The basic functions of the vertical elements - Walls, Windows and Doors

Civil Engineering Reference

In-Depth Information

copper and brass) can be calculated

using Dose Responsive Function

relationships. This may help to

assess long term durability of a

proposed solution in any particular

location.

Metals perform best in a clean, dry

environment. While it is never

possible in walling to achieve these

ideal conditions, the design should

be such as to prevent, as far as

possible, the lodgement of dirt, dust

and moisture on the surface. This, in

general, means the avoidance of

horizontal or near-horizontal

surfaces. And while it is not possible

to avoid surfaces getting both wet

and dirty, designs should be free-

draining to reduce time of wetness.

Ideally, cladding should be washed

regularly. Fully exposed surfaces will

benefit from washing by rain.

However, the lee side of a building

will require monitoring.

The conditions under which

walling is required to perform

become more onerous with

increasing height. Great care will be

needed to ensure that the walling is

sufficiently weathertight such that

joints, crevices etc in the design do

not permit corrosion to occur unseen.

It would be prudent, particularly at

high levels, to assume that there will

be some moisture ingress, to check

for water entrapment, and to ensure

that the frame material is adequately

corrosion resistant or corrosion

protected.

The presence of thermal

insulation behind the walling could

also complicate the situation as

moisture could collect in some

materials and be retained as it were

within a poultice in contact with the

frame. While it may seem that the

major risk is from rain penetration,

one should not rule out the

possibility of condensation

increasing the risk of corrosion, both

to the frame as well as to the internal

face of metal cladding. Ventilation of

a cavity is the best way of reducing

this risk, though of course there

should be no inadvertent reduction

of thermal insulation value.

Ferrous metals

Ferrous materials, except stainless

steels, are normally in themselves

insufficiently durable for use

externally and require additional

corrosion protection. This additional

protection can either be in the form

of a metallic coating (eg zinc) or an

organic coating (eg PVC) or a

combination of both (a duplex

coating). The life of such ferrous

metals is directly related to their

protective coatings.

The most common metallic

coating is zinc, applied by

galvanising or sherardising, although

other electro-plated or hot-dipped

coatings (eg of aluminium zinc

alloys) and aluminium are available.

The life of a zinc coating is

dependent on the environment to

which it is exposed and is

proportional to the thickness of the

zinc. The thickness of the zinc

coating required to give protection

depends upon many factors, but

generally cladding sheeting is

formed from pre-galvanised sheet;

the methods of forming the sheet

(after galvanising) generally restricts

the total zinc coating weight to not

more than 275 g/m 2 including both

sides. With thicker coatings, the zinc

may crack and spall on bending. The

life of zinc coatings is reduced in

contact with mortar and moisture,

and thicker (or duplex) coatings are

thus required for ties and lintels used

in the outer leaf of masonry. Wire for

this purpose is electrogalvanised and

is ductile. See also Chapter 2.2.

One type of ferrous metal which

can be used in certain circumstances

without additional protection is a

weathering steel. These steels have a

low rate of corrosion, and can

weather to an attractive colour.

However, there is a major drawback

as the run-off from such material is

rust coloured and will cause staining

to adjacent materials. Weathering

steel cannot under any

circumstances be used as part of a

curtain walling system into which

glass, particularly insulated glass

units, will be inserted. Buildings with

such cladding need provision for

ensuring that run-off products can be

dispersed effectively.

The only satisfactory steel for

permanent use in and on external

walls is austenitic (18-chrome

8-nickel) stainless steel. This is

stable in all but very special

circumstances, does not cause

staining, and is widely used for

fixings. It should be replaced by

molybdenum/chrome/nickel steel

where high chloride levels are

expected. In very special conditions

such as swimming pool interiors,

stainless steel may not be suitable;

nor ferritic stainless steel as it is

prone to pitting corrosion. See also

Chapter 4.5 in Roofs and roofing (24) .

For reinforced concrete members

and rendered claddings, it may be

prudent to use protected

reinforcement and stainless steel

mesh in high rise buildings and in

marine areas; otherwise, corrosion

can be severe. But it can also be

severe in relatively sheltered areas

when the alkaline environment

giving protection to steel

reinforcement disappears (Figure

1.66 on page 68).