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cavity walls, brick, block, cmuDesign Guide for Taller Cavity Walls

Page 6

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Cavity Walls 6

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    BIA Tech Notes Online

    NCMA Techical Notes

 

DETAILS COMMON TO ALL CAVITY WALL SYSTEMS

PARAPET DETAIL

Detailing the parapet can be difficult. Designers have tried different ways to detail parapets to minimize cracking, leaking and displacement. Figures 20 and 21 illustrates two parapets coping details.

The cavity should be continuous up to the top of the parapet, and expansion joints should extend up and completely through the parapet. In addition, the parapet wall should be doweled to the structural deck. Vertical reinforcing is required when the height of the parapet exceeds three times its thickness. Parapet copings should be of stone, hard-fired clay or precast concrete caps. The coping should be sloped and should provide a drip on both sides of the wall. Place a continuous (through the wall) flashing membrane under the mortar bed immediately beneath the coping. to avoid displacement, the coping is secured to the wall by using an anchor and dowel rod connection. Fill and seal alternate coping head joints with foam backer rods and a sealant (keep joint totally void of mortar). The back of the parapet should be constructed of durable materials.
 

MOVEMENT JOINTS

There is a common misconception in the construction community. Many designers, builders, and contractors do not know the distinction between control joints and expansion joints. Control joints are placed in concrete or concrete masonry walls. They control cracking by reducing restraint and accommodating wall movement resulting from shrinkage to initial drying. Locate vertical control joints a maximum of 20'-0" on center horizontally. Other provisions which dictate the placement of control joints are discussed in NCMA TEK 5-2A,10-1 & 10-2B.

There are several ways to form a control joint. The most practical method of forming a control joint is to rake back a vertical mortar joint creating a plane of weakness. The joint should be raked back to a minimum depth of 3/4 inch and extended continuously up the wall surface. Apply sealant where required.

Since shrinkage, due to drying is not found in clay masonry construction, control joints are not necessary for brick masonry walls. Brick expands when subjected to moisture gain. Brick also expands and contracts when experiencing temperature variation. So expansion joints are required for brick.

An expansion joint is a continuous break in the exterior wythe of masonry. The break is in the form of a soft joint which is totally void of any mortar or material capable of resisting movement. This joint will accommodate movement due to temperature variations, moisture expansion or differential structural movement. An expansion joint is usually placed in the vertical direction although horizontal expansion joints are required under shelf angles.

Construct a minimum of one-1/2 inch expansion joint every 30'-0" on center horizontally. Consult BIA Technical Note #18A for more accurate placement information. Typical details of movement joints are shown in Figure 22.

Many mid rise and high rise buildings are designed with balconies. Water penetration problems often occur at the sliding door/cavity wall juncture. This is due in part, to the pitch of the concrete deck. Figure 23 illustrates a flashing detail which could eliminate this moisture problem, using a prefabricated end dam with self adhering flashing.
 

fig20

FIG 20 - Coping detail for low parapet

fig21

FIG 21 - Coping detail for high parapet

fig23

FIG 23 - Flashing at sliding door

fig22

FIG 22 - Expansion and Control Joints

THE BOTTOM LINE

Aside from the finished product being of considerable beauty and formidable strength, other initial and long-term benefits are gained when cavity wall systems are coupled with the structural entities previously cited.

INITIAL BENEFITS

  • The statement that "Masonry is too expensive" is just a myth. Cavity wall systems are initially lower in cost than many glass curtain walls, metal panel curtain walls, granite panels, marble panels, and architectural precast concrete walls. Add to this a reduction of $26.00 per lineal foot of shelf angle deleted, and additional savings occur.
  • Limiting the number of crafts involved promotes rapid construction resulting in savings due to early occupancy.
  • All materials required are usually available locally, which eliminates costly shipping charges and untimely postponements.

LONG TERM BENEFITS

  • Cavity walls are energy efficient when considering the life cycle cost of a building. A
  • typical "R" value can be increased if greater energy-efficiency is desired.
  • A structure built with the type of systems previously discussed provides a built-in,  2-to-4 hour, fire-rated barrier. Annual fire insurance premiums can be reduced by nearly 1/3, depending upon the type of construction chosen, and its occupancy.
  • Masonry construction is very economical with respect to long-term maintenance.

 

REFERENCES

1. Technical Note 16, Rev "Fire Resistance", Brick Industry Association, Reissued Oct. 1996.

2. Technical Note 18 series, "Differential Movement, Cause and Effect, Expansion Joints, Flexible Anchorage", Brick Industry Association, Jan. 1991, December 1991.

3. Technical Note 21 series, "Brick Masonry Cavity Walls, Insulated, Detailing, Construction", Brick Industry Association, Aug. 1998, Feb. 1999, Feb. 2002.

4. NCMA TEK 3-12 "Loadbearing Concrete Block in High Rise Buildings" National Concrete Masonry Association, 1998.

5. NCMA TEK 5-2A, 10-1A, & 10-2B "Control of Wall Movement with Concrete Masonry" National Concrete Masonry Association.

6. NCMA TEK 14-10 "Lateral Support of Concrete Masonry Walls" National Concrete Masonry Association, 1994.

7. NCMA TEK 5-6A. "Details" 2001: NCMA TEK 5-7A "Floor and Roof Connections to Walls" 2001.

8. "Manual for the Design of Hollow Core Slabs", Prestressed Concrete Institute, 1985.

9. "Introduction of Cavity Walls - Letters" Chartered Surveyor Weekly, (published in U.K.), vol.10 p.856 March 28, 1985, vol.10 pp. 707, 708 March 14, 1985.

10 "Reinforcing Steel in Masonry", Masonry Institute of America, January 1982.

 

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