In recent years, reports of natural disasters seem to be ever more common. On Aug. 24, 2017, Hurricane Harvey made landfall at San Jose Island, Texas, breaking a record 4,323-day drought in major hurricanes making landfall in the U.S. Harvey rivaled Hurricane Katrina as one of the costliest storms with an estimated $125 billion of damage. Harvey was followed by Hurricane Irma, which made landfall in the Florida Keys, and then hurricane Maria devastated Puerto Rico soon after.
In 2018, Hurricane Michael became the first Category 5 hurricane to make landfall in the continental U.S. since hurricane Andrew in 1992, all but destroying the town of Mexico Beach, Fla. Although hurricanes and major storms are not preventable, the related damage can be mitigated through improved design of new buildings and retrofitting of existing buildings.
Building enclosure integrity plays a key role in protecting a structure from hurricane damage. If a building enclosure is breached, internal building pressures are increased, which has an additive effect to external wind pressures and can lead to a complete structural failure. If you do business in hurricane-prone areas, now is the time to evaluate and harden your building enclosures to withstand these events.
Establish Performance Criteria
The first step in considering a hurricane hardening retrofit of an existing enclosure is to establish a desired outcome in terms of the design event, as well as acceptable post-event performance. What types of events is the enclosure to be designed to withstand? What level of post-event performance is acceptable? Design events will be dependent on the building location and occupancy type.
The required/desired post-event performance level is ascertained by the building function, as well as the budget and individual owner preferences. For example, the performance requirement for a post- disaster structure, such as a hospital, may be to sustain minimal damage and remain operational following a design event; whereas the requirements for a residential building may be that the structure remain intact and repairable following a design event. Some owners may elect to establish higher performance levels than others for individual reasons. The key is to establish an acceptable outcome for a given design event and evaluate the existing building based on acceptable outcome.
Typical categories for desired performance post-disaster are:
- NO PERFORMANCE: Substantial damage to building; occupancy may pose life risks; may not accommodate disaster evacuation.
- LOW PERFORMANCE: Substantial damage to building; repairs are necessary; delays in re-occupancy; risk to occupants.
- MODERATE PERFORMANCE: Moderate and repairable damages; minimal impact to occupancy; low risk to occupants.
- HIGH PERFORMANCE: No substantial damage to building; fully operational through disaster.
Evaluate the Existing Building
The building enclosure is the first line of defense against harsh weather. Wind loads are carried by the enclosure back to the primary building structure. The building enclosure and primary structure must work together as an integrated system. If the enclosure fails, the structure is exposed to increased wind pressures and water infiltration, resulting in damage and possibly failure of the primary structure. Likewise, the ability of the building enclosure to carry wind loads is limited by the primary structure. The enclosure can never carry any more loading than the primary structure that supports the enclosure. In designing a retrofit, it is key to follow the load paths, identify the weakest link in the chain and initiate the retrofit design from this point.
When considering a retrofit for high winds, first establish a design wind speed and calculate the associated wind pressures. Design wind speeds and pressures are established using wind-speed maps and equations from ASCE 7-10 “Minimum Design Loads for Buildings and Other Structures”. Following ASCE 7-10 provides design wind pressures for the building structure and the exterior cladding elements.
Design wind pressures are used to evaluate the building structure. If the primary structure is not able to withstand the design pressures, the cost and feasibility to retrofit the structure is evaluated and a feasible level of primary structure retrofit established. This provides a performance level for the primary structure and determines the level of retrofit needed for the building enclosure.
As an example, ASCE 7-10 calculations may determine a design uplift pressure of 100 pound per square foot for the roof, but the existing roof may have a significantly lower structural capacity. Retrofit of the roof structure to 100 psf may not be feasible. In this case, replacement of the roofing system with a new system rated to 100 psf would be of no value because the roof structure itself would fail prior to the roofing system.
Enclosure Elements to Consider
The following building-enclosure elements should be reviewed and considered when retrofitting a building for high winds: