708 



STEEL BUILDINGS. 



Advantages. A great advantage in the use of 

 steel for construction is the space which is gained 

 by the reduced thickness of the walls. Two feet 

 saved in the thickness of a wall means nealy 600 

 feet of floor space gained in each of the lower 

 stories of a building 50 by 100 feet. Increased 

 light obtained by putting windows on all sides as 

 the structure rises above its surroundings is another 

 great advantage of the tall steel building, and with 

 this come the advantages of more air and the view 

 obtainable from upper stories. Another reason, not 

 often considered, which has influenced the erection 

 of the great office building, is that its very size and 

 capacity invites tenants because it makes it possible 

 for so many classes of business to find quarters 

 under one roof. The average mammoth steel office 

 building contains a restaurant, a cigar counter, a 

 news stand, a barber shop, a telegraph and district 

 messenger office, a typewriting bureau, stationer 

 and printing office, and other conveniences all 

 under one roof and within easy reach of every 

 tenant. Of course, the high value of land in the 

 center of a great city does much to influence the 

 erection of a tall building to increase the rental, 

 but the advantage of affording tenants so many 

 conveniences under one roof is probably the largest 

 factor of their commercial success. 



Designing. The construction of the great steel 

 office building requires, as a preliminary, designing 

 by both an architect and a mechanical engineer or 

 bridge engineer. The steel construction and founda- 

 tion work is very similar to that of bridges, and calls 

 for the same kind of experience as that gained by en- 

 gineers in the designing of steel truss bridges. The 

 work of the architect is so mixed in with that of 

 the engineer that the two find it necessary to work 

 together in the development of the plans. The size 

 of the structure being determined, it is necessary 

 to begin designing from the top and work down- 

 ward, because in no other way can they certainly 

 place the beams and girders so as to know that they 

 are properly proportioned for the weight they are 

 to carry. When the tower and superstructure are 

 planned, their weight can be figured out, and the 

 beams of the upper story figured of a size and num- 

 ber to support them properly. Each story, as the 

 plans descend, has to be made a little stouter than 

 the one above, because it has that much more 

 weight to support. By the time the street level is 

 reached, the weight, wind surface, and all such 

 items can be computed. The designers, knowing 

 just what they have to support and being made 

 familiar with the nature of the soil on which the 

 building is to stand, through borings made at 

 numerous points to discover what is below, are in a 

 proper position to decide what is the best character 

 for the foundations. Among the duties of the 

 engineer are the verifying of the actual size of the 

 lot, which might differ slightly from the surveyor's 

 measurements appearing in the deeds. Then he 

 has to determine both the dead load and the live or 

 moving load, the latter requiring twice as much 

 strength of material as the dead load. The law in 

 New York prescribes an allowance for live load 

 which is greatly in excess of that which is actually 

 found in office buildings, and this obliges stronger 

 construction than is really necessary, thus increas- 

 ing the factor of safety. The engineer must also 

 calculate the wind pressure, usually at 30 pounds to 

 the square foot, and make due allowance therefor 

 in the bracing. He must locate the principal 

 columns, girders, and beams so as to carry the 

 strains according to his calculations, which must be 

 verified at every step. 



Construction. The steel building consists of a 

 skeleton of steel beams and girders made commonly 

 with Z or I sections to resist bending, and riveted 



together, knee braces being put in here and there 

 to increase the resistance to wind pressure. Many 

 of the upright members are made in two-story 

 lengths, and the steel work is put up by means of 

 an erecting plant which is usually elevated two 

 stories at a time. From this erecting plant are op- 

 erated cranes and other hoisting devices, and here 

 are supported the forges used to heat the rivets, 

 which are inserted red hot and fixed in place by 

 pressure or hammering. Sometimes tubular col- 

 umns of cast iron are preferred to steel columns. 

 Cast iron is stronger than steel cr wrought iron in 

 compressional strength, though inferior in most 

 other qualities. The cost of each is about the same. 

 Where steel columns are preferred it is often because 

 their Z or other angular section affords convenient 

 space in which to carry pipes, wires, and the like, 

 as well as giving better opportunity to bolt on or 

 attach any small structures that are not so readily 

 affixed to tubular cast-iron columns. In the 

 foundations of the steel building the canti- 

 lever principle is ordinarily employed in or- 

 der to distribute the weight of the walls. 



Cantilever Truss 



This is best illustrated by a draw- 

 ing, which shows a cantilever trus> 

 in the reverse position in which it 

 " is used on bridges. The wall being- 

 made to rest on the extreme edge of the truss, the 

 weight is carried inward so that it centers on the 

 large pier. If the weight were placed directly on 

 the pier it would tend to tip it over ; but by thia 

 arrangement the weight of a 2-foot wall may be 

 distributed over piers 10 or 12 feet thick. Canti- 

 lever girders usually extend the entire width of a 

 building, and some of the largest weigh 80 to 100 

 tons each. The piers may rest either on piles, en 

 caissons, or on steel grillage. If the soil is hard 

 sand or earth, much saturated with water, bundles 

 of piles may be driven and tied together at the top 

 by beds of cement, and on these the piers can lie 

 erected. A mud bottom usually requires a steel 

 grillage which is made of long steel I beams crossed 

 in a bed of cement that fixes the whole so that it 

 becomes like one large plate on which the whole 

 structure may rest. For very large buildings where 

 the foundation rock is 50 to 70 feet below the street, 

 and there is constant danger of undermining ad- 

 jacent buildings a condition frequently found on 

 Manhattan Island the caisson foundation is used. 

 This construction is borrowed from the bridge 

 builder and was first used in making the foundation 

 for the Manhattan Life Insurance Company's build- 

 ing. A caisson or great steel box, open at the bot- 

 tom and with a door in the top, is let down into 

 the foundations by placing men inside to dig away 

 the earth so that it gradually goes down as the ex- ' 

 cavation proceeds, with the very least disturbance j 

 to the surrounding soil, which is kept in place by 

 the stout sides of the caisson. In this manner the \ 

 caisson is worked down to the bod rock, which is 

 leveled or stepped to receive it. When it is prop j 

 erly set the workmen withdraw and cement is 

 poured in and allowed to harden, thus forming a 

 great stone block incased in steel and as solid ami 

 level as is possible by any known means. A sei ie> 

 of these caissons sunk in the foundations serve to 

 support the piers from which the structure rises. 

 As the steel framework is raised, the stone and terra 

 cotta are built on, a common rule being that there 

 shall be at least 8 inches of this on the outside ami 

 4 inches inside to protect the steel. For about 75 



