Case 2

Good Practices

Curtain wall

Design

Design wind pressure

• The curtain walling shall be designed to basic wind loads of +1.7kN/m² to -1.7kN/m²
• The building frame should be designed for wind life pressure (combination of positive pressure beneath and negative pressure above the frame.)

Building movement

1. Lateral movement in any direction of the building of maximum H/600 where H is the height above ground level, with a maximum differential movement of adjacent floors of storey height/500 [3]
2. Midspan vertical deflection of the edge of building concrete slab relative to the column support positions of L/500 or 20 mm whichever is less, where L equals beam span between columns.

Thermal movement

Provision shall be made in the curtainwall and installation for noiseless expansion and contraction resulting from an exterior surface ambient temperature range of 15°C and 84°C and a building interior temperature range of 150°C- 35°C without buckling, opening of joints, glass breakage, undue stress on fasteners, or other detrimental effects.

Allowances for both horizontal and vertical movements [3].

Fabrication and assembly

• All components of curtain wall shall be fabricated in accordance with approved drawings.
• Fabrication method and materials used shall conform to the national standards and satisfy the performance specification of the project. Fabrication tolerance [4]

Glass

Material

• Glass shall be of the appropriate type, thickness and colour to achieve the required performance requirements of the specification.
• Glass shall be cut and polished to the required tolerances.
• Cutting, drilling and surface treatments should be carried out before heat treatments.

Sealant

Design

Joint design is important in influencing the performance of gasket and sealant in the curtain wall system.

Three type of joints [4]:

1. Butt joint
2. Lap joint
3. Fillet joint

Calculation for butt joint design width is based on the theoretical total movement amplitude experienced by the joint and the sealant’s movement accommodation factor (MAF). The formula of design joint width:

minimum joint width required = (Movement x 100 / MAF)

Material selection

Each sealant type depending on the formulation has its own relative characteristics and merits or limitations. Below provides information on the various types of sealants applicable to curtain walling system [3-5].

Movement capability

Sealant must have a cyclic movement capability of ± 12% to ± 25% through its useful service life (Refer ASTM C719 or equivalent).

Weather resistance

Sealants should not exceed a Shore A hardness of 45 after heat ageing for 6 weeks at 70 °C. (Refer ASTM C719 or equivalent).

Sealant compatibility [3-4]

The following test should be carried out in advance of installation to evaluate:

• Adequacy of adhesion of sealant to substrates and other sealant types (ASTM C794)
• Possibility of sealant causing staining to certain substrates (ASTM C1248)
• Compatibility of sealant with gaskets, spacers setting blocks etc. (ASTM C1087) Application [5-7]

The substrate should be properly cleaned, dry and free of loose particles, contaminants etc., before applying sealant, primer.

Apply masking tape to the surfaces next to the joint to keep excess primer and sealant off the areas where they are not intended.

The sealant shall be applied over bond breaker/backer rods, set into the joint to control the depth of sealant and to prevent three-sided adhesion.

Minimise movement of joint during cure and allow adequate sealant exposure to air for cure. Adequate cured sealants should be checked for good adhesion by choosing joint locations at random, on site and using a simple peel adhesion test.