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I mentioned a few days ago that railroad stations were among the first buildings in the US to need the services of structural engineers in design. The other group consisted of gyms and other large indoor spaces. Wood trusses were the traditional method of creating long-span roofs, but not compatible with late-1800s new ideas about creating non-flammable structures and not strong enough to carry any load above other than a roof. The answer was to import steel trusses from bridge and railroad design, which meant importing structural engineers along with them.

The picture above is the gym at the 1901 YMCA on West 23rd Street in New York. Today, we’d probably span that distance with ordinary beams or plate girders, but here we’ve got big warren trusses. A close up, from the running track:

I particularly like the way the track hangers make a slight change in the bottom-chord rivet pattern. If you look at the center-right connection (above the ladders) you can see that the vertical connection has only two rows of rivets, versus four for the diagonals, because the verticals are only transmitting a relatively small of roof load down to the connection, to reduce bending in the upper chord.

(As a side note, it’s worth mentioning that the emphasis on gym design at that time was for gymnastics, as can be seen in the equipment. Organized sports were a secondary concern, and if the geometry was not quite right for a basketball court, that was okay, as the space was devoted to physical activity, not games.)

The YMCA is a steel-frame building, with the trusses connected to columns hidden in the masonry piers on the side walls, but similar trusses were used in low-rise bearing-wall buildings. Here’s the 1900 armory at the US Naval Academy in Annapolis:

Those are portal-frame trusses, designed to have enough rigidity to not exert significant outward thrust on the masonry walls despite the lack of a tie between the gable eaves. The skylights are a common part of the design of these roofs, but are often mounted on a monitor projecting up from the main planes of the roof. Having the skylight in the plane of the roof, as here, looks better but is more difficult to waterproof.

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