Floor construction

The fire proof floors that had been used in earlier mill buildings had shortcomings and the development of lightweight terracotta flooring solved these problems and was a fundamentally important innovation necessary for the evolution of the skyscraper.

The floors in early mill buildings had two disadvantages. The simplest way of constructing these was either with brick arches or with concrete cast onto a corrugated iron arch. In both cases the arch was sprung between the flanges of the beams, but both of these methods left the bottom flanges of the beams exposed to any potential fire. It was also the case that such floors were rather heavy with a dead weight of about 75 lbs/sq.ft. (366 kg/sq.m). The development of terracotta flooring solved both these problems and was a fundamentally important innovation necessary for the evolution of the skyscraper.

Fire proof terracotta partitions and floors Johnson Developed and the Tribune Building in New Tork (1869) is said to be the first to use this device. Hollow blocks of terracotta were made to form'flat arches' which spanned between the lower flanges of the beams.

The early Pioneer blocks had their voids parallel to the span of the beams, while the slightly later Lee terracotta flat arch had blocks with the voids at right angles to the beams so that the walls of the blocks provided a greater arch area. Both of these methods were used in subsequent systems. The blocks were arranged to project below the lower flange of the beam and were either provided with flanges of terracotta to cover the iron, or designed to support small cover slips of terracotta to perform the same function. Some systems used terracotta floor tiles while others used a concrete floor cast over the arches.

The weight of these floors naturally depended upon the distance between the steel joists and could vary from less than 30 lbs./sq.ft. (147kg/sq.m) to as much as 50 lbs./sq.ft. (244kg/sq.m) depending upon the particular system and the depth needed for the spacing of the joists. These new terracotta based floor systems gave a weight saving of about half that of brick and concrete arch floors. This meant a significant reduction in the self weight of the structure, and hence the load to be carried by the walls, columns and foundations, which was particularly important in Chicago with its poor subsoil conditions.

Later, other floor systems were introduced including arched construction and flat concrete floors.

Later, other floor systems were introduced:

• arches of a lightweight cinder concrete cast onto expanded metal as permanent shuttering or
• flat concrete floors spanning between the top flanges of the beams.

Birkmire described the Columbian floor comprising close spaced cruciform and double cruciform steel bars encased in concrete, and the Roebling floor arch; a "wire-cloth arch stiffened by steel rods" to carry concrete. In Britain there was a similar development of lightweight floorings to be used with steel joists but so called 'filler joist' floors were commonly used early in this century. These comprised joists set at between 22" (559mm) and 33" (838mm) spacings with concrete cast in between and to a depth of up to 4" (102mm) above their top flange and such a construction had the merit of providing both the floor and a flat ceiling.

The protection given to the steel by these forms of construction meant that reasonable levels of fire resistance were now possible. But the improvements in efficiency of these new flooring systems were, to some extent, negated by the lack of consensus on what floor loadings framed buildings should be designed to withstand.

American fire tests required that the floors maintained their integrity for four hours at a temperature of 1700°F (927° C).  However, the live floor loadings which these systems were designed to support varied in different cities and even by the turn of the century there was no general agreement on the appropriate loadings to be assumed. Regulations varied from 100 lbs./sq.ft. (488kg/sq.m) in Chicago and Boston, to 75 lbs./sq.ft. (366kg/sq.m) in New York. In Britain  the regulations were even more conservative, and as late as the 1920s the loading for offices was still 120 lbs./sq.ft. (586kg/sq.m).