structural RETROFIT Wind Retrofit Resources for Structural Engineers

By William L. Coulbourne, P.E., F.SEI, F.ASCE

Every year, the nation experiences many high wind events, including hurricanes and tornadoes, and signicant straight-line winds in some years. ese high wind events frequently damage buildings; that damage must either be repaired or the building demolished and re-built. e visual image this damage creates often makes owners of similar buildings wonder about the wind resistance of their own buildings (both residential and commercial). As a result, some owners seek expertise in determining the wind resistance of their buildings and look to retrot the building with improved wind resistance.

ere is not much information published about wind retrotting. is article intends to provide some helpful resources to the practicing structural engineer should they be retained to provide wind retrotting expertise.

is article addresses resources that can be used for all buildings, including residential and commercial. However, retrotting a residential building is not frequently attempted; it seems to be more common to build a residential building back stronger once it has been damaged. On the other hand, commercial buildings are often retrotted, especially buildings used as critical facilities in a community. Manywind damaged schools, re stations, and hospitals are examples of completed wind retrot projects.

Several federal and state grant programs fund wind retrots. Improved wind resistance of critical facilities is intended to improve resiliency in the community and help speed up recovery from disasters. Understanding the problem and using all available retrot resources for wind retrotting is essential for the structural engineer.

e following is a review of all of the resources the author researched. A summary from each resource is provided so the reader can determine how each resource might benet their particular project.

Hurricane Michael

FEMA’s Recovery Advisory 1 (2019) focuses on the performance of critical facilities during Hurricane Michael. It provides good examples of successes and failures of retrotting and includes a helpful ve-step process for improving wind resistance.

is document does a good job of explaining the care that must be given to assessing all components of the wind resistance elements so that retrot dollars are spent wisely. An excellent example is illustrated in the damage shown in Figure 1, where the door frames failed during the high winds, which caused the doors to fail. e internal wind pressure caused by the door failure also damaged the roof system parallel to the doors. Ultimately, grant funds were spent on new doors.

is recovery advisory suggests a ve-step process for improving wind resistance by retrotting. is process is shown in Figure 2 as a owchart. e initial step is a comprehensive vulnerability assessment so the engineer can determine what the most critical retrots might be and how much other parts of the building might be aected if the most critical element was damaged by wind.

There are numerous examples throughout the resources that indicate thorough assessments were not made initially. erefore, when high winds impacted the buildings, the retrotted elements did not perform as needed or expected because some other element was not suciently strong to resist the wind load. Figure 1 is a good example – new strong doors were installed, but the door frames were not reinforced to be able to resist the wind pressures imparted to the doors.

Table 1 lists some common high wind vulnerabilities with associated possible common failure modes and some common retrot methods used for listed elements.

Home, Commercial, and Multifamily Programs

FORTIFIED, by Insurance Institute for Business and Home Safety (IBHS) (last revised 2022), has programs for three dierent building types –homes, commercial, and multifamily buildings. e program designations are similar to FEMA P-804 (described later). ere are construction standards for each building type that must be met for the building owner to receive insurance premium reductions for improving the wind resistance of their building. Some of the standards are easier to complete if included in new construction; others can be successfully completed as retrot projects. ere are three program levels: Roof, Silver, and Gold.

Roof focuses on roof system improvements, including stronger roof surfaces better attached to the substrate, sealed decks for steep-sloped roofs, strong skylights to resist water intrusion and wind-borne debris, and roof-mounted equipment designed for higher wind pressures.

Element Common Failures Retrot Methods

Glazing

Roof Coverings

Roof structure

Rooftop equipment

Sectional garage and rolling doors

Sof ts

Wallcoverings

Breakage from wind pressure, breakage from wind-borne debris, damage to glazing frames

Covering is peeled back from roof, wind-borne debris punctures cov-ering, attached coverings pull over attachments such as nails or screws

Plywood substrate is pulled off roof, wood or steel trusses collapse, gable end walls collapse, roof structure fails at roof-to-wall connection

Rooftop equipment is lifted or moved off its base

Doors implode from wind pressure, door tracks are bent, allowing the door to slide out of tracks

Sof ts are pushed up into attic space or pulled out of sof t sup-port

Wallcovering removed by wind pressure, water inltration occurs when siding is removed, wall cov-ering punctured with wind- borne debris

Silver focuses on glazed opening protection from wind-borne debris and wind pressure, strengthening anchorage of attached structures, impact-resistant wall systems, and parapets and false fronts adequately secured or braced.

Gold focuses on load path continuity throughout the structure.

FORTIFIED Multifamily has the same protection levels as homes, except the focus is on commercial buildings. is building type was recently added. Figure 3 below is an application ow chart of the wind improvement method for multifamily buildings.

Wind Retrofit Guidance for Residential Buildings

FEMA’s P804 , Wind Retro t Guide for Residential Buildings (2010), focuses on existing residential buildings. It covers three typically

Replace with impact-resistant glazing, strengthen attachment of glaz-ing frames to structure, cover with impactresistant shutters/covers

Replace membrane roof coverings, provide secondary membranes to avoid puncture damage, re-fasten coverings with more nails/screws

Add fasteners through the sub-strate to trusses/framing when roof covering is replaced, add bracing to gable end walls, add roof-to-wall hurricane clips

Secure equipment to roof curbs and roof framing

Add reinforcement to large doors, add reinforcement to door tracks, jambs, and headers

Add supports to sof t framing to keep sof t in place

Add fasteners through siding to wall substrate, replace siding with more wind-resistant siding; if sid-ing is removed, replace substrate with more debris impact-resistant substrate vulnerable areas: roof and wall coverings, openings (windows/ doors), and load path connections. However, it uses wind design information from ASCE 7-05, Minimum Design Loads for Buildings and Other Structures , which dates the FEMA document since there have been three revisions to ASCE 7 since the ASCE 7-05 publication.

e retro t ideas are grouped into “packages” similar to the method used by IBHS noted earlier. e retro t process is initiated with a condition assessment of the existing home before suggesting the most appropriate retro t method.

e Basic Package focuses on the roof covering and roof structure. It includes methods to improve the performance of the roof covering (including replacing it), strengthening vents and so ts ( Figure 4 ), and strengthening overhangs at gable end walls.

e Intermediate Package focuses on protecting openings (both windows and doors) with impact-resistant glazing or coverings and bracing tall gable end walls.

e Advanced Package focuses on load path continuity and protecting openings from design wind pressures.

Applicants who seek FEMA grant money for wind retrotting must comply with the wind retrot requirements of P804. e costs and benets of each package are also discussed.

Wind Vulnerability Assessment

FEMA P-2062, Guidelines for Wind Vulnerability Assessment of Existing Critical Facilities (2019), provides considerable detail and depth on various wind vulnerabilities and materials used for construction. is document describes how those materials might fail under high wind pressures and how to test their vulnerability to high wind pressures. e vulnerabilities cover both vertical and horizontal load paths and building envelope and building equipment issues.

A table in P-2062 provides the expected wind performance of a building in various design-level wind events dened by Mean Recurrence Intervals (MRI). e table is shown here as Table 2

is guidance document covers the widest variety of building materials of any of the researched publications. It covers masonry (concrete block and brick), Exterior Insulation and Finish Systems (EIFS), stucco, metal panels, ber cement siding, vinyl, and many roof system materials, including single-ply membranes, asphalt shingles, standing seam metal panels, and others.

ere is a discussion of water inltration and the impact such water has on building damage. Structural failures are described, such as that shown in Figure 5 of an end wall collapse in a relatively new metal building system.

Summary

ere are several good resources for the structural engineer to use for guidance on wind retrotting; four of the best and most complete that the author has found have been noted and summarized in this article. ese resources recommend beginning a retrot project with a vulnerability assessment of the existing building so that retrot dollars are spent on the most eective retrot projects. Each of the four resources provides a step-by-step process that varies by building type and age. In all cases where retrot grants or insurance discounts are being pursued, the chosen retrot method must be followed step-by-step.■

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