Tornado Damage Investigation, Greensburg, Kansas, 1699 DR-KS
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were coordinated with Jim Donley of FEMA Region VII and Chris Hudson of FEMA Headquarters Mitigation.
URS was tasked with assessing the damage in Greensburg and with classifying the tornado using the Enhanced Fujita tomado damage scale. The methods used in conducting this work are described in the section titled Study Methodology.
No attempt was made to compare the tornado wind speeds experienced in Greensburg with any building code design wind speeds. Most of the buildings in Greensburg were designed and constructed to older building codes, such as the Uniform Building Code (UBC), although a few newer buildings may have been designed and constructed in accordance with the International Building Code (IBC). Regardless of the code used (IBC or older code such as the UBC), there are currently no design guidelines in the IBC for catastrophic wind events of the magnitude experienced by Greensburg (although there are high wind provisions in the building code for hurricane-prone areas), so a building code comparison was judged to not provide any useful or concluding information.
The work also included developing three recovery advisories that the community, or any community that is impacted by a tornado, can use as reconstruction guidance. One tornado risk advisory was developed to provide the community with a better idea of how the risk of tornadoes in Greensburg might compare with other areas of the country and how that risk might affect decision-making about shelter locations, design, and construction. A second tornado risk advisory was written to provide storm shelter design criteria describing the design concepts important for tornado events. The third tornado risk advisory was written to provide residential sheltering guidance to residents who want to create a safe refuge inside their home. These advisories will soon be available on FEMA's website.
2.1 STUDY METHODOLOGY
In order to assess the damage in Greensburg, two site visits were conducted (May 10-11 and May 21). The observations made during the site visits were used for two purposes. First, the observed damage for 46 residential structures was used to rate the damage according to the EF Scale and thereby derive estimated wind speeds (see Section 3.1). The estimated wind speeds were then mapped (Section 3.3). In addition to these 46 residential structures, additional DOD assessments were made in the field; these assessments included damage to seven non-residential structures and trees in the tornado path.
Second, specific damage information was collected at four sites to perform structural analysis and evaluate the failure stresses in the materials. These buildings were selected where all or a portion of a building was damaged or destroyed, where the damage had not been disturbed, and where, in the Study Team's opinion, there was a possibility of back-calculating the pressures causing the failures to determine the approximate wind speed. Detailed information was collected for: the elementary school pre-engineered metal building (PEMB), the John Deere PEMB, the precast concrete tee roof failure at the hospital, and the glulam beam failures in the First United Methodist Church. After performing a materials analysis to calculate the wind speeds required to cause the observed failures (Section 3.2), the calculated wind speeds were mapped and compared to the wind speeds derived from the EF Scale assessment (Section 3.3).
2.2 FIELD OBSERVATIONS
- 1. The primary site observations were obtained from an initial study of overhead imagery and initial site inspections conducted on May 10-11, 2007:
- a. The most severe swath of damage through the center of town was approximately five to six blocks wide. The damage on either side of this swath was less for about one to two blocks on each side, with even less damage observed for another one to two blocks on the extreme outer edges of the damage swath. See Figure 1 for an aerial view of Greensburg after the event and Figure 8 for a map of the damage swath.
- b. Many of the residential buildings were observed to have basements and some of these buildings were shifted on their basement foundations.
- c. The roofs of many of the buildings located near the edge of the storm's center and along the storm's path had roof coverings that either completely or partially survived the tornadic winds.
- d. Most residential and commercial buildings in town were older than any of the model building codes. Exceptions were the John Deere Building built in the mid-1990s and the elementary school PEMB erected in 2002 (see Figure 8 for locations). The residential and commercial buildings observed during the site visit did not have any specific design features that would have been intended to protect building occupants from the effects of this tornado except for basements or below ground areas and any small interior room, such as a closet or bathroom, used as a best available refuge area. No "tornado shelters" designed to resist the wind and debris associated with a tornado were identified during this effort.
Wind Speed Map for Greensburg, KS Tornado
one surface (i.e., the top of the roof). Because the overhangs are extensive (3 feet), the uplift on the tee was considered to be in 2 segments—the flat portion and the overhangs. The effective wind area for the overhangs is 10 sf; the effective wind area for the main roof area is 113 sf. The GCp for the overhangs is -2.8 and the GCp for the flat roof is -1.1.
The total uplift pressure of 90 psf is the sum of the uplift on the flat roof and the uplift on the overhangs. P1 is the overhang pressure and P2 is the flat roof pressure. A partially enclosed building is considered appropriate in this case because wind did get inside the building through broken windows and other building envelope penetrations.
P1 = q(-2.8-0.55) = -3.35q
P2 = q(-1.1-0.55)=-1.65q
90 psf = [P1(20sf) + P2(113)]/133 sf= [3.35q(20) + 1.65q(113)]/133
Solving for q =47 psf
Calculating the wind speed from the equation q = 0.00256* I*Kd*Kz*Kz*V²
V = 147 mph
JOHN DEERE BUILDING PEMB:
The failure mode studied in this building was buckling of the steel roof purlins, which are 'Z' shaped. The thickness was 1/16 inch or 0.0625 inch. The 'Z' shaped channel was 8 inches deep, 16 feet long, and had 3-inch wide flanges that formed the 'Z'. The moment of inertia about the axis that buckled (y axis) was calculated to be 0.281 in | A mathematical formula should appear at this position in the text. See Help:Fractions and functions for formatting instructions |
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Calculating the wind speed from the equation q = 0.00256* I*Kd*Kz*Kzt*V² and p = q(GCp — Gcpi). Again the MWFRS is used to determine GCp. The gust factor G is 0.85; Cp = -0.7 is the coefficient for uplift on the roof when the slope is about 15°; GCpi is 0.55.
p = q(-1.14) and q = 27.3/1.14 = 24 psf. Calculating wind speed from q yields V-122 mph.
If the Components and Cladding coefficients were more appropriate, in the field of the roof for an 80 sf effective wind area, GCp = -0.8, which would result in p = q(-1.35) and q=20.2 psf. Wind speed required for this pressure V = 96 mph.
This work is in the public domain in the United States because it is a work of the United States federal government (see 17 U.S.C. 105).
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