Cavity Wall Design Guide Page 2

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cavity walls, brick, block, cmu Design Guide for Taller Cavity Walls

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	CAVITY TYPE BEARING WALLS

GENERAL

When engineered, a cavity wall system can be designed to provide both structure and the enclosing skin. Building this system consists of constructing a series of single story structures, one on top of the other. The structure can be erected at a rate of one floor per week by implementing a tight schedule and sufficient man power. Combining load bearing cavity walls and precast concrete plank floors can make for efficient, economical and speedy construction.

The system relies upon composite reaction between the masonry walls, the precast concrete plank floor, and the roof system. Concrete masonry and precast concrete plank connections transfer wind induced shear stresses through the floor diaphragm to interior masonry shear walls (which may also be utilized as bearing walls). This type of construction is ideal for low and mid-rise construction like the Green Castle apartments shown here.




	FIG. 3 Green Castle (Elmhurst, IL) is constructed of cavity type bearing walls and spans 7 stories high

LOAD CAPACITY INVESTIGATION

The following calculations examine the load-bearing capacity of a six

story cavity type bearing wall system. The criteria used is as follows:

Brick ..........................4" thickness, 6000 psi min. compressive strength

CMU ..........................6" thickness f'm = 1350, wt 26 #/ft2

Concrete Plank ..........8" thick, 24'-0" span, wt = 60 #/ft2

Mortar ........................Portland/lime or mortar cement, type designated by physical property

CALCULATIONS

Floor loads on 6" CMU

8" Concrete plank = 60 #/ft2

Partitions & misc. = 20 #/ft2

Dead load = 80#/ft x 24/2 = 960 #/ft

6" CMU = 8x26 = 210 #/ft

Live load = 40 #/ft2 x 24/2 = 480 #/ft

Use Live Load Requirement

Roof Loads:

Let drainage fill + roofing = 20 #/ft2

Dead load = 60 + 20 = 80 #/ft2 x 24/2 = 960#/ft

Live Load = 30x24/2 = 360 #/ft

Wall Design

Use ACI 530-99/ASCE 5-99/TMS 402-99)

Assume:

• Wall height 8' 0"

• 8" concrete plank bears fully on 6" CMU

At Roof > P = .96K/1 + .36K/1 = 1.32K/1

e = 5.6 / 2 - 5.6 / 3 = .93" 1"

Allow. load. = 6.64K/1 > 1.32 OK

At 2nd Floor > P1 = 1.32 = 5(.21) = 4(.96) + 4 (.75 x .48) = 7.65 K/1

e = .93"

P2 = 1.44K/1 x (.93) = 1.34K/1

P1 + P2 = 9.09K/1

ev = 1.34 / 9.09 = 0.15"

P1 + P2

Allow load = 9.48K/1 > 9.09K/1 OK

CONCLUSION:

The calculations indicate that a 6 inch hollow CMU cavity type bearing wall system will support the given loads. The clear height of the wall must not exceed 8' 0" and the concrete planks


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