Building Design and Construction Handbook - Merritt - Ventech!

7.124 SECTION SEVEN
The whole operation of lintel adjustment is one of coordination between the
several trades. That this be carried out in an orderly fashion is the duty of the
construction engineer. Furthermore, the desired procedure should be carefully
spelled out in the job specifications so that erection costs can be estimated fairly.
Particularly irksome to the construction engineer is the lintel located some distance
below the spandrel and supported on flexible, light steel hangers. This detail
can be troublesome because it has no capacity to resist torsion. Avoid this by
developing the lintel and spandrel to act together as a single member.
CORROSION PROTECTION
Protection of steel surfaces has been, since the day steel was first used, a vexing
problem for the engineers, paint manufacturers, and maintenance personnel. Over
the years, there have been many developments, the result of numerous studies and
research activities. Results are published in the ‘‘Steel Structures Painting Manual.’’
This work is in two volumes—Vol. 1, ‘‘Good Painting Practice,’’ and Vol. II, ‘‘Systems
and Specifications’’ (Steel Structures Painting Council, 40 24th Street, Suite
600, Pittsburgh, PA 15213). Each of the paint systems covers the method of cleaning
surfaces, types of paint to be used, number of coats to be applied, and techniques
to be used in their applications. Each surface treatment and paint system is
identified by uniform nomenclature, e.g., Paint System Specification SSPC-PS7.00-
64T, which happens to be the identity of the minimum-type protection as furnished
for most buildings.
7.44 CORROSION OF STEEL
Ordinarily, steel corrodes in the presence of both oxygen and water, but corrosion
rarely takes place in the absence of either. For instance, steel does not corrode in
dry air, and corrosion is negligible when the relative humidity is below 70%, the
critical humidity at normal temperature. Likewise, steel does not corrode in water
that has been effectively deaerated. Therefore, the corrosion of structural steel is
not a serious problem, except where water and oxygen are in abundance and where
these primary prerequisites are supplemented with corrosive chemicals such as soluble
salts, acids, cleaning compounds, and welding fluxes.
In ideal dry atmosphere, a thin transparent film of iron oxide forms. This layer
of ferric oxide is actually beneficial, since it protects the steel from further oxidation.
When exposed to water and oxygen in generous amounts, steel corrodes at an
average rate of roughly 5 mils loss of surface metal per year. If the surface is
comparatively dry, the rate drops to about 1 ⁄2 mil per year after the first year, the
usual case in typical industrial atmospheres. Excessively high corrosion rates occur
only in the presence of electrolytes or corrosive chemicals. Usually, this condition
is found in localized areas of a building.
Mill scale, the thick layer of iron oxides that forms on steel during the rolling
operations, is beneficial as a protective coating, if it is intact and adheres firmly to
the steel. In the mild environments generally encountered in most buildings, mill
scale that adheres tightly after weathering and handling offers no difficulty. In buildings
exposed to high humidity and corrosive gases, broken mill scale may be det-