it is suggested that the calculated maximum movement may be halved.
Joints between soft metals like aluminium (particularly the spigot and socket joints in curtain walling) have been found to be severely restrained by friction and often show little or no movement and are wholly ineffective as movement joints unless provided with a plastic sheath. Occasionally a very sudden type of movement (stickslip movement) may occur and this sudden movement is particularly likely to cause failure of mastic joints.
Provision for carrying loads
Plastic mastics, and elastic mastics until cured are incapable of carry- ing loads without extruding and these loads are usually carried me- chanically by spacers, setting blocks or other devices.
With concrete slabs, for example. the load is generally carried by the fixing devices on the structural frame but sometimes mortar is used to transfer the load either to other slabs or to projecting floors.
In windows and certain walls. the weight of the glass is carried by setting blocks, two of which are normally set at quarter points to minimise strain
on the glass. If. however, the frame is likely to how excessively under the weight of glass. they are often placed closer to the corners of the frame. The position of setting blocks and spacers in open. ing lights may vary according to the way the windows are hung and the manufacturer's recommendations should be followed.
On tall exposed buildings, the wind load on the panels will need to be calculated before joint design can be finalized. The assessment of wind load is discussed in Digests 98 and 122 (first series), and Example 3 at the end of this Digest illustrates the method of calculation. It should be remembered that the whole of the wind load on the panels, and part of the weight of the glass in opening lights, is borne by the relatively nar- row band of mastic at the edge.
spacers may need to be at foot in- tervals. It is important to bed the glass and bead firmly down on to the spacers, and oval or diamond- shaped spacers facilitate this. These precautions, however, are usually unnecessary
for elastic mastics, when cured. and gaskets, though it re- mains necessary to ensure that the finished job, including the fixing of beads. gaskets and the unit itself to the structural frame. is adequate to carry safely the loads to be encoun- tered.
Durability
on
Modern uses of mastics have in- creased the long-term demands their durability; frequent renewal of multiple joints on a large building is troublesome. yet neglect can lead to much damage. To some extent the durability of mastics has been improved to meet these demands. but it is still important that the joints should be designed to afford maximum protection to the mastic. shielding it as far as possible from light and air. Since many mastics harden from the surface they should not be applied too thinly; in the shal- low joints they may harden quickly and lose their efficiency. This is particularly true of mastics based on drying oils or on bitumen. General- ly the depth of the mastic should not be less than in.
If more than one type of mastic is used in a joint. the materials must be compatible with one another and
Table 1
or
also with the adjacent materials such as setting blocks, spacers com- pounds used in assembling herme- tically sealed insulated glazing similar units.
or
ex-
In glazed units, adhesion between the glass and the mastic is often lost at places where the interface is posed to sunlight, especially on the inside of clear glass and in a narrow band on uhe outside. Thiokol'. in particular. is susceptible to this effect. In general, however, only slight see- page of water results from the loss of adhesion because the overall effec- tiveness of the seal depends largely on the inner portion of mastic the outside of the glass. The inter- face in this position is not reached by sunlight. It is suggested there- fore, that the use of expensive mas- ties like Thiokol' might be confined to this position and a cheaper mas- tic used elsewhere, if the extra la- bour costs are considered justified by the saving.
on
The following precautions are necessary to avoid cracking of dark- coloured or heat-absorbing glass:
1. Clearances must be adequate and the glass must not be rigidly restrained by the mastic. by setting blocks, fix- ing bolts or temporary fixing devices inadvertently left in. 2. Mastics must remain pliable for long periods at the tem- peratures reached by the glass (see Table 1).
ESTIMATED TEMPERATURES ON BUILDINGS IN ENGLAND
Part of construction
Light-coloured masonry wall (outer 3 in.) exposed concrete caves, edges of floor slab
Similar construction, but dark coloured
Black glass, ceramic tiles, or metal. insulated behind
White glass, ceramic tiles, or metal, insulated behind
Black metal tray, exposed behind clear glass and insulated behind
Temperature
Maximum Minimum
°C *F °C
"F
50 120
65 150
80 180
-20
--20
-5
-25 -15
60
1.40 -25 -15
130
260 -10
15
Clear glass in front of dark insulated background such as tray above
80
180 -25 -15
on
CX-
Aluminium mullion in a curtain wall (natural colour or white)
50
I 20
-15
5
the
NOTE Figures are 'rounded off and not exact Fahrenheit-Centigrade conversions.
The need for and distribution of load-carrying spacers depends the particular mastic and the posure; in severe exposure
138
THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 4
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