Cavity Wall Design Guide - brick block cmu

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	Design Guide for Taller Cavity Walls


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INTRODUCTION

Simply stated, a cavity wall is two wythes of masonry, separated by a cavity of varying dimension. The masonry wythes may consist of solid brick, structural clay tile, or concrete masonry units and are bonded together with masonry ties. The cavity (ranging from 2 inches to 4 1/2 inches in width) may or may not contain insulation. See Figure 1. Combining these elements with a sound structural design, appropriate details, quality materials and good workmanship will result in high performance cavity walls.

HISTORY

Cavity walls are not new, they have been observed in ancient Greek and Roman structures. At the Greco Roman town of Pergamum, on the hills overlooking the Turkish town of Bergama, a stone wall of cavity type construction still exists. Sometime in the early part of the 19th century, the cavity wall was probably reinvented by the British. Plans dating as early as 1805 suggest a type of construction, featuring two leaves of brickwork, bonded by headers spanning across a 6-inch cavity. An early British publication (dated 1821) suggests the use of cavity walls as a means of protection against moisture penetration. The use of metal ties was introduced in Southern England sometime after 1850. These original ties were made of wrought iron.

Cavity walls were first built in the United States late in the 19th century. Figure 2 illustrates an alternate type of cavity wall system originally featured in an 1899 text book assembled for people engaged in the engineering professions and construction trades. However, it was not until 1937 that this type of construction gained official acceptance by any building or construction agency in the United States. Since then, interest in and use of cavity walls in this country has increased rapidly. This has resulted in extensive testing to determine cavity wall properties and performance.

The early use of cavity walls in this country was limited primarily to exterior load-bearing walls in low rise construction. In the 1940s, designers began to recognize the advantages of cavity walls in high-rise buildings. Today, masonry cavity walls are used extensively throughout the United States in all types of buildings. The primary reasons for their popularity are superior resistance to rain penetration, excellent thermal properties, excellent resistance to sound transmission and high resistance to fire.

PROPERTIES OF CAVITY WALLS

RESISTANCE TO MOISTURE PENETRATION

No single unreinforced 4" wythe of masonry is totally impervious to moisture penetration. A cavity wall is designed and built as a moisture-deterrent system. This system takes into account the possible moisture penetration through the outer wythe. Moisture will penetrate masonry walls where hairline cracks exist between masonry unit and mortar. Water which runs down the exterior wall surface will be drawn towards the inner cavity due to wind pressure exerted on the exterior of the wall and the negative pressure present within the cavity. Providing a clean air space will allow this moisture to flow unobstructed down the cavity face of the outer wythe. Flashing installed at recommended locations will then divert this moisture back to the building's exterior through weepholes. Proper drainage of moisture will reduce the chance of efflorescence and freeze-thaw damage.

THERMAL ENERGY EFFICIENCY

At one point in time, energy conservation was not a major consideration in building design. Cavity walls were primarily built for their structural and moisture diverting qualities. During the mid 1970's, designers became aware of the life cycle cost of buildings so the design of energy efficient walls were initiated. The cavity became an excellent place to insert insulation, minimizing heat loss and heat gain. Both wythes act as a heat reservoir, positively affecting heating and cooling modes. The isolation of the exterior and interior wythes by the air space allows a large amount of heat to be absorbed and dissipated in the outer wythe and cavity before reaching the inner wythe and building interior.

This ability is further increased by the use of closed cell rigid insulation in the cavity. A foil faced, polyisocyanurate insulation is the most beneficial for three reasons: it yields an R value of 8.0 per inch of thickness, its R value is not affected by the presence of moisture, and its foil back enclosure creates a reflective air space that increase the walls overall R value by approximately 2.8. The R value of a typical cavity wall may range from 14 to 26 depending on the type and thickness of insulation selected.


	Table 1 - R Value of Brick and Block Cavity Wall

Exterior Air Film	0.17
4" Brick	0.44
R of reflective air space	2.80
2" Polyisocvyanurate	14.4
6" cmu	0.45
1 1/2" air space	0.68
TOTAL R VALUE	21.25

FIRE RESISTANCE

Results of the ASTM E-119 Fire Resistance Tests and the contents of both the Fire Protection Planning Report (CMIFC)2 and the Fire Resistance Ratings. Report (AISG)3 clearly indicate that masonry cavity walls have excellent fire resistance. All cavity walls have a fire rating of 4 hours or greater.

STRUCTURAL PROPERTIES

Masonry's capacity as a load bearing material is superb, yet its structural potential is often overlooked.Three principle factors affecting the overall compressive strength of a wall are: the compressive strength of the individual units, the type of mortar, and the quality of workmanship. Tables 2 and 3 lists the assumed compressive strength (f'm) for brick and concrete masonry. For large projects prism testing is preferred since actual values are usually higher than the assumed strengths.

The tables indicate that a standard concrete masonry unit with a type N mortar (1:1:6 by proportion) will yield a minimum f'm of 1500 psi. This strength is sufficient for most mid to low-rise bearing wall structures.

In addition to its excellence capacity as a bearing element, concrete masonry's performance as a back-up system is superb. Each wythe in a cavity wall helps resist wind loads by acting as a separate wall. The cross wire of the horizontal joint reinforcement transfer direct tensile and compressive forces from one masonry wythe to the other. Tests have indicated that joint reinforcement also provides some transfer of shear, approximately 20 to 30 percent, across the wall cavity. For a reference on allowable heights of cavity wall see Table 4.


	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

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


	After the cavity wall has been designed to meet the structural requirements, connections between the precast concrete plank and the masonry wall must be detailed. Other details, such as flashing, must also be developed. The wall/floor connections provide the wall with lateral bracing against wind loads. This connection should also assist in the transfer of shear stresses, and in the case of bearing walls, transfer gravity loads to the foundation


	FIG 4. Typical Bearing Wall Section

CONNECTION FOR LOAD BEARING

One way to anchor precast concrete plank into load bearing concrete masonry is to create a positive tie with reinforcing bars bent at 90 degree angles, see Figure 5. A structural engineer should determine the size and spacing of the reinforcement required.

The reinforcing bar is set into the layway formed between the concrete planks and grouted solid. The exposed portion of the reinforcement fits into the cell of the concrete masonry unit. In the next course, a positive connection is formed when the cell is grouted.

If lateral forces are low, an alternative connection should be considered,see Figure 6. This connection bonds the precast concrete planks to the masonry with a solidly grouted joint. Plugging the cores of the precast concrete planks creates a continuous grout cavity. When the grout is poured it flows into the grout pocket formed at the end of the planks. After the grout cures a positive key connection is formed between the planks and the concrete masonry units. All the precast planks should be in place and the grout fully set before the wall construction continues. Because this detail relies on the bearing pad's frictional resistance to help transfer shear stresses, a structural engineer should determine when this connection is

Fig 7 - Lateral Bracing Option 1

Fig 8 - Lateral Bracing Option 2

CONNECTION FOR NON-BEARING

Non-bearing walls (which span parallel to the floor planks) must also be laterally braced by the concrete plank floor system. One method requires holes to be broken in the top of the plank at designated intervals, see Figure 7. Specify the plank adjacent to the wall to bear on the wall a minimum of 3 inches. The cures of the plank are plugged on both sides of the hole with inserts to form a grout packet. A strap anchor is installed so that one end projects down into the grout pocket and the other end projects up into the cell of a concrete masonry unit. The grout pocket and cell of the concrete masonry unit are grouted solid. This connection transfers sear stresses through the floor diaphragm to interior shear walls while providing lateral support for the exterior wall. An alternative connection requires cutting or breaking the precast concrete plank continuously and butting the plank against the wall,see Figure 8.

Reinforcement is aligned and set into the head joints of the concrete masonry and bent at 90 degrees into the core of the precast plank. The core of the precast plank is then grouted solid when the grout cures it forms a positive connection. The significance of base flashing can never be over emphasized. The success of any cavity wall system depends on proper flashing details at the base of the wall. Figure 9 illustrates a properly flashed cavity wall at the foundation. Weepholes are required at 16" or 24" on center to divert moisture from the cavity to the exterior of the building.

Figure 10 suggests one method of construction for a window-head condition. A bond beam is used in lieu of a steel angle lintel. Flashing should be extended beyond the jamb lines with both ends damned. Solid masonry jambs should be avoided. However, for steel windows, the jamb must be partially solid to accept most standard jamb anchors. Stock sizes of windows may be used in cavity walls, although sometimes additional blocking is needed for anchorage. Window spans may be limited for this type of construction.

FIG 9 -Base Flashing Detail - Cavity Wall

FIG 10 -Lintel Detals


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