ALWEG - Footprint
The Seattle ALWEG monorail beam was constructed with the beam-on-column method so common with today's ALWEG-style monorails. The supporting columns were set on top of appropriate foundations (cast-in-place, driven pile or spread foundations.) Most of the supporting structure consists of T-shaped reinforced concrete columns. The columns average 24 feet high, with 4 feet embedded into the ground. Minimum clearance under the cantilevering crossarms is 15 feet, and under the rail beams approximately 20 feet. At the time the system was built, the concrete quality was 3750 psi. These columns were designed to be up to 35 feet high.
The beamway itself consists of prefabricated, prestressed concrete beams. These were hollow girders approx. 3' wide and 5' high, with the approximate shape of an "I" beam. Each of the 90-ft straight beams weighs about 55 tons. The 77-ft curved beams also weigh about 55 tons, since they have slightly thicker walls. Maximum span for straight beams is 100 feet. Curved beams of 600+-ft radius can span 75 feet, while curved beams of less than 300 feet average 60 feet.
The construction procedure was very simple. After the foundations were in place, one crane and one labor crew could set 12 columns/day. Then, two cranes plus one labor crew could set 18 beams/day.