LEVELING THE PLAYING FIELD

The site for the new 18,800-seat Target Center sports arena was a difficult one. While most of the site was limestone bed­rock, the northwest corner extended into a preglacial river channel. These soil conditions made a single-foundation design not practical. So the engineers used three footing types.

Typical spread footings were placed away from the channel, where the limestone was in place. They bear directly on the limestone with a pressure of 20 tons/sq ft, which re­duces the need to remove bedrock. Because the shale below the limestone was of lower strength, minimum thicknesses of limestone were required between the footing and shale. Where the limestone thickness was ineffi­cient, footings were extended to the sand­stone. About half of the arena is supported on 180 spread footings.

Drilled piers designed for founding on sandstone because of the limited limestone thickness, were placed near the channel's edge. The sandstone in this area functioned as the bearing stratum because of limited lime­stone thickness. Ranging in di­ameter from 36 to 78 in and in length from 5 to 50 ft, these piers support 78 columns. The 5 ft piers are founded on good-quality limestone that extends beyond the estimated edge of the channel.

In the channel, concrete-filled 9⅝ in di­ameter pipe piles carry 80-100 tons per pile. Protective conical points helped the piles embed themselves into the sandstone-bear­ing stratum. There are from three to 20 piles per group driven to the depth of 50-130 ft.

The arena has the first structurally sup­ported, elevated, movable floor in the U.S. It was the health club with two levels below grade that determined the column spacings beneath this playing floor. The engineers used a 15 x 22 ft concrete column grid pattern to support the 500 lb/sq ft live load.

The movable floor area (228 x 90 ft) is two levels above the lowest slab. When lowered, movable seats pull out on all four sides for basketball and concerts. In the up position the seats pull back and the floor is lifted with a series of 98 jacks.

The top 6 in of concrete on the floor con­tains the coolant piping for the ice hockey slab. Directly below this slab is 6in rigid insulation and a 10in structural slab of con­ventionally reinforced cast-in-place concrete.

When lowered, the floor functions as a one-way structural slab that spans between the jack support beams. In the up po­sition, it functions as a two-way slab sup­ported on the 98 jacks. The jacks are supported by a series of conven­tionally reinforced cast-in-place beams that, in turn, are supported by the cast-in-place columns. The dead load of the movable floor is 2,625 tons, with a live-load capacity of 2,625 tons when lifted up and 5,250 tons when lowered.

The trusses of the roof system are 30 ft deep at the ends and slope to 32 ft deep at the centre. They are made of W14 sections rotated with the weak axis vertical. The top and bottom chords are grade 50 steel with web members grade 36 steel.

The trusses were all fully preassembled with the drilled bolt connection shop fabricated then disassembled and transported to the site. This eliminated fitting problems during erection. The 330 ft trusses weighed an av­erage of 250 tons each. All truss connections were bolted in-situ with 1in diameter A325 bolts.

Notes:

coolant piping n. система труб охлаждения

disassemble v. демонтировать

drilled pier буронабивная свая-столб

health club оздоровительный клуб

ice hockey хоккей на льду

live-load capacity допустимая временная нагрузка

pipe pile трубчатая свая

preglacial river channel русло доледниковой реки

support beam опорная балка

Ответьте на вопросы к тексту

1. Why was the site a difficult one?

2. What were the three footing types the engineers used?

3. What did the use of spread footings result in?

4. Why were drilled piers placed near the channel's edge?

5. What is the structure of the arena floor?

6. How do the cast-in-place columns under the floor area function?

7. What are the trusses of the roof system made of?

8. How are they joined?