Life's a beach
By: Maribeth Bradfield
The South Carolina coast is a setting of rare beauty for the new oceanfront Waddell home. The Mediterranean-style house uses an exterior insulation/finish system over concrete masonry walls, providing a complete thermal and water-resistant envelope and allowing detailed architectural shapes to be economically incorporated into the façade. While the owner's main desire was to provide a relaxing family retreat in a beautiful location, the home's designers also had to consider the significant design effects of the coastal South Carolina weather.
According to the Federal Emergency Management Agency (FEMA), the South Carolina coast can expect a hurricane to make landfall about once every seven years. Although the high winds associated with strong coastal storms are often the cause of catastrophic building failure, damage can be caused by many other factors such as well--flooding, high-velocity storm surge, breaking waves, erosion of underlying soil, intense rainfall, as well as wind- and water-borne debris.
In 1989, for example, Hurricane Hugo, one of the strongest hurricanes known to have struck South Carolina, caused almost $7 billion worth of damage. However, even in extreme events such as Hugo, damage and destruction is not a foregone conclusion. FEMA's post-storm investigations have consistently concluded that properly sited, designed and constructed buildings generally perform well.
Nature of high wind
When they strike, hurricanes can subject buildings to winds more than 100 mph, imposing large lateral as well as vertical uplift forces. The effect of high winds on a particular building depends on a number of factors, including the wind speed and duration; building height (wind loads increase with height above grade); how exposed the building is to the prevailing winds (a building on an open plain is subjected to higher loads than one surrounded by buildings or other obstructions); the strength of the building elements and connections; the building shape; and the sizes and locations of openings such as windows and doors.
Often, building failure during high winds begins with a relatively minor failure. For example, if one roof fastener pulls out or breaks, the load on the surrounding fasteners increases, often leading to their failure. A portion of the roof can be pulled away from the walls in this way, leaving unsupported walls that were designed to be supported at the roof. In fact, the most common source of failure in high-wind events is wind uplift on the roof.
To combat this threat of progressive failure, the building must be designed and constructed to maintain structural integrity during high winds.
This requires that the building incorporate continuous paths that allow structural forces to be transferred from the roof to the underlying soil, where the forces are dissipated. These continuous load paths allow wind forces on the roof to be properly distributed to the walls and then the foundation. This structural continuity relies heavily on properly sized, located and installed connectors between the building's various structural elements.
The Waddell residence
The Waddell residence, which was designed by Derrick Mozingo, AIA, of Pzingo + Wallace Architects LLP, relies on a foundation of concrete masonry piers. The depth of the foundation is such that their stability will not be compromised in the event that a significant amount of sand beneath the home is washed away during a storm. The ground level of the home is primarily open to allow storm surge to flow under the home without imparting large loads on the foundation. An alternate design for coastal homes uses washout walls, which are intentionally loosely connected to the building structure so that high wave flow removes the walls without damaging the supporting piers and the structure above.
The Waddell home's 8-inch concrete masonry exterior walls are reinforced to withstand all structural loads. In addition, special connections are used to ensure the walls are structurally continuous with the roof, floor and underlying piers. Connections for homes built to withstand high winds must be able to resist gravity loads, shear and/or pullout due to wind uplift or suction.
Connections between structural elements must be strong and durable enough to allow load transfer during high winds, maintaining the continuous load path through the home and into the ground below. Critical areas include: between the roof or floor and the walls; between intersecting walls; and between the walls and foundation. Connectors in each of these applications must be strong enough to support design loads, be properly installed and be adequately protected from corrosion for long-term durability.
The designer should carefully review manufacturers' data when choosing connectors for buildings subjected to high winds. Manufacturers' specifications will include structural ratings for the various connectors, usually in the form of ultimate capacities for resisting shear, uplift and gravity forces (for connections to wood elements, these values typically vary with the wood species used). The manufacturer will also include specific information on the appropriate number and type of fasteners to be used with each connector.
For example, in the Waddell residence proprietary hurricane clips are used to attach the roof trusses to the top plate. The hurricane clip manufacturer provides allowable uplift and lateral loads for the clip, based on the clip being installed using eight 8d-by-1.5 nails to fasten it to the roof truss, and eight more to fasten it to the top plate. Connectors are then spaced as needed to carry the design uplift and lateral loads. Anchor bolts at 32 inches are used to attach the double 2-by-8 top plate to the concrete masonry wall.
Wall to wall connections, such as at outside corners of the home, are provided by interlocking block coursing as shown in Figure 1. Two continuous No. 5 (M # 16) reinforcing bars are grouted into each corner to provide increased continuity between the intersecting walls, as well as provide a vertical load path between roof and piers. Horizontal joint reinforcement overlaps at all corners as well.
Walls are connected to the concrete masonry foundation piers by extending vertical wall reinforcement into the piers. The reinforcing bars are extended a full 3-feet into the piers to provide adequate anchorage.
The concept of the continuous load path requires not only that individual elements are tied together, but also that loads are easily transferred from one element to another. This is most directly accomplished by placing the various connections in a direct line from the roof to the foundation. For example, when considering a typical wall elevation, the roof truss to top plate connection should be directly above the top plate to wall and wall to foundation connections. When structural elements must be offset, reinforcement should be detailed such that it is structurally continuous, as shown in Figure 2 for changing the location of vertical reinforcement.
Steel connectors must be protected from corrosion to help ensure long-term strength. Coastal environments not only subject connectors to corrosive salts, but they are also more likely to be exposed to water due to wind-driven rain. For masonry connectors, Specification for Masonry Structures requires carbon steel joint reinforcement, anchors and ties to be protected from corrosion by means of mill or hot-dipped galvanizing or epoxy coatings (the specification contains specific minimum coating thicknesses based on the type of coating and the steel element to be protected). As an alternative, stainless steel connectors can be used. In fact, FEMA recommends the use of stainless steel connectors, or thicker-than-required corrosion protection coatings, for connectors in locations where rapid corrosion is expected.
Typical coastal South Carolina homes are constructed using wood framing and clad with an exterior insulation and finish system (EIFS). Concrete masonry was chosen for the Waddell residence to avoid the associated problems with moisture entry and the subsequent deterioration due to wind-driven rain often experienced in other local homes.
Although chosen primarily for its durability and decay resistance, the use of reinforced concrete masonry walls for this oceanfront vacation home also provides excellent structural durability against high winds. Attention to detail--particularly to connections and providing a continuous load path--has helped produce a beautiful home well able to weather coastal storms for years to come.