Precast concrete structures - Elliott.pdf - Konstrukcje - Rickson

3 Precast frame analysis

3.1 Types of precast concrete structures

Preliminary structural design, in what many people refer to as the feasibility stage,

is more often a recognition of the type of structural frame, which is best suited

to the form and function of a building, than the structural design itself. The

creation of large ’open plan’ accommodation giving the widest possible scope

for room utilization clearly calls for a column and slab structure, e.g. in Figure 3.1,

where internal partitions could be erected to suit any client’s needs. The type of

structure used in this case is often referred to as ’skeletal’ - resembling a skeleton

of rather small but very strong components of columns, beams, floors, staircases,

and sometimes structural (as opposed to partition) walls. Of course a skeletal

structure could be designed in

cast in situ concrete and structural

steelwork, but here we will

consider only the precast concrete

version.

The basis for the design of pre-

cast skeletal structures has been

introduced in Figure 1.7. The

major elements (=the precast

components) in the structure are

shown in Figure 3.2. Note that the

major connections between beams

and floors are designed and con-

structed as ‘pinned joints’ and

therefore the horizontal elements

(slabs, staircases, beams) are all

simply supported. They need not

Figure 3.1: Precast skeletal structure showing large unobstructed

spaces for the benefit of both the construction workers and client.

Precast Concrete Structures

1 Mainspandrel beam

2 Hollow-core unit

3 Internal rectangular beam

4 Gable spandrel beam

5 Gable beam

6 Main edge beam

7 Landing support beam

8 Staircase and landing

9 Ground beam

10 Column

11 Wall

12 Double-tee unit

13 Internal beam

14 Main edge spandrel beam

Figure 3.2: Definitions in a precast skeletal structure.

always be pinned (in seismic zones the connections are made rigid and very

ductile) but in terms of simplicity of design and construction it is still the pre-

ferred choice. Vertical elements (walls, columns) may be designed as continuous,

but because the beam and slab connections are pinned there is no global frame action

and no requirement for a frame stiffness analysis, apart from the distribution of some

column moments arising from eccentric beam reactions. The stiff bracing elements

such as walls are designed either as a storey-height element, bracing each storey

in turn, or as a continuous element bracing all floors as tall cantilevers.

In office and retail development

distances between columns and beams

are usually in the range 6m to 12m,

depending on the floor loading and

intended use. In multi-storey car parks

where the vehicle loading is always

about the same it is around 16m. The

exterior of the frame - the building’s

weatherproof envelope, could also be a

skeletal structure, in which case the

spaces between the columns would

be clad in brickwork, sheeting etc.

Alternatively, the envelope might be

constructed in solid precast bearing

walls, which dispenses with the need

Figure 3.3: Wall frame with 3.6m wide hollow core floors

(courtesy A. Curd and Partners, USA).

Precast frame analysis

for beams, and is referred to as a

'wall frame'.

An example of a building where

a precast wall frame would be the

obvious choice is shown in Figure

3.3 - the walls are all load bearing

and they support one-way span-

ning floor slabs. There is less archi-

tectural freedom compared to the

skeletal frame, e.g. walls should

(preferably) be arranged on a rec-

tangular grid and a fixed modular

distance, usually 300 mm, between

walls is quite important economic-

ally. A wall frame may be more

economical and may often be faster

to build especially if the external walls are furnished with thermal insulation and

a decorative finish at the factory. Figure 3.4 is a good example of this. Distances

between walls may be around 6m for hotels, schools, offices and domestic hous-

ing, and 10 m to 15 m in commercial developments. Given this description, wall

frames appear to be very simple in concept, but in fact are quite complicated to analyse

because the walls have very large in-plane rigidity whilst the connections between

walls and floors are more flexible. Differential movement between wall panels and

between walls and floors has resulted in major serviceability problems for more than

a 25-year life, often leading to a breakdown in the weatherproof envelope and the

eventual condemnation of buildings which are nevertheless structurally adequate.

A third category of precast building is the 'portal frame', used for industrial

buildings and warehouses where clear spans of some 25m to 40m I or T section

prestressed rafters are necessary (Figures 3.5 and 3.6). Although portal frames are

nearly always used for single-storey buildings they may actually be used to form

the roof structure to a skeletal frame, and as this book is concerned with multi-

storey structures it gives us a reason to mention them. The portal frame looks

simple enough and in fact is quite rudimentary in design, providing that the

flexural rotations at the end of the main rafters, which we can assume will always

cause cracking damage to the bearing ledge, are catered for by inserting a flexible

pad (e.g. neoprene) at the bearing. As mentioned earlier in this section, pinned

connections between the rafter and column are the preferred choice - they are

easy to design and construct. But the columns must be designed as moment

resisting cantilevers - which might cause a problem in some structures as

explained later in Section 6.2. A moment resisting connection is equally possible

allowing moment distribution in the column. However, unless the columns are

particularly tall, say more than about 8m it is not worth the extra effort.

Figure 3.4: Exterior facade to wall frame.

Precast Concrete Structures

Prestressed concrete or

cold rolled steel purlins

and eaves gutter

Prestressed concrete splitter

beams for brickwork support

Prestressed I section rafter

with 4° to 60° roof slope

Typical span 20-30 m

Gable columns

Typical bay 6-8 m

Concrete spine beam may

eliminate need for some

interior columns

Edge columns with haunch or

corbel for rafter support

Typical height 4-8 m

Figure 3.5: Definitions in a precast portal frame.

Figure 3.6: Examples of a precast portal frame (courtesy Crendon Ltd, UK).

Precast concrete structures - Elliott.pdf

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