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The decrease in clearance. assum- ing free expansion of the glass over the full range of 195°F. and with 20 ft of mullion restrained during ex- pansion over 115°F. is about 1 in. It is not feasible to provide clearances of this order in normal curtain walls. Usually the section of the mullion and the restraint of the transoms and the restraint of the transoms and in- fill prevent

buckling. Expansion joints in soft metals like aluminium often stick. even when filled with mastics, but nevertheless no buckling or other defects have been observed in normal curtain walling. If the expansion joints must move freely. the use of nylon or other slip layers might be considered.

Sagging of transoms

With double-glazed units, parti cularly, the bottom transoms are liable to sag under the weight of glass. An American design figure requires the sag at the midpoint of the transom to be not more than 1/360 of the span: for a 4 ft length the sag must therefore not éxceed

about 0.13 in. If this additional need for clearance cannot be met. setting blocks can be moved nearer the corners of the frame at the cost of higher strain in the glass, or more robust transoms can be used. Limits are fixed mainly to ensure that the corners on the bottom edge of the glass do not contact the transom, us- In ing the normal setting blocks. the present case, the requirement that not more than 3/8 in. of black glass is covered by the bead to minimise risk of cracking means that thicker setting blocks than normal would have been used and the reduction of clearance between glass and tran- som in the frame below may be more important.

Example 3

Wind load on panel 4 x 4 ft 200 ft above ground

Wind load on buildings is dis- cussed in Digests 98 and 122 (first series; and more recently by Scruton and Newberry in Proc. Intn. Civ. Engrs. June 1963. For particular sites. it is best to obtain the advice. of the local Meteorological Office.

For general guidance, obtain the basic wind pressure from Table 1 of Digest 122 and multiply by a factor of 1.5 to obtain pressure or suction on a wall.

Or

Thus, the basic pressure 200 ft

is above ground in coastal areas 31 tb/sq. ft and the total load on the panel is 4 x 4 x 31 x 1,5 or 741 lb. If the width of the mastic on the panel were in., its area would be 95 sq. in. and the mastic would be required to carry a suction pressure of about 8 lb/sq. in. with- out extruding. No simple dead-load test can take account of the fluctuat- ing load of wind gusts and in some cases of flexing of the panel, but in the absence of special apparatus, a dead-load test at 50% above the cal culated figure might be used. cause of the cooling effect of the high wind speed, a test temperature of 25°C (77°F) is adequate for most constructions.

Be-

Prepared at the British Building Research Station. Garston, Herts, Crown Copyright.

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THE HONG KONG & FAR EAST BUILDER-VOLUME 18, NUMBER 4