By Phillip Cutler, P.E., and Mason Nichols
It has been nearly three years since Joplin, Mo., experienced the wrath of Mother Nature. The city was hit by an EF5 tornado, a monster twister (see the sidebar “Fujita Tornado Intensity Scales Explained”). Joplin was literally leveled by the storm. More than 160 people lost their lives on that horrific day, and the city is still recovering and rebuilding from the devastation.
This time, let’s rebuild smarter with precast concrete
As part of this rebuilding, city planners, community officials and directors of construction are planning for the future, and precast concrete will be part of that future. Precast concrete is being chosen as the preferred solution in the building of high-wind shelters or precast concrete safe rooms for schools.
According to Mike Johnson, director of construction for Joplin Schools, there were plans for 11 safe rooms in all to be constructed with cast-in-place concrete. “They were cast-in-place because of the job opportunities it would offer the local work force.” Johnson said. “There are no precast companies here in the immediate area, so we bid it both ways on our first batch of safe rooms to see which was more cost effective. And it was a slam dunk, the precast came in well under the cost of cast-in-place. That’s how we wound up with precast. We looked at it both ways and precast won the day.”
The Joplin School District has partnered with Nabholz Construction Services as the construction manager for its 11 high-wind shelters. These projects will result in a safe room at each of 10 Joplin elementary schools. Some of these precast concrete structures will also serve as an auxiliary gym facility, a student locker room and high school concession services.
“Nabholz has eight precast storm shelters under construction and one that is constructed of load-bearing masonry,” said Jeff Gattis, senior project manager with Nabholz Construction Services. The remaining two shelters are in the final stages of design. A prestressed concrete construction with numerous special connections and reinforcing make up the shell.
The safe rooms are built to FEMA 361 guidelines (see the sidebar “Precast Concrete Safe Rooms are FEMA Compliant”), plus local codes as determined by the architect and project engineer, added Gattis. The guidelines and codes are also integrated into the spec depending on the safe room location.
Precast concrete provides a safe haven
Safe rooms must be safe, and so the big question is what precast concrete brings to the table. “Plant controlled production of the shell ensures high concrete strengths, and one-component wall systems can provide integrated insulation and finishes at both faces,” explained Gattis. “Concrete provides a weight and durability advantage, which, along with prestressed reinforcing, results in a wall-and-roof system that meets the requirements for the shelters – and does so economically.”
Precast allows for the structure to be built in a controlled environment while site preparation is being completed. Ideally the completion of site prep and the off-site precast construction is completed around the same time so that the erection can start immediately. “These 8,000-sq-ft structures are being erected inside of two weeks, which is great for the project schedule,” said Gattis.
“This project is one that the Nabholz team is very proud to be a part of,” continued Gattis. “The town of Joplin was so devastated by the tornado. Being a part of a team that helps to provide a safe haven for the kids of Joplin and the surrounding community is very fulfilling.” The precast producer for the project was Prestressed Castings Inc. in Springfield, Mo.
If we intend to protect citizens – and particularly our vulnerable school children – from the horrifying death and damage experienced in the Joplin tornado, we need to rebuild storm-damaged cities with stronger, more durable infrastructure. Precast concrete safe rooms are just one of the stronger, safer and smarter solutions precast can provide storm-prone areas. Find more precast solutions at precast.org or contact a local precast manufacturer at precast.org/find.
Phillip Cutler, P.E., is NPCA’s vice president of Technical Services.
Mason Nichols is NPCA’s communication coordinator.
Sidebar 1 – Precast Concrete Safe Rooms are FEMA Compliant
By Evan Gurley
To ensure that safe rooms are structurally sound units that provide near-absolute protection from adverse elements, FEMA has developed design, construction and operation criteria for architects, engineers, building officials, local officials, emergency managers and prospective safe room owners/operators. The two design guidelines are FEMA 320 and FEMA 361.
FEMA 320 outlines the design criteria for the development of residential safe rooms (16 persons or less), while FEMA 361 covers the development of public and community safe rooms (more than 16 people).
Using the FEMA guidelines as a standard, design and construction professionals led by the International Code Council (ICC) and the National Storm Shelter Association (NSSA) have joined forces to produce the first ICC/NSSA Standard for the Design and Construction of Storm Shelters (ICC-500). Manufacturers of products meeting this standard assure prospective owners that their safe rooms will be able to provide life-safety protection. While fully supporting this effort, FEMA has continued to promote FEMA 320 and FEMA 361 guidelines to communities and individuals seeking further guidance.
Due to the implementation of the ICC-500 standard and other national, state and local protection initiatives, FEMA identified a need to distinguish between a “safe room” and a “shelter,” as the terms have been used almost interchangeably in the past. While FEMA and ICC criteria are both designed to ensure life-safety protection, only units meeting FEMA criteria provide “near-absolute” protection from extreme wind events. Therefore, FEMA refers to the term “safe room” as all shelters, buildings or spaces that are designed to the FEMA criteria. Buildings, shelters or spaces designed to the ICC-500 standard are termed as “shelters.” So all safe rooms designed to the FEMA criteria meet or exceed the ICC-500 requirements.
Design basics
Residential and community safe rooms designed to meet FEMA 320 and FEMA 361 criteria follow these basic principles:
FEMA 320
- Located in an area that is quickly accessible
- Built in an area where flooding will not occur
- Readily accessible from all parts of the home, business or critical facilities (building or facility occupied by large numbers of people)
- Free of clutter and obstacles
- Adequately anchored to resist overturning and uplift (if specified by design)
- Built with connections that can resist failure
- Built with walls and roof that can withstand windblown objects (designed for 250 mph winds)
- Designed to resist a 15-lb wooden 2×4 in. board traveling horizontally at 100 mph and vertically at 67 mph (ASCE 7-05).
FEMA 361 (additional criteria to FEMA 320)
- Designed for all cases as partially enclosed buildings
- Special life-safety protection elements when occupancy is 50 or more people
Evan Gurley is a technical services engineer with NPCA.
Sidebar 2 – Fujita Tornado Intensity Scales Explained
By Phillip Cutler, P.E.
To put an EF5 tornado in perspective, we need to take a look at the wind power it can generate. Regularly, we hear about earthquakes and likely have a much better understanding of the ratings for the strength of earthquakes based on the Richter scale. It is a very simple rating scale and is based on a logarithmic function using powers of 10. Simply put, a 5.0 on the Richter scale is 10 times stronger than a 4.0.
Table 1: Fujita Tornado Intensity Scale
In contrast, the rating for the strength of a tornado is not as easy to understand, because it is based on the Fujita Tornado Intensity Scale (see Tables 1 and 2). The Fujita scale is based on the amount of damage a tornado could potentially produce.
For example, an F0 tornado is referred to as a “gale force” tornado and has a sustained wind speed of approximately 73 mph (think about how it feels to stick your arm out the window of a car traveling on the Interstate). As we progress up the Fujita Scale, you begin to get some relative perspective on the destructive power of wind as their velocities increase. F4 and F5 tornados accounted for only about 1% of all tornados that occurred between 1950 and 1994. EF5 tornados, like the one that destroyed the city of Joplin, are extremely rare weather events.
Enhanced Fujita Scale
As most measurement equipment in use today would likely not survive the destructive force inside an F3 tornado, the EF Scale was developed. The EF Scale, or Enhanced Fujita Scale, is based on estimates of the maximum wind speeds in the various categories. An EF5 tornado signifies total destruction for most structures in its path.
Table 2: Enhanced Fujita Scale
Phillip Cutler, P.E., is NPCA’s vice president of Technical Services. Contact him at [email protected] or (800) 366-7731.
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