A Practical Treatise on Engineering and Building Foundations: Including Sub ... by Charles Evan Fowler

Author:Charles Evan Fowler
Language: eng
Format: epub
Publisher: Wiley
Published: 1920-03-25T05:00:00+00:00

Fic. iSS.— Retaining Wall Diagrah.

of the pressure of the water is to turn the wall up<m the point O (Fig. 189). Calling H the height of the wall in feet, and t its thickness in inches, and assuming i cubic foot to weigh 120 pounds, the

moment of stability is — for rectangular walls, per foot of length; 2.4

Ht^ 2 siP

or (Pxio.4 must not exceed —; or fi=~-. This enables the

2.4 H

thickness of a wall to be determined, that will exactly balance the pressure. Safety would be just secured by adding 50 per cent to the thickness at the base, and subtracting ihe same amount from the top thickness, thus giving the wall a sloping back as per dotted line, increasing the acting leverage of the CG (center of gravity) of the section, so that the resultant pressure will now fall within the middle third of the w&U thickness. This method is a[^roximate and useful

)vGoO'^lc

OALCULik'nON OF PIEBS, FOOTINQS AND BETAINmO WALI^ 2S9

for small construction. Lai^e masonry dams require more special consideration, and may be designed by calculations at each few feet of depth, designing the wall in a series of steps and drawing a final shape to aldose the whole in one harmonious section, which must also fulfill the conditions of strength proper to the material and crushing stress. Special care is necessary that water does not get in between bed joints, as it would greatly assist to turn a wall over.

It is usually oon-^dered desirable that the resultant of pressure on a wall should fall within the middle third of its thickness. Thus in Fig. 190 if ab to any scale represents the total pressure 31.24J' concentrated at the center of pressure upon the face of a wall, and be to the same scale is the weight of the wall, the line be being drawn through the CG of -the cross-section, then if the diagonal of the rectangle abed cuts the base of the wall within its middle third of thickness, the wall may be assumed fully safe. If the diagonal fall outside the middle third, the thickness of the wall must be increased. It may be made safe by building the wall higher, and thus loading it; but the danger of this method is that a greater area is exposed to wind

Fig. 189.— Pbessurz of Water o

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