Article No: 142

2006-05-03 08:23:29
Designed for strong winds
By: National Concrete Masonry Association


Photography courtesy of National Concrete Masonry Association

As an area threatened with hurricanes each year, one of Florida's newest coastal residences uses concrete masonry to stand up against high winds.

The warm waters of the Florida coast have been attracting beach lovers for decades. The small town of Cocoa, known for its close proximity to Cape Canaveral, brings an enjoyable living environment to residents. Whitley Bay is a 13-story concrete masonry luxury condominium complex located on the Indian River Intercoastal Waterway. The complex includes a 10-story building with two lower levels of parking and a three-story retail and marina building that features an elevated swimming pool on the third floor.

According to Maath Bennett, vice president of Benko Construction, "We use concrete masonry for these buildings because the material takes no abuse. Our customers like the look and feel, the solidness of concrete masonry."

The building owners have been using concrete masonry for more than 70 years in this area because of its inherent qualities. "We have been involved with construction in this area for more than 70 years, and there is no other building material like concrete block," said Michael Mervis, assistant to the chairman of Zilber, Ltd., the owners of Whitley Bay. "We have an extremely high level of laborers in our area. With a great product like block and a great labor force, there is no other way to construct a building. The durability, ease of design, flexibility and general quality of construction make concrete block the material of choice."

Filling in with concrete masonry
The condominiums at Whitley Bay are designed using high-strength (f'm = 2000 psi) 8- and 12-inch reinforced concrete masonry. Sean C. Burlingham, P.E. of Burlingham Engineering, served as the structural engineer for the project. "We used several types of masonry construction methods," says Burlingham. "Some of the walls were constructed with the building. For example, the walls were built with the building, prior to the floor slabs above them. But the vast majority of the building's masonry walls were built using infill masonry."

Infill masonry is a typical design method for multi-story buildings. Infill concrete masonry walls use the concrete masonry as cladding and interior partitions between concrete or steel frames, which form part of the structural load-resisting system. Concrete masonry walls are often used in this application because of the cost effectiveness and ease of construction.

At the Whitley Bay complex, the concrete masonry walls are the structural elements that transmit the wind into the building structure, loads which are then transmitted into the foundation system through the concrete shearwalls. Most of the wind pressure on the building is being applied to its masonry walls, which must be structurally adequate to handle these heavy and dynamic loads. Reinforced concrete masonry performs this vital task quite well.

"We cast grout ports into the fill slabs, through which vertical reinforcing and grout was placed into the cells of the walls built between the floors," Burlingham explains. "This method allows us to quickly erect the building skeleton (the columns, shearwalls, elevator shafts, stairwells and floor slabs) while leaving the building sidewalls open for access to the flying forms used for the floor slabs.

"The infill construction method also keeps the masons onsite and busy full-time because they are building walls at the working deck level and, at the same time, constructing infill walls several floors below the working deck."

Battling strong winds
In addition to designing a home that met the needs of its residents, the designers had to consider the high wind activity that often hits coastal Florida. While Florida typically experiences only one hurricane per year, according to the National Hurricane Center, the result of these high winds associated with strong coastal storms can be devastating and costly - to the tune of millions of dollars in damage. As with all building construction, a multi-story building with proper siting, design and construction performs well during these high-wind events.

"The Whitley Bay project was designed to withstand a 133-mile-per-hour hurricane wind without damages," says Burlingham. "It is also rated as 'Exposure D,' which is for hurricane winds coming across open water, the most stringent of wind design requirements."

Exposure D requires the designer to consider much higher wind pressures for a given wind speed. For example, suppose you have two buildings designed for 133-mile-per-hour wind. One is rated as "Exposure B," which is for urban areas surrounded by houses, small buildings and wooded areas (a typical suburb). The other is rated as "Exposure D" (such as Whitley Bay), which is for wind flowing over open water before hitting the building. The design wind pressures for the building rated as Exposure D are 30 to 40 percent higher than those for the Exposure B building. Both are designed for the same wind speed, but the physical location of the building becomes a large factor in determining the actual wind pressure that the building is designed to withstand.

Connecting all the pieces
Connections between individual building elements - roof, walls, floors and foundation - are critical to maintaining structural continuity during a high-wind event. The critical damage to buildings in such events typically occurs because of uplift on the roof, resulting in the loss of crucial diaphragm support at the top of the wall. A primary goal for buildings subjected to high winds is to maintain a continuous load path from the roof to the foundation. This allows wind uplift forces on the roof to be safely distributed through the walls to the foundation. If one part of the load path fails or is discontinuous, building failure may occur.

Connections between building elements are key to the performance of the structures and should therefore be considered carefully during the design process. Connections should be simple and easy to construct and, where necessary, should accommodate building movements from expansion and/or contraction of building materials.

Concrete masonry multi-story buildings have been capable of withstanding some of the most severe windstorms with little damage. Burlingham noted the concrete masonry in the Whitley Bay Project helps put the safety and strength behind it: "This building is where I would hide if another Hurricane Andrew were to take aim at our county."

References:
TEK 3-12, Construction of High-Rise Concrete Masonry Buildings (1998)
TEK 5-11, Residential Details for High Wind Areas (2003)