Masonry Magazine January 1974 Page. 43
DEFLECTED SHAPE
PRESSURE (q) IN PS.F.
0
20
40
8
50
WIND LOAD
L-20-0
CAPACITY OF WALL
F 23 PS
21133
CLE
W-5 PSP
WIND VELOCITY
30 TO 35 MPH
q-0.00256 x V²x CH
CH=1
10 30 50 70 90 110
WIND VELOCITY IN MP.H.
130
BRACE
SHALECTED
WIND LOADE
PSF
CAPACITY OF WALL
PLAN VIEW
PLANK
M
F46 PI
F
W133
W-24 PSP
WIND VELOCITY
70 TO 80 MPH
ALLOW STRESS
1430
Fox 1280
VERTICAL
SEC. PROPERTY
5-6.67
A-20
L-16-0
DIAGONAL
-96>50
INEGLECT)
LOAD CAP AFC
LOAD CAP 106
VER 15-20-300-
SPC
The spacing of these braces varies with the number of contractors using them. The most common type (4) has a vertical member placed against the wall, supported by a diagonal at the top and a horizontal near the base. The diagonal is usually secured to the ground by means of a stake. The spacing varies anywhere from 10-0 centers to 30-0 centers, or more.
To rationaize the use of this system, an analysis was made (4) to determine the spacing needed utilizing the strength of the wood planks. Prior to any analysis, several assumptions must be made:
1. Assume proper anchorage to the ground.
2. Assume the horizontal member at the base supports the vertical but does not transmit this load to the diagonal.
3. Neglect the timber code requirement regarding slenderness ratios of of compression members.
4. Assume good structural grade lumber is used.
5. Use a 15 psf wind load.
The analysis shows these braces would have to be spaced at 4-0 on center. This is neither practical nor economical, and therefore could not be expected of a contractor.
However, even if the spacing were adequate, the problem of anchorage is a major concern. A stake driven in sand or mud is almost useless, a stake cannot be driven in rocky ground, a stake driven in clay has bearing value only until the ground gets wet. This alone should negate the use of this system, but contractors say, "We've been doing it this way for 25 years, why should we change?"
Another bracing system being used is a series of cables. It consists of steel cables with turnbuckles that run over the wall and are anchored to the ground on both sides. The theory behind the system is to add downward load (P) to the structure, thus reducing tension in the members. The formulas shown in (1) still govern. However, this system again depends entirely on adequate anchorage to the ground, tightness of cable, and the inability of the wall to slip under the cable. Satisfying these conditions consistently is virtually impossible; therefore, this system should also be discarded.
It can be concluded from the foregoing that the braces should be spaced so the wall will not fail between them (yet not so close as to be impractical), the wall should be forced to span horizontally instead of vertically, the brace should not depend on soil conditions for anchorage, and it should be both practical and economical. A large order, but it has been accomplished. The authors, utilizing a combination of sound engineering principles and years of construction experience, developed a patent-protected system of temporarily bracing masonry walls.
The system is all steel, reusable, easy to install and remove. It uses a simple beam and strut arrangement designed to change the span direction of the wall from a vertical cantilever to a horizontal simple span, and to transfer the wind load directly to the foundation. It is built in stages and installed as the wall progresses. It does not interfere with scaffolds, material handling, or progress of work.
To prove the system, Underwriters Laboratories, Inc. in conjunction with Wiss, Janney, Elstner & Associates, a nationally known testing company, were retained to conduct a full-scale field test (5) using our bracing system. Field conditions were simulated by constructing a wall, 20-0 high and 24-0 long on a concrete foundation, using 8" concrete block, 4" face brick and type S mortar. A heavy wood vacuum chamber was built on one side of the wall and both the chamber and the wall were sealed with a polyvinyl covering. A gap of 2½" separated the chamber from the wall. The braces were installed on both sides of the wall, spaced at 16-0 on center, leaving a 4-0 wall projection or cantilever at both ends and stopping 2'-0 (Please turn page)
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