Structural Drawing

Wind Pressure.—There are obvious dangers associated with tall structures, which arise chiefly from the effects of wind pres sure upon the exposed elevated faces. Stability of the structures as a whole is now generally obtained by a combination in the building of rigidity and weight. While the steel frame in itself is sufficiently rigid to reduce distortion under wind pressure to a negligible amount, in all calculations the rigidity of the walls is taken into consideration, as it is a considerable factor in resisting distortion. The necessity for maintaining a rigid form of frame which would resist distortion under side pressure has led to the development of the braced structure and of the structure with rigid connections. Both these types demand of the designer a knowledge of the most advanced sections of mechanics and struc tural engineering. Much experimental research upon problems associated with tall structures has been carried out in the uni versities of America and by American engineers, from which satisfactory bases of design have been developed or theories verified. The progress of framed construction in tall buildings has enabled such a high degree of achievement to be reached, that careful experiments upon one of the tallest buildings recently erected in America have fully demonstrated that, even in the strongest gales, the little vibration that occurs is of no conse quence in such structures.

Electric welding, to replace riveting, is still in the experimental state. While no additional strength or rigidity is necessary than is provided by the riveted structure, two objects are being sought : the elimination of noise and the saving in steel that would result from not needing such heavy angles at the connections.

Corrosion.

During the later development of skeleton con struction considerable attention was given to the dangers which might arise from the corrosion of important parts of these steel structures. In the earlier days of steel construction applied to buildings, insufficient care was given to the protection of hidden and inaccessible parts, and connections and members were often so designed and fabricated that protection was difficult to apply. A few failures have occurred owing to corrosion, but the recent demolition of some of the earliest tall buildings in America has demonstrated that, with reasonable care, the life of the ordinary type of steel-framed building may be practically indefinite if the work is properly done. At the same time the investigation has led to the recognition of the value of concrete as a suitable and effective encasing material for the protection of steel. Reliable and durable protective paints have been produced and are now in wide use for the treatment of steel framework to be enclosed in masonry. They should be freely applied in at least two coats to all parts where actual encasing in solid concrete is not specified. Some hopes are entertained that a solution of the problem of corrosion will be found in the production of non-corrosive steel, having the necessary structural properties and yet economical in production.

Building Regulations.

The extent to which the adoption of suitable building regulations has assisted in the development of efficient and economical methods of construction has varied in different countries. In Great Britain, the type of local by-law which has generally been in force has lagged seriously behind the best accepted modern practice and has often acted as a deterrent to such development. In relation to the construction of walls the regulations have had a restrictive effect since they have com pelled the erection of exceptionally thick walls. In relation to other parts of buildings such as roofs, floors and columns these by-laws are usually inadequate and arbitrary and, while they may be of a character to prevent the erection of unsafe structures, they have not tended to encourage the adoption of better forms of construction.

The first and the most outstanding regulations to govern steel building construction in Great Britain were embodied in the Lon don County Council (General Powers) Act of 1909, and are usually referred to as the L.C.C. Regulations for Steel-Framed Buildings. These regulations, while containing some peculiar features and restrictions, had a marked effect upon the develop ment of this form of construction and there has since been a steady increase in the number of buildings erected in accordance with them. At present, practically all large buildings erected in London, apart from reinforced concrete structures, are designed and erected as steel-framed buildings.

In consequence of the manner in which the L.C.C. regulations were framed, they applied only to new buildings in which the main structure was to be of steel, while buildings in which steel was combined structurally with other materials were still subject to the older regulations. This lack of co-ordination has tended to discourage forms of composite construction which, for smaller buildings would often have produced more economical results. Under the London County Council (General Powers) Act 1926 efforts are being made to remove these and other anomalies.

In the United States a much closer relation has been established between regulations and practice, and many thorough revisions of local building codes, particularly in their effect on steel construc tion, have been undertaken during the last twenty years in American cities. The general methods of procedure and super vision adopted by authorities in the United States are more elastic in practice than in Great Britain, so that the regulations do not tend so much to discourage the best available methods offered by the engineers and contractors; the system in vogue is indeed equivalent to a continual revision of the regulations, thus encouraging constant progress and improvement.