Origin of the Types. Iron has been employed extensively in buildings for many years. The first building in this country of what is now known as the skeleton type of construction, was the Home Fire Insurance Company Building, built in Chicago in 1883, of which Mr. W. L. B. Jenney was the architect.

As this was an epoch-making event, it is important to know a few of the details of this building. In an account published in The Engineering Record of January 6, 1894, Mr. Jenney says: "The problem presented by them was to so arrange the openings that all stories above the second or bank floor could be divided to give the maximum number of small offices - say about 12 feet in width - each with its windows conveniently placed and sufficient to abundantly light the entire room. The work was planned quite satisfactorily, but the calculations showed that a material with very much higher crushing strength than brick was necessary for the piers. Iron naturally suggested itself, and an iron column was placed inside of each pier." The chief departure was in making the columns bear all the loads, the walls between the piers supporting only their dead weight for a single story in height. Mr. Jenney states that the difficulty which was feared from the expansion and contraction of the iron columns led to the supporting of the walls and floors independently on the columns. The columns were of cast iron of box section, and the walls were supported on cast-iron box lintels, resting on brackets on the columns. The floor loads were carried by iron beams, although a few Bessemer steel beams were used, these being the first to be used in this country.

Since the connections were by bolts, the beams were connected together by a bar running through the cast-iron columns, in order to secure a more rigid frame.

The chief advance from that day is in the substitution of steel for all members in high-building construction, and in the development of details in the connections of the members.

Types in Use. There are three main types of high buildings:

1. The class in which the exterior walls are self-supporting, and are designed also to support the ends of the girders carrying the floors. The floor loads inside the walls are carried by steel beams and girders framed between steel or cast-iron columns.

2. In the second class, the exterior walls are self-supporting but the wall ends of floor girders are carried by steel girders and columns.

3. In the third class, the steel frame is a complete unit in itself, and carries all floor loads, and, also, the load of the walls themselves. This latter is the pure skeleton type and the more common form of construction.

Effect on Foundations. The different types have an important effect on the design of the foundations, and in some cases fix their character.

In the first type, the benefit of isolated columns with independent foundations is largely lost, as unequal settlements in the walls themselves and in the walls and columns are likely to result.

In the second type, as all loads are carried on columns which have isolated footings, more equal settlement will probably result, and in the event of the walls settling unequally with respect to the columns, would not affect the steel frame.

In the third class all foundations are generally in effect of the character of isolated piers which can be proportioned to give nearly uniform settlements.

When a party wall makes it desirable to keep all foundations inside of the building by means of a cantilever construction it can be more readily done in buildings of the third class than in any other type.

LA SALLE STATION, L. S. & M. S. AND C, R. I. & P. RAILROADS, CHICAGO

LA SALLE STATION, L. S. & M. S. AND C, R. I. & P. RAILROADS, CHICAGO.

General view of building site during the foundation work. Cast-iron column bases set in place, ready to receive steel columns.

Concrete and brick foundation walls being started. For foundation below bases, Bee illustrations on page 154.

ELEVATION OF STEEL FRAME, SHOWING WIND BRACING OF OFFICE BUILDING FOR THE CHICAGO & NORTHWESTERN RAILWAY COMPANY, CHICAGO

ELEVATION OF STEEL FRAME, SHOWING WIND-BRACING OF OFFICE BUILDING FOR THE CHICAGO & NORTHWESTERN RAILWAY COMPANY, CHICAGO.

E. C. & R. M. Shankland, Engineers, Chicago.

See also typical floor-framing plan on page 138.

Effect of Wind Pressure. Probably the most distinctive problem in high-building construction is the provision for lateral strains in the framework, due to wind pressure. The amount of these strains varies, of course, with the relation of the height of the building to the dimensions of its base and to its exposure on different sides. In the earlier designs, much more complete provision was made for such strains than is now the practice. The laws of some cities, Chicago and Boston for instance, now limit the height to about 125 feet above the street. In other cities, notably New York, buildings of 350 feet or more are allowed. In New York, in buildings having an exposed height of four times or less the least dimension of the base of the building, no special consideration of wind strains is proscribed.

In buildings where the walls are of solid masonry construction and of moderate height, it is not necessary to consider the effect of wind pressure, as the dead weight of the masonry and the stiffness afforded by cross walls and partitions are sufficient to resist the effect of the wind, under ordinary conditions. With the light steel skeleton buildings carried to the height of the modern buildings, the elasticity of the steel frame makes it necessary, under certain conditions, to consider wind pressure. The walls being merely thin coverings, and the partitions also thin and not bonded to the walls, it is apparent that the frame itself must provide all the resistance.