed areas and open water extending one mile or greater.

Effects Of Wind

To understand wind turbulence, wind speed is considered as two components: mean wind speed and fluctuating component.

Wind turbulence is caused by wind speed variability and short term wind variability. The speed is always fluctuating, and therefore the energy content of the wind constantly changes. The extent of the variation depends both on the weather and on local surface conditions and obstacles.

The fluctuating aspect is caused by meteorological movement and/or ground roughness/mechanical turbulence. In the boundary layer at high wind speed, mechanical turbulence predominates. Mechanical turbulence is higher in rough terrain than in smooth terrain and decreases with increasing height.

Another factor is aerodynamics. This is the effect of the wind flowing over and around a building which produces pressures on the external surfaces and may also produce internal pressures caused by the effect of wind/structure interaction.

Photos courtesy of ATAS International

As air accelerates past sharp corners, the air flow separates creating very high negative pressures at the roof and wall corners, eaves, and ridge.

Windward walls can experience inward acting pressures called positive pressures. Local pressures are caused by the acceleration which occurs as air flows past sharp corners. Due to air flow separation, very high negative pressures develop at wall corners, eaves, ridge, and roof corners.

For canopies, the wind effects depend on the size of the canopy and its location on the windward wall. Pressures on the canopy’s upper and lower surfaces are approximately the same as those exerted on the adjacent wall surfaces. The overall wind load may be downward or upward depending on the location of the canopy on the windward wall.

Channeling between two buildings leads to increased wind speed and lower negative pressures on the facing walls of the building. Pressures are highest near the windward edge of the wall. Channeling would tend to affect walls cladding more than the roof cladding.