Dome over Orlando

With municipal services that should have been under one roof situated in several build­ings, Orlando clearly needed a new City Hall.

The new City Hall is now under construction. This $32.5 million structure is a nine-storey re­inforced concrete tower, clad in granite-ag­gregate precast concrete panels and surmounted by a 120 ft diameter copper dome.

While the dome is the structure's most important visual feature, the view at ground level will be impressive as well. The main entrance will be through the "Great Room", a circular space 80 ft in diameter and 35 ft tall. Beneath the main tower is a basement added for structural rather than architectural reasons.

The soil at the City Hall site is quite dense, fine sand down to about 38 ft, with a mix of sand and clay of increasing hardness below. Ground water was about 12 ft below the surface.

We designed a system of 60 and 125 ton ca­pacity precast concrete piles, with each pile group rigidly connected to individual columns by 5,000 psi reinforced concrete pile caps about 5 ft deep. The design required for the piles to be driven about 100 ft below ground.

The only shallow foundation feasible was a mat foundation. It was placed by Brasfield & Gorrie General Contractor, Inc., Orlando. To make the site ready, they dewatered the soil to a minimum of 30 ft be­low ground level for 2 weeks before excava­tion. After compacting the soil at the bottom of the excavation, they poured a thin concrete mat to provide a good surface for steel placement.

We designed the foundation for a maximum allowable gross bearing pressure of 5,000 psf. The mat had to be 4½ ft thick, with a con­crete strength of 7,000 psi. Some 4,400 cu yd of concrete were placed in all, along with 290 tons of reinforcement. The mat was concreted in a single, 13-hour continuous pour without construction joints.

The attached low-rise section rests on in­dividual spread footings. This part of the building was constructed after the tower was finished, in order to minimize differential settlements. A construction joint with dowels and reinforcement connects the two sections.

The typical floor plate in the tower is square, varying from 154 ft at the fourth floor to 148 ft 4in at the ninth. The building perimeter steps inward at the third, fourth, sixth, eighth and ninth floors. From the fourth to the ninth, the interior framing for each floor is basically the same. However, the perime­ter framing cantilevers outward on all four sides, with spans varying from 15 ft at the fourth floor to 12 ft 2in at the ninth.

The framing scheme is a rein­forced concrete joist system with post-tensioned girders. By prestressing the girders, we were able to span longer distances. For wind resistance, wind beams spanning parallel to the joists and located on column grids serve as lateral-force-resisting beams and are post-tensioned at the perimeter cantilevers.

We designed two adjacent bays north and south of the interior core for significantly higher loads. These areas measure 52 ft by 36 ft and can be used for heavier rooms.

Above the large space of the Great Room, a tower column rises seven floors to the domed roof. To transfer the load from the tower column, two perpendicular pairs of long-span, post-tensioned girders are sup­ported on tall, slender columns.

The four long-span transfer girders work together to support the tower column. We designed each with the proper amount of prestress force and reinforcement for its part of the transferred load from the col­umn. Because of the high level of prestress force required for the final service load, stage stressing was to be used.

Framing of City Hall's 120 ft copper dome began in mid-October. The dome's skin rests on structural steel trusses, supported by a centre circular steel frame with 12 columns bearing on a rein­forced concrete frame. The frame rises 21 ft 8in above roof level. A perimeter circular steel frame with 12 columns bearing directly on the roof gird­ers also supports the trusses.

Notes:

City Hall здание муниципалитета

dewater v. осушать

dome n. купол

domed roof купольное покрытие

excavation n. земляные работы (при строительстве здания)

granite n. гранит

gross bearing pressure совокупное опорное давление

impressive adj. впечатляющий

make ready подготовить

municipal adj. муниципальный

view n. вид

visual adj. визуальный

Ответьте на вопросы к тексту

1. Why does Orlando need a new City Hall?

2. What are the features of the new building?

3. What kind of soil is there on the site?

4. What types of foundation were used?

5. What preliminary (предварительная)work did the construction of the mat foundation require?

6. How was mat foundation produced?

7. What are the characteristics of the low-rise section foundation?

8. What does the building perimeter feature?

9. What is the framing scheme of the structure?

10. Why were the girders of the frame prestressed?

11. How is the load from the tower column transferred?

12. What is the structure of the dome?