Foundations for Radar Towers.

a. General.   This design procedure provide minimum footing dimensions complying with criteria fo tilting rotations resulting from operational wind loads

Design of the footing for static load and survival win load conditions will comply with other appropriat sections of this manual.

b. Design procedure.   This design procedur is based upon an effective modulus of elasticity of th
foundation.  The effective modulus of elasticity i determined by field plate load tests as described i subparagraph  d  below.  The design procedure als requires seismic tests to determine the S-wave velocit in a zone beneath the footing at least 1 1/2 times th maximum size footing required.  Field tests on existin radar towers have shown that the foundation perform nearly elastically when movements are small.  Th
required size of either a square or a round footing t resist a specific angle of tilt,  α,  is determined by th following:


The design using equations (10-5) and (10-6) is only valid if the seismic wave velocity increases with depth.,
If the velocity measurements decrease with depth, special foundation design criteria will be required.  The discussion of these criteria is beyond the scope of this manual.

c. Effective modulus of elasticity of foundation soil (Es).  Experience has shown that the design modulus of elasticity of in-place soil ranges from 1000 to 500, kips per square foot.  Values less than 1000 kips per square foot will ordinarily present severe settlement problems and are not satisfactory sites for radar towers.  Values in excess of 5000 kips per square foot may be encountered in dense gravel or rock,  but such values are not used in design.

(1) Use equations (10-5) and (10-6) to compute-

(a) Minimum and maximum footing sizes using Es = 1000 and 5000 kips per square foot, respectively.
(b) Two intermediate footing sizes using values intermediate between 1000 and 5000 kips per
square foot.

Use these four values of B or D in the following equations to compute the increase (or pressure change) in the live load, ∆L.


(2) The Es value depends on the depth of the footing below grade, the average dead load pressure on the soil, and the maximum pressure change in the live load,  ∆L, on the foundation due to wind moments.  A
determination of the E,  value will be made at the proposed footing depth for each footing size computed.

(3) The dead load pressure, q0, is computed as the weight, W, of the radar tower, appurtenances, and the footing divided by the footing area, A.


The selection of loadings for the field plate load test will be based on qo and ∆L.

d. Field plate load test procedure.   The following plate load test will be performed at the elevation of the bottom of the footing, and the test apparatus will be as described in TM 5-824-3/AFM 88-6.

(1) Apply a unit loading to the plate equal to the smallest unit load due to the dead load pressure q0.  This unit loading will represent the largest size footing selected above.
 (2) Allow essentially full consolidation under the dead load pressure increment.  Deformation readings will be taken intermittently during and at the end of the consolidation period.
(3) After consolidation under the dead load pressure, perform repetitive load test using the live load pressure ∆L computed by the formulas in paragraph 10-7c.  The repetitive loading will consist of the dead load pressure, with the live load increment applied for 1 minute.  Then release the live load increment and allow to rebound at the dead pressure for 1 minute.  This procedure constitutes one cycle of live load pressure application.  Deformation readings will be taken at three points: at the start,  after the live load is applied for 1 minute, and after the plate rebounds under the dead load pressure for 1 minute.  Live load applications will be repeated for 15 cycles.
(4) Increase the dead load pressure, q0, to the second lowest value,  allow to consolidate, and then apply the respective live load increment repetitively for 15 cycles.
(5) Repeat step 4 for the remaining two dead load pressure increments.
(6) An uncorrected modulus of elasticity value is computed for each increment of dead and live load pressure as follows:


(7) The above-computed uncorrected modulus of elasticity will be corrected for bending of the plate as described in TM 5-824-3/AFM 88-6, where E' is defined above, and E, is the effective modulus of elasticity for the test conditions.

e. Selection of required footing size.   The required footing size to meet the allowable rotation criteria will be determined as follows:

(1) Plot on log-log paper the minimum and the maximum footing size and the two intermediate footing sizes versus the required (four assumed values) effective modulus of elasticity for each footing size.
(2) Plot the measured effective modulus of elasticity versus the footing size corresponding to the loading condition used for each test on the same chart as above.
(3) These two plots will intersect.  The footing size indicated by their intersection is the minimum footing size that will resist the specified angle of tilt.

Table 3-6.  Values of Modulus of Subgrade Reaction (ks) for Footings as a Guide to Order of Magnitude

0 comentarios:

Post a Comment