Fort Lewis Barracks

  The award-winning Whole Barracks Renewal project in Fort Lewis, Washington has always had loftly goals. WJA Design Collaborative took on the challenge to design a new barracks that could provide energy efficient, aesthetically pleasing, functional living areas for 300 Army soldiers while also complying with safety requirements, such as anti-terrorism force and fire protection.

At the time of the proposed design, “the lead time on structural steel was about nine months, and the construction schedule just didn’t allow that type of delay,” says Steve Borman, president, Keystone Masonry Inc., the masonry contractor. “Rather than use steel, we proposed they use loadbearing CMU with hollowcore concrete plank floors.”

The Army Corps of Engineers agreed to use concrete masonry as the structural building material. The base of the building uses burgundy, split-face block, which continues up to the first floor window sills. From these sills on up, burgundy half-high, smooth-face CMU were used while the upper floor uses cream-colored units. “Using CMU definitely expedited the schedule,” states Borman.

“I believe we put that building up faster with CMU than it could be done framed.” “Budget and schedule are critical elements to the Army,” says Dan Callan, principal, WJA Design Collaborative. “We were able to stretch the  budget by using a structural concrete block masonry wall system that emulates the look and color of adjacent brick veneer structures.”

Another big advantage of using CMU on this type of project is that “it helps meet the progressive collapse and force protection requirements that can be pretty stringent for military projects,” says Mike Steinthal, Absher Construction Company. “If you use woodframe construction, you have to go back and beef up the structure quite a bit to meet those same requirements. By using concrete masonry units and placing a little extra reinforcement and solid-grout, it allowed us to easily comply.”

This structure has been a huge success and is the first at North Fort Lewis to attain LEED Silver certification for its energy savings and sustainable features. The project was awarded a 2007 National Design Build Award. Regarding the success of using CMU on the structure, Ted Lewis, project engineer, Army Corps of Engineers says, “It met all of our needs and gave us adequate force protection. It blends in well with other structures at North Fort Lewis. It turned out really nice.”

 

Owner
Army Corps of Engineers, Fort Lewis, WA

Architect
WJA Design Collaborative, Seattle, WA

Structural Engineer
WJA Design Collaborative, Seattle, WA

General contractor
Absher Construction Company, Seattle, WA

 

“ [Using CMU] helps met the progressive colapse and force protection requirements that can be pretty stringent for military projects.”

Mike Steinthal, Director of Government and Defense Contracting Group, Absher Construction Company

 
 

Blast Resistance with Concrete Masonry

Concrete masonry has long been considered an excellent material for building “secure” structures—from the backyard bomb shelters of the 1950s, to the high security prisons and seismic resistant buildings of today. In recent years, protection from terrorist attacks has become a higher priority for many buildings.

While the type and size of a terrorist attack  cannot be predicted, guidelines for improving building performance are available. The mass of concrete masonry is beneficial for blast resistance. Masonry walls also protect against ballistics and shrapnel (flying debris from the bomb). Properly designed concrete masonry and glass masonry products provide protection for people, essential facilities, computers, and security systems.

The overriding design philosophy for blast resistant structural systems is to prevent progressive collapse of a structure subjected to a blast load. Structural redundancies should be provided to carry additional loads that may be imposed after a bomb attack. For example, beams, girders, and columns should be detailed to carry the loads of damaged slabs or columns. The Oklahoma City Bombing: Improving Building Performance Through Multi-Hazard Mitigation (Emergency Management Agency Mitigation Directorate and American Society of Civil Engineers.) recommends the use of one of the following structural systems for seismic and/or blast resistance: compartmentalized building, special moment resisting frames, or dual systems which are a combination of the two. These systems provide the mass and toughness necessary to reduce the effects of extreme overloads on buildings, and have typically shown good earthquake resistance.

As the name suggests, compartmentalized buildings are composed of structural “compartments,” which can act somewhat independently. Reinforced structural walls are typically used to provide structural integrity in case part of the building is damaged, thus preventing progressive collapse. The design results in a stiff, massive structure capable of withstanding significant loads. Concrete masonry is well suited to compartmentalized buildings. In fact, masonry shear wall structures designed to current standards have outperformed frame systems in limiting damage from earthquakes and hurricanes.

Concrete Masonry Shear Walls (TEK 14-7. National Concrete Masonry Association) provides more detailed design information. Special moment resisting frames rely on detailing the building joints so that elements adjacent to the damage will continue to function as designed. Hence, damage is prevented from spreading. Detailing requirements are thorough and restrictive to help ensure adequate protection. The design results in a relatively flexible structure that can withstand significant deformation without failure. In buildings designed as special moment resisting frames, concrete masonry is often used as infill between the frames, providing in-plane shear transfer, thereby stiffening the frame. Unlike compartmentalized buildings, special moment resisting frames can provide large open spaces that may be more desirable for some building types.

Dual systems make use of two or more structural systems in combination to resist seismic or blast loads. For example, a large office building may have a large atrium or other open area at the front. This building may utilize a special moment frame at the front of the building and a shear wall for the back.

Dual systems are subjected to rigorous design requirements to ensure structural compatibility between the systems used. For instance, in the example above, the design must adequately account for the differential movement between the flexible moment frame and the stiff shear wall.

The Federal Emergency Management Agency/American Society of Civil Engineers report on the Murrah building states that buildings currently designed and detailed to resist seismic events will provide some measure of blast resistance.

Seismic design requirements are included in the 1994 edition of NEHRP (National Earthquake Hazard Reduction Program) Recommended Provisions for Seismic Regulations for New Buildings. NEHRP requires that walls in the highest seismic categories contain minimum areas of both the horizontal and vertical reinforcement of at least 0.007 times the gross cross-sectional area of the wall. It also requires the sum of the horizontal and vertical reinforcement areas to be at least 0.02 times the gross-cross-sectional area of the wall. In addition, minimum reinforcement must be placed in certain areas of the wall where stress concentrations may exist as indicated in the figure above.

For further information on the blast resistance, see TEK 14-21 Blast and Bullet Resistance of Concrete Masonry. This and all other NCMA TEK are available  free on line courtesy of NCMA sponsoring members. A list of e-TEK sites is available at www.ncma.org.

A new publication will be available later this year from NCMA, Blast Resistant Design Guide for Concrete Masonry Structures. CMD

 

Progressive Collapse

Progressive collapse is defined as a situation where local failure of a primary structural component(s) leads to the collapse of adjoining members, which in turn leads to additional collapse. Hence, the extent of total damage is disproportionate to the original cause. Another way of describing progressive collapse is a chain reaction or propagation of failures following damage to a relatively small portion of a structure. Regardless of the definition, blast loading or other unforeseen events can cause progressive collapse due to damage of some key element(s) which can either make the structure unstable or trigger the failure of the main portions of the gravity structural system.

Source: “An Engineers Guide to: Concrete Buildings and Progressive Collapse Resistance”, Portland Cement Association, www.cement.org.