Civil engineering and building works

Chapter 3

of safety related structures are conveniently similar. The concrete sizes, reinforcement proportions anil distribution thus derived are likely to be shown by analysis to be suitably resistant to collapse when threatened by the occurrence of the SSE.

The .serious student of the design of reinforced concrete power plant superstructures would be well advised to consult references [23] and [24], The first of these comprehensively covers the broader aspects of the structural design and analysis of nuclear power stations including impulsive and impactive (missile) loading; the second covers in depth reinforced concrete analysis, behaviour and detailing (particularly with respect to aseismic design and American research and practice).

Commercial (non-safety-related) structures (such as administration buildings and stores) at nuclear power plants could also be designed to ACI 318M, but it is probably more convenient to use BS8110 as for conven­ tionally-planted thermal stations. Here again reference [24] contains very useful information on the behaviour and detailing of reinforced concrete elements; an understanding of this is required to achieve safe, serviceable and durable structures.

rot-proof compressible boarding (as used in concrete retaining walls, slabs anil similar structure), to ilic complicated roller, rocker or sliding joints provided at the end of bridge or similar beams.

Expansion gaps in walls and slabs can be made water­ tight if necessary by incorporating a suitable water bar and sealant. Expansion joints in buildings must be carefully located and made weatherproof by providing cappings at joints in roofs and in the use of copper water bars or similar methods of sealing vertical walls.

.< Care must be taken with the treatment of internal finishes at the positions where expansion joints are located, plates being used on the floors and cover strips or other features being used on walls or ceilings to permit sliding and prevent cracking of plaster or other finishings.

It should be noted that construction joints are not considered to be movement joints. They are introduced to divide the structure into conveniently sized sections for casting. The reinforcement is continuous across the joint and the face of the joint is prepared to encourage continuity of the concrete. In practice, however, shrinkage frequently occurs at construction joints con­ verting them cll'cclivcly into contraction joints.

13.5 Movement joints

Movement joints are joints which can accommodate relative movement between the adjoining parts ot a member or structure. The principal movements which may have to be accommodated are as follows:

(a) Expansions and contractions due to thermal movement and drying shrinkage of the concrete. An indication of the possible magnitude of these movements is given in BS8110 Part 2 [3].

(b)Rotation and/or translocation at a connection.

(c)Long term increase in deflection due to creep of the concrete.

If the movements noted under item (a) are restrained, stresses will build up which can crack the concrete. Where cracking cannot be reduced to acceptable levels by other means, movement joints are introduced.

The following types of movement joint are com­ monly used in power station construction:

Contraction joint This type of joint is generally used in slabs and walls to permit contraction of the concrete. Initially there is no gap between the concrete on either side of the joint and the reinforcement may be con­ tinuous or discontinuous across the joint. Where it is necessary to achieve a watertight joint, a suitable rubber or neoprene water bar and joint sealant is incorporated.

Expansion joint Expansion joints are designed to accommodate both expansion and contraction. They can vary in complexity from a simple gap filled with

13.6 Curing

Curing is the process ot keeping the concrete saturated whilst maintaining the temperature within suitable limits during the early stages ol hardening.

Concrete hardens as a result of hydration ol the cement. Hydration can only take place when the pores in the concrete are adequately wet and the tinal strength partially depends on the extent to which hydration has taken place. The temperature of the concrete determines the rate at which hydration takes place and hence the rate of strength development.

Keeping the concrete saturated during the early stages of hardening is achieved by preventing loss of moisture by evaporation from the surface. The most Common methods of preventing loss of moisture include retaining formwork in position, spraying or flooding the surface with water, covering the surface with a damp absorbent material, covering the surface with a waterproof membrane such as polythene and spraying the surface with a sealing compound. Curing should commence immediately the concrete has set to prevent premature drying out of the surface.

The perio.d of curing required depends on the type of cement, the ambient conditions and the temperature of the concrete. BS8110 Part 1 lays down the minimum curing periods taking these factors into account. For example, the minimum curing period for concrete made with ordinary Portland cement, under average ambient conditions and at an average surface tempera­ ture of 5-10°C, is four days.

252

Reinforced concrete

The next generation of UK nuclear plant

starting

W

=

operating wind load

with

Sizewell

B will

have pressurised

water

reactors

Ro =

pipe and equipment reactions

(PWRs). Analysis of the seismic response of the main

structures for Sizewell is discussed in ‘Seismic Design

To

=

thermal

effects

during

normal operation

or

Approach for the Sizewell B Nuclear Power Plant [21],

shutdown

which includes the background to soil-structure interac­

Ess

=

load

effects of safe shutdown

earthquake

tion (SSI)' analysis. However, it should be noted that in

the early stages of analysis the ground motion is applied

(SSE)

Wt

=

loads

generated

by

extreme

wind

or related

directly to the foundations in order to estimate

the

gross loads for calculation of stresses. This procedure

internal forces and moments

will

be shown

to be

conservative by comparison

with

U

=

required

strength

to

resist

factored

loads

or

the global SSI analysis at a later date. Civil design and

related internal forces and moments

analysis work may thus proceed independently of the

Then typical design load combinations are:

SSI analysis so that the iterations which would other­

wise be necessary

between these

two parts of

the

U

=

1.4D

+ 1.4F + 1.7L + 1.7H + 1.7W +

1.7Ro

seismic design process are avoided. Whilst the section

U

=

D +

F + L + H + To + Ro + Em

sizes are determined by adopting a fixed base type of

analysis it

still remains

possible

to

benefit

from a

U = D + F + L + H + T„ + Ro + W,

subsequent SSI analysis in the design of reinforcement.

Although

the

section

strengths

determined

in

accord­

The demonstration

that

structures

can withstand the

ance

with

ACI

349

are

similar

to

the ULS

strengths

loading which

is imposed

on them

during the SSE is

of

BS8110,

appropriate

adjustment

must be

made

to

carried out using analysis procedures in line with inter­

account

for

the

different

materials and strength testing

nationally accepted standards and practices. As there is

methods in the UK. Appendix C of ACI 349 deals with

a CEGB commitment that the plant will be manifestly

special provisions for impulsive and impactive effects.

licensable in the USA (the country of origin), Sizewell

Plant

in

the

second category

comprises items which

and

probably

future

PWR plant

concrete structures

are

not

themselves required

to

function after

the SSE

(excluding

reactor

containment) in the UK

will be

but whose

failure could imperil other plant of the first

analysed and designed in accordance with AC1 349-80

category. The

design

bases

have to

ensure

that where

[22].

relevant:

There are two categories of plant at a PWR station

for which a capability to withstand the SSE must be

• The failure of whole or part of a second-category,

demonstrated. Plant in the first category is required to

structure docs not jeopardise the integrity of a first-

function after the SSE so that safe shutdown can be

category structure, system or component.

achieved. The

design

bases are

defined to ensure

that

•

Radiological

protection is maintained as appropriate

where relevant:

for structural elements or the structures as a whole.

• The safety

related

structures

or

structural

members

It is often impractical to provide sufficient separation so

do not

collapse

or severely

distort

such that

con­

that second-category structures would not impinge on

tinued

operation

of

safety

related

plant

is

pre­

first-category structures if they collapsed. Either they

judiced.

must be designed as first-category structures (which is

•

Structural

integrity

is

maintained

to ensure

that

not always economic) or their integrity must be demon­

there are no releases of air or liquid-borne radio­

strated by inelastic dynamic analysis to show that under

activity

in excess

of

the targets

defined,

or losses

the SSE the deformations are well below those levels

of radiological protection.

that lead to collapse. This second (inelastic) design

This is most conveniently achieved by working to the

option is the most demanding of expertise, since it

methods and design criteria of ACI 349 which will

requires rigorous assessment of the damping and duc­

ensure that when subjected to various combinations of tilities appropriate to the structural behaviour and

gravity, wind, thermal, seismic and accidental loads	advanced numerical analysis techniques. The reinforce­
the structures will behave generally in a linear-elastic	ment distribution and detailing must be correct to
manner. Elastic behaviour is considered as limited by	achieve the ductility and hence energy absorption im­

the yield stress of the effective load-carrying structural plicit in the inelastic analysis. In order to arrive at the

	materials or ultimate capacity of an element.			proportions and reinforcement of a second-category
		Thus using the nomenclature of ACI 349, if:		structure which can then be analysed for resistance to
	D	=	dead loads	collapse, it is probably most convenient to work within
				the code requirements of ACI 318M-83 [23], from
	F	=	lateral andvertical pressure of liquids	which ACI 349-80 was developed. The format is
	L	=	live loads	common to both codes (though appendices are differ­
				ent as they relate to separate areas) and therefore the
	H	=	lateral earthpressure	methods for the design and analysis of both categories

251

Civil engineering and building works

Chapter 3

FINE CONCRETE

FILLING

REINFORCEMENT

PRECAST

PRESTRESSED

BEAMS

LIGHT CONCRETE

IN-FILL BLOCKS

FLOOR FINISH

HOLLOW BEAM FLOOR

BOX SECTION

PRECAST UNITS

WITH ENDS CUT

AWAY TO FIT

TOP FLANGE OF

BEAM

STEEL BEAM WITH

SHELF ANGLES

FLOOR FINISH

PRESTRESSED BEAM AND IN-FILL UNIT FLOOR

STEEL BEAM WITH

BOTTOM FLANGE

ENCASED IN

CONCRETE AS

ALTERNATIVE

METHODOF

SUPPORT TO FLOOR

UNITS

Fig. 3.43 Precast concrete floors

254