Homecrete Homes are Strong and Safe
Hurricanes and Tornados:
Debris driven by high winds presents the greatest hazard to homeowners and their homes during hurricanes and tornados. Recent laboratory testing at the Wind Engineering Research Center, Texas Tech University, compared the impact resistance of residential concrete wall construction to conventionally framed walls. The frame walls failed to stop the penetration of airborne hazards. The concrete walls successfully demonstrated the strength and mass to resist the impact of wind driven debris.
What was tested?
Various wall specimens were subjected to the impact of a 2 x 4 wood stud traveling at up to 100 miles per hour. This is equivalent to the weight and speed of debris generated during a tornado with 250 miles per hour winds. This testing covers the maximum wind speed generated in 99 per cent of the tornadoes occurring in the United States. Wind speeds are less than 150 miles per hour in 90 per cent of tornadoes.
Ten wall specimens were constructed, each representative of the type of construction now used to build frame homes and concrete homes in the U.S. Tables 1 & 2 describe each wall assembly tested.
The Wind Engineering Research Center's compressed air cannon was used to propel the wood stud debris "missile" at the test walls. The stud was propelled along its axis with the leading end hitting the specimen. Electronic timing devices measured the speed of the debris as it traveled from the cannon to the test walls located 16'-6" away.
How did the frame walls perform?
The frame walls lacked the weight and mass to resist the impact of the wind driven debris. In each case, the debris traveled completely through the wall assembly with little or no damage to the "missile."
Table 1: Frame Wall Test Results
| Wall Type: | Test Wall Description: | Speed of Debris: | Results: |
|---|---|---|---|
| Wood Frame: | 5/8" gypsum board interior finish, 2 x 4 wood studs at 16" o.c., 3-1/2" batt insulation, 3/4" plywood sheathing, vinyl siding exterior finish. | 109.0 mph | The debris "missile perforated completely through the wall assembly. Little damage to missile. |
| 5/8" gypsum board interior finish, 2 x 4 wood studs at 16" o.c., 3-1/2" batt insulation, 3/4" plywood sheathing, 4" brick veneer with 1" air space. | 69.4 mph | The debris "missile perforated completely through the brick veneer, and the interior finish. Minor damage to missile. | |
| Steel Frame: | 5/8" gypsum board interior finish, steel studs at 16" o.c., 3-1/2" batt insulation, 3/4" plywood sheathing, vinyl siding exterior finish. | 103.5 mph | The debris "missile perforated completely through the wall assembly. Little damage to missile. |
| 5/8" gypsum board interior finish, 2 x 4 wood studs at 16" o.c., 3-1/2" batt insulation, 5/8" gypsum board sheathing, synthetic stucco exterior finish. | 50.9 mph | The debris "missile perforated completely through the wall assembly. No damage to missile. |
How did the concrete walls perform?
The concrete stopped the debris from traveling through the wall. Exterior finishes were damaged by the impact, but the concrete walls were unscathed. Even the narrowest, 2" thick section of "waffle grid" ICF wall was undamaged by the direct impact of the debris at over 100 mph.
Table 2: Concrete Wall Test Results:
| Wall Type: | Test Wall Description: | Speed of Debris: | Results: |
|---|---|---|---|
| Concrete: | 6" thick reinforced concrete wall, #4 vert. reinforcing bars, 12" o.c. No finishes. | 109.0 mph | No cracking, front face scabbing or back face spalling of concrete observed. |
| 6" thick reinforced concrete wall, #4 vert. reinforcing bars, 24" o.c. No finishes. | 102.4 mph | No cracking, front face scabbing or back face spalling of concrete observed. | |
| ICF: | Block ICF foam forms, 6" thick flat concrete wall, #4 vert. reinforcing bars, 12" o.c. Vinyl siding. (Tested a second time with similar results.) | 103.8 mph | Debris penetrated vinyl siding and foam form. No cracking, front face scabbing or back face spalling of concrete wall observed. |
| Block ICF foam forms, 6" thick flat concrete wall, #4 vert. reinforcing bars, 24" o.c. 3" brick veneer with ties spaced 1'-0" ea.way. | 99.0 mph | Debris penetrated and cracked brick veneer. Foam form dented. No cracking, front face scabbing or back face spalling of concrete wall observed. | |
| Panel ICF foam forms, 4" thick flat concrete wall, #4 vert. reinforcing bars, 24" o.c. vinyl siding. | 96.7 mph | Debris penetrated vinyl siding and foam form. No cracking, front face scabbing or back face spalling of concrete wall observed. | |
| Block ICF foam forms, variable thickness "waffle" concrete wall, 6" max. thickness, and 2" min. thickness. #4 vert. reinforcing bars in each 6" vertical core at 24" o.c. Synthetic stucco finish. (Tested a second time with similar results.) | 100.2 mph | Debris penetrated synthetic stucco finish, and foam form. Impact of wall at 2" thick section. No cracking, front face scabbing or back face spalling of concrete wall observed. |
Note: All concrete tested: 3000 PSI compressive strength, maximum aggregate size 3/4", 6" slump.
Hurricane wind velocities will be less than the equivalent maximum speeds modeled in these tests. Missile testing designed to mitigate property damage losses from hurricanes use a criterion of a 9-pound missile traveling about 34 miles per hour.
Earthquakes
Built according to good practices, concrete homes can be among the safest and most durable types of structures during an earthquake. Homes built with reinforced concrete walls have a record of surviving earthquakes intact, structurally sound and largely unblemished.
In reinforced concrete construction, the combination of concrete and steel provides the three most important properties for earthquake resistance: stiffness, strength, and ductility.
Why buildings survive?
Scientists study damage from earthquakes to determine what types of buildings best withstand seismic forces.
Modern earthquake-resistant design relies several recent studies:
| Year | Earthquake | Magnitude | Studies |
|---|---|---|---|
| 1989 | Loma Prieta | 7.1 | University of California, Berkeley |
| 1994 | Northridge | 6.8 | NAHB Research Center National Institute of Standards and Technology |
| 2000 | Yountville/Napa | 5.2 | Stanford University |
Studies of earthquake damage consistently show well-anchored shear walls are a key to earthquake resistance in low-rise buildings.
Optimal design conditions include shear walls that extend the entire height and located on all four sides of a building. Long walls are stronger than short walls, and solid walls are better than ones with a lot of openings for windows and doors. These elements are designed to survive severe sideways (in-plane) forces, called racking and shear, without being damage or bent far out of position. Shear walls also must be well anchored to the foundation structure to work effectively. Properly installed steel reinforcing bars extend across the joint between the walls and the foundation to provide secure anchorage to the foundation.
Why buildings fail ?
Low-rise buildings most vulnerable to earthquakes do not have the necessary stiffness, stregth, and ductility to resist the forces of an earthquake or had walls that were not well anchored to a solid foundation, or both. Three types of buildings sustained the most significant damage:
Multi-story buildings with a ground floor consisting only of columns.
Most of these buildings were 3 to 4 stories tall with a parking garage or a living area with many large windows on the ground level. The columns may have been strong enough to hold up the structure, but did not provide an adequate amount of racking resistance during a seismic event. When the earthquake shook the building side-to-side, the upper stories sometimes tipped over to one side. Whether build to wood, steel, or concrete-they all suffered damage.
Wood-frame houses with weak connections between the walls and foundation.
Wood-framed buildings are inherently ductile (flexible), which is an attribute during an earthquake. However, the shaking sent some of these houses sliding to one side. Frequently, the shear walls were strong enough, but the connection to the foundation was a weak point that gave way.
Un-reinforced masonry or concrete buildings.
Masonry or concrete walls not reinforced with steel bars were not ductile enough to be effective shear walls. And if there is no steel connecting them to their foundation, the joint between walls and foundation can be a weak point.
Why reinforced concrete is safe ?
Reinforced concrete walls are a composite system:
Concrete resists compression forces, and reinforcing steel resists tensile forces produced by an earthquake. The concrete is cast around the bars, locking them into place. The exceptional ductility of the steel to resist tensile forces, coupled with the rock-like ability of concrete to resist compression, results in an excellent combination of the three most important earthquake resistance properties: stiffness, strength, and ductility. A study at Construction Technology Laboratories revealed that even a lightly reinforced concrete shear wall has over six times the racking load resistance as framed wall construction.
It’s no wonder that modern reinforced concrete buildings were found to survive these recent earthquakes with rarely any significant damage.
Concrete Homes are Built to Last
When building a house in areas of high seismic risk, always follow good design practice. Make sure the exterior walls are properly designed and constructed —relatively continuous, unbroken walls of stout construction that includes reinforcing steel. Install strong, durable connections of these walls to the foundation.
Studies have shown that properly designed reinforced concrete walls offer greater earthquake resistance than other types of construction.