Technology Brief 8
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Technology Briefs > Brief 8
Concrete Homes: Structural Load Resistance: ICFs
vs. Steel & Wood-Frame
Research was conducted to compare the ability of insulating concrete form
(ICF) walls, and conventionally framed walls, to resist structural
stresses that can occur during earthquakes. All of the walls were
tested by applying a sideward or lateral force along the top edge
of each wall, in line
with the plane of the wall, while restraining the bottom. This "racking" of
the specimens creates internal tearing or shear forces as the
top of the wall tries to move while the bottom of the wall remains
stationary. The concrete walls demonstrated significantly higher
structural capacity and stiffness to resist the in-plane shear
forces than wood or steel frame walls.
What type of walls were tested?
One wall specimen for each of five types of exterior residential
wall systems were tested, each 8 feet high and 4 feet wide.
Three of the wall specimens were built with ICFs, creating one
flat concrete wall, a waffle grid concrete wall, and a screen
grid concrete wall, all shown below. The walls were reinforced with
grade 60 steel rebars. Additional steel extended from the top
of the footing into each wall. The nominal compressive strength
of the concrete used in the wall panels was 2500 psi. No finishes
were applied to the surfaces of the wall specimens. Loads were
applied to a concrete beam secured to the top of the wall
panels with high strength anchor bolts to transfer the lateral
forces to the top of the specimens. The foam plastic formwork
was removed from one side of each panel to permit the performance
of the concrete to be observed.
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Flat ICF Wall |
Waffle Grid ICF Wall |
Screen Grid ICF Wall |
The two frame walls consisted of: a 2 x 4 wood stud specimen,
and a 20-gauge steel stud specimen. The framing was covered
with 7/16-in. OSB (Oriented Strand Board) on one side and gypsum wallboard
on the other. The frame walls were secured to concrete footings with
steel hold-downs anchoring the ends,
and with embedded anchor bolts at two intermediate locations. A
6-inch deep timber beam was firmly attached to the top of the
frame wall specimens to transfer the lateral forces to the top of the panels.
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How was in-plane shear resistance measured?
The structural details for the test specimens were adopted based
on design recommendations and guidelines for typical exterior
wall panels in earthquake zones 1 or 2, and for minimum wind speed of up
to 70 miles per hour. The test setup and procedure followed general guidelines
of ASTM E564-95, Standard
Practice for Static Load Test for Shear Resistance of Framed Walls
for Buildings.
A hydraulic ram was used to transfer lateral load to the beam at
the top of each wall. A calibrated instrument measured the increasing
magnitude of this load. Additional devices were used to measure any movement of the panels or footing. The amount of load was gradually
increased while any major distress,
cracking, or damage was observed and recorded. The loading was typically
continued beyond the peak loading capacity of each wall and testing
terminated when the strength of each wall was significantly reduced
due to excessive damage to the wall specimens.
How does the shear resistance compare?
The frame walls showed initial damage at relatively light loading and had
a much lower maximum lateral resistance. The ICF walls resisted
a maximum lateral load 6 to 8 times the maximum loads resisted by the frame wall panels. Under lateral loads of about twice as much as
the maximum resistance of the frame
walls, the ICF panels were still very stiff, with extremely small
deformation, and showed no damage. The table below summarizes the actual results for each tested wall
panel.
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In-Plane Shear Wall Testing and Loading Results:
| Wall Panel |
Global Lateral Stiffness (lbs/in)1 |
Load at First Major Damage (lbs) |
Displacement at First Major Damage (in) |
Maximum Lateral Resistance (lbs) |
Displacement at Maximum Lateral Resistance (in) |
| Wood Frame |
18,500 |
3,500 |
0.51 |
4,553 |
0.89 |
| Steel Frame |
30,000 |
3,500 |
0.54 |
4,004 |
0.76 |
| ICF Flat |
708,000 |
8,500 |
0.06 |
34,245 |
2.66 |
| ICF Waffle Grid |
662,000 |
9,000 |
0.07 |
28,946 |
1.64 |
| ICF Screen Grid |
526,000 |
8,600 |
0.05 |
27,889 |
1.71 |
1 Global Lateral Stiffness at 1/3 the maximum lateral
force resistance for the wood and steel frame, at a lateral load
of 5,000 lbs. for the ICF walls.
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What does this difference mean?
These results suggest that when subjected to lateral in-plane loading from
sources such as wind or earthquake, the ICF wall panels are
not only considerably stronger but also much stiffer than the framed
wall panels. The higher strength of ICF walls enable concrete homes
to resist winds and earthquakes of much higher magnitudes.
The higher stiffness demonstrated by the ICF wall panels at the
loading limits of the frame wall systems, would result in smaller
lateral deformation and prevention of potential damage to non-structural
elements of a home such as finishes and trim. In the case of
moderate earthquakes, the repair cost of the damaged non-structural
components is usually the major, and sometimes the only part
of the restoration costs. ICFs offer great potential for reduced property loss from strong wind and earthquakes.
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More Information?
The following publication is available from the Portland Cement
Association. To view, please click on the link below.
"In-Plane Lateral Load Resistance of
Wall Panels in Residential Buildings",
by Armin B. Mehrabi, Construction Technology Laboratories for Portland
Cement Association, 2000, Serial No. 2403
This report documents the results of comparative in-plane shear
wall testing conducted on wood and metal frame, and insulating concrete
form wall specimens. The panels represented some of the typical wall
systems being used for current construction of residential buildings.
Full results of the testing is reported. The test setup and procedure
followed the general guidelines of the ASTM E564-95, Standard Practice for Static Load Test for Shear Resistance of Framed
Walls for Buildings. Concrete Homes Helpline: (888)
333-4840
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