Control
of Wall Movement
All
building materials that are used in general construction are subject to
movement. When this movement occurs and is restrained, unsightly cracks may
develop. The movement in masonry materials is generally due to changes in
the moisture content and temperature. Normally, the net effect of this movement is shrinkage. The quantity of movement is associated with the type of masonry
unit used, the quantity of reinforcing used, the shrinkage coefficient
of the masonry units and the length to height ratio of the wall. There are three main changes that cause these stresses in
concrete masonry:
1. Physical change due to loading
2. Thermal movement
- Changes
in temperature cause structures to expand or contract as the temperature
increases or decreases. A change of 50 degrees F. will affect a 100-foot concrete wall
with forces of about 410 PSI as much as ΒΌ inch. Since the tensile strength of the wall is generally less than
that, cracking generally occurs.
3. Changes in moisture content
- Shrinkage
due to moisture changes from uncontrolled means also cause changes in
wall length. The amount of
moisture content in the masonry units at the time it is placed in the wall
determines the effects of shrinkage. The
environment and weather conditions do and can change during
construction.
Control
Joints
Control
joints are vertical joints, which provide continuous separation in the
masonry to allow freedom of movement so as to relieve any build up of
stress. These joints should
be located where cracking (or stress) would be most likely to occur, such as in long straight walls, abrupt changes in wall
thickness or heights, chases for pipes, at openings, columns or
pilasters and any other points of potential excess tensile stress. They may also be used at wall intersections in main walls and
partitions. Various types
of control joints are shown in this section.
Factors
that Affect Joint Spacing
Reinforcing- Use of horizontal
reinforcing helps to accommodate movement by increasing the tensile
strength of the wall. Steel
reinforcing may be added either by incorporating bond beams into the
structure or by using joint reinforcing. Bond beams are also normally used to carry structural loads, such
as over openings in a wall and the steel reinforcing is normally sized
according to the magnitude of these loads. If bond beams are to be used as tensile reinforcing they should
be reinforced with a minimum of two No. 4 bars; however, structural
engineers should always be referred to when making these decisions. Reinforcing serves to increase the tensile strength of the
structure. It may be spaced
8, 16 or 24 inches apart. The
enclosed tables show the effect of joint spacing when joint reinforcing
is used.
Length To Height Ratio- The
length to height ratio for a masonry wall also affects the distance
between control joints until the proper spacing is achieved. Care should be taken not to exceed recommended joint spacing for
these ratios to ensure good performance.
In
estimating the proper spacing of control joints, consideration
is given to the height of the wall or vertical distance between major
horizontal restraints, such as floors, roofs, etc. Usually this
distance is equal to story height.
This illustration at the right shows the simple application of
control joints as related to the height (h) and length (L) of a
wall. The amount of horizontal joint reinforcement has a direct
affect on the spacing control joints as related to L/h. Recommended control joint spacing not to exceed 25ft (7.62m) Ref. NCMA TEK 10-2B |
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