Post-stressing concrete mat foundation reduces excessive vibrations in machinery. The mat is stressed in compression after the machinery is mounted, thus closing any cracks in it, altering the distribution of the soil subgrade reaction on the mat, and changing the mat-subgrade natural frequency

Fistedis, S. H.

English

NASA Technical Reports Server

July 1967	 Brief 67-10237 
AEC — NASA  TECH BRIEF 
AEC-NASA Tech Briefs describe innovations resulting from the research and development program 
of the U.S. AEC or from AEC-NASA interagency efforts. They are issued to encourage commercial 
application. Tech Briefs are published by NASA and may be purchased, at 15 cents each, from 
the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Post-Stressed Concrete Foundation May Reduce Machinery Vibration 
(a)T 
(b)
(d)	 Fr-. 
te)
The problem: 
To reduce excessive vibrations in machinery 
mounted on a concrete mat supported by a soil 
subgrade. The vibration characteristics of a con-
crete mat and subgrade system are difficult to 
determine. Therefore, the natural frequency of a 
mat-subgrade system may inadvertently coincide 
with the vibration frequency of the mounted 
machinery, resulting in excessive reinforced vibra-
tion. 
The solution: 
A concrete mat which may be stressed in com-
pression after the machinery is • mounted, thus 
closing any cracks in the mat, altering the dis-
tribution of the soil subgrade reaction on the 
mat, and changing the mat-subgrade natural 
frequency. The post-stressing is accomplished by
tightening cables passing through parabolic pas-
sages provided during construction in the concrete 
mat. 
How it's done: 
This vibration reducing technique depends upon 
the assumption that closing the cracks and alter-
ing the soil subgrade pressure distribution on the 
bottom surface of a concrete mat will change the 
mat-subgrade system natural frequency. 
If a machine is mounted on a rigid mat with a 
centered block, as in figure (a), the bearing pres-
sure of a noncohesive subgrade such as sand on 
the bottom of the mat is as shown in (b). Thus, 
the bearing pressure is greatest at the center of 
the mat, reducing to zero at the edges. When 
cables are passed through preformed parabolic 
passages in the mat and tightened, as in (c), the 
cables introduce a horizontal compression and an 
-	 (continued Overleaf) 
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upward load distributed as in (d). In effect, the 
cables tend to lift the center of the mat and 
force down the edges. As a result, the pressure 
distribution of the sand substrate on the mat 
tends toward a uniform distribution as shown in 
(e). It is not important, however, exactly what 
pressure distribution between the subgrade and 
mat results after cable tightening. The essential 
feature is that the rigidity of the mat and the 
distribution do change, thus altering the mat-
subgrade natural frequency. 
For cohesive subgrades, such as clay, the initial 
pressure distribution on the mat is the opposite 
of that for noncohesive subgrades, resulting in a 
minimum pressure at the mat center and a maxi-
mum at the edges. The cables therefore pass 
through a parabolic passage which is concave 
downward. 
This technique promises simple implementation 
in the field. When the subgrade is identified as 
either cohesive or noncohesive, a mat with the 
appropriate cable passage concavity is easily con-
structed. The machine is then mounted and en-
ergized. If no excessive vibration is experienced, 
no further adjustment is necessary. If vibration 
is excessive, cables are mounted in the passages, 
fastened at one end, and tightened at the other 
end by a hydraulic jack reacting against the edge 
of the mat. Cable tension can be varied until 
vibration is minimized, whereupon the cables are 
securely fastened to the mat edge and the jack 
removed. 
Several cables may be mounted parallel to each 
other, and passages may be provided to allow 
tightening from front to back, and from side to 
side.
Notes: 
I. The assumption that altering the rigidity of the 
mat and the subgrade pressure distribution will 
change the mat-subgrade system natural fre-
quency has not yet been validated by testing. 
2. This technique should provide a simple and 
effective correction procedure for equipment 
vibration until more accurate methods are 
devised for determining soil vibration charac-
teristics. 
3. Additional details are contained in: (I) Pro-
ceedings of World Conference of Prestressed 
Concrete, Univ. of Calif. July 29-Aug. 2, 1957, 
pp. 10-1, 10-6; and (2) Patent #3,015,912 
available from U.S. Patent Office at $0.50 
each. 
4. Inquiries concerning this innovation may be 
Directed to: 
Office of Industrial Cooperation 
Argonne National Laboratory 
9700 South Cass Avenue 
Argonne, Illinois 60439 
Reference: B67-10237 
Source: Stanley H. Fistedis, 
Reactor Engineering Division
(ARG-130) 
Patent status: 
Inquiries about obtaining rights for commercial 
use of this innovation may be made to: 
Mr. George H. Lee, Chief 
Chicago Patent Group 
U.S. Atomic Energy Commission 
Chicago Operations Office 
9800 South Cass Avenue 
Argonne, Illinois 60439
[1 
S 
Brief 67-10237	 Category 05


Post-stressed concrete foundation may reduce machinery vibration

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