A fair degree of precision is necessary with joints to ensure that the joint is never less than the cal- culated minimum width, but not so much greater that the mastic is likely to slump.
Example 1 - Mastic for dense precast concrete panels
Figures are given in Table 5 of Digest 99 (first series) for thermal and moisture movement of facing materials. It is assumed that the drying shrinkage of
precast panel and of the framework behind is substantially completed. The temperature range is taken from Table 1 of the present Digest.
a
Dimension of panel 5 ft Range of temperature 70°C. 125°F
0.47 in
Thermal movement Range of moisture
content Moisture movement
Wet to dry 0.02 in
If the joint is made when the panel is cold and dry or when it is warm and wet. moisture and thermal movements can operate together and total 0.067 in. It may also happen that. by the way the panels are hung. the full movement of both panels on either side of a joint operate to open or close the joint. giving a total movement of about 0.13 in.
of
Assuming these figures represent the maximum joint movement 25% or 75% which the best mastics can tolerate in butt or lap joints respectively (Table 2), it follows that the minimum mastic width will need to be of the order of 0.6 in. or 0.2 in.
Attention must also be given to the problem of the tolerance (T) in the size and position of the panels. If a soft butyl mastic has been chosen to fill a butt joint 0.6 in. wide, the design width of the joint should be (0.6|2T) to ensure that no joint is narrower than 0.6 in. (Fig. 1). Since the mastic may. tend to slump if the joint width is much more than 1 in. wide. T should preferably not exceed in. This serves to illustrate the need for precision if mastic joints are to be successful and of the advantage of methods of erection which allow
142
adjustments to be made for size variation of panels.
―
Example 2 Insulated black glass in 4 x 4 ft aluminium frame
Consider a curtain wall.
wall, with mullions forming the vertical sides of the frame, and transoms forming the horizontal sides are attached to the mullions. Expansion joints in the mullions occur in some frames but only at 20 ft intervals instead of the more usual 8 ft
or 10 ft. Thermal movements alone need to be considered.
The use of beads of or 4 in. depth. and an edge cover to the black glass restricted to in.. is assumed.
Prolonged heating
In this example the glass and aluminium expand freely in reach- ing their maximum temperatures. having been glazed in the coldest condition. The maximum possible thermal movement can be calculated as in Example 1, or from the coeffi- cients of expansion.
If the black glass expands by 5 X 10-6 of its length for 1°F rise in temperature, the expansion of a 4 ft length caused by a temperature rise of 195°F is 5 X 10-6 X 48 X 195 in., i.e. 0.047 in.
For aluminium with a coefficient of expansion of 13.5 X 10-6/°F the expansion of a 4 ft length caused by a temperature rise of 115°F is 13.5 X 10-6 X 48 X 115 in., ie. 0.075 in.
+
If the temperatures of frame and glass rise or fall together. they ex pand or contract together and the clearance changes by the differencs between their movements. Heating increases the clearance; cooling de- creases it. This is because of the greater coefficient of expansion of the surrounding metal frame. From the above figures the maximum pos- sible change in clearance is thus 0.028 in., or about 1/32 in.. which is not excessive; a clearance of 3/16 in. for glass over 30 in. in the longer dimension, as recommended in CP 152:1960, is ample. Further shear joint of 1/8 in. between mov- ingt surfaces, which is commonly
a
used in curtain walls, will tolerate 3/32 in. movement with the best mastics. Fairly soft butyl and "Thiokol mastics should give good performances under these conditions tures that can be reached by black glass. good softness retention is essential.
Intermittent heating
Well insulated units of low ther- mal capacity can heat up rapidly in sunny periods while more massive components heat up slowly. The tem- perature of insulated black glass. for example, may rise by 5°C (9°F)/min. and that of aluminium mullions by 10°C(18°F)h. In this slightly exaggerated example. with the sun rising or suddenly emerging from behind a cloud. the aluminium experiences only slight rise in tem- perature, which is neglected, while the temperature of the glass increases the full range of 195°F. In this case the clearance is reduced by 0.047 in. Again, this is not exces- sive. for clearance haviour.
over
or mastic
Position of expansion joints
be-
In frames where expansion joints occur in the mullions an increase in temperature will tend to close the joints and bring the transoms of frames containing them closer to- gether. thus reducing the clearance between the glass and its upper tran- som. The clearance between the glass and the lower transom is. of course. fixed by setting blocks. In this example the mullions are fixed to the frame of the building at 20 ft intervals with the joints midway between. Assuming that the expan sion of glass and aluminium is un- restricted through their maximum
of temperature ranges
195°F and 115°F it can be calculated that the reduction in clearance in frames containing expansion joints is 0.42 in. The demands for clearance in this case are too high and it would be necessary either to employ more expansion joints or to incorporate them in split transoms.
Failure of expansion joints
If expansion joints in light mul- lions stick or are omitted altogether the mullions may buckle in the heat.
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 4
Page 150Page 151
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