Civil engineering and building works

dees not make them economic lot powei slation frames. When comparing the economics of a steel structure with a reinforced concrete structure the cost of the original protection and subsequent treatments must be considered. As will be seen from other parts of this chapter, other factors must also be considered before the type of frame is decided.

In the past, steelwork, after fabrication at the works, was painted with one coat of red oxide or red lead paintbefore being despatched to the site. It was found, however, that in many cases the paint was being applied over mill scale and although the steelwork was erected and painted in the normal way, repainting was very soon required because the mill scale and paint film became detached. It has been the practice, therefore, to deliver steelwork to site unpainted and allow it to weather, thus removing loose mill scale before cleaning and painting commences. The paint treatment in this case consists of;, coat of rust-inhibiting primer followed by one or two undercoats and a Iinislling coal of gloss paint.

However, failure to achieve clean conditions of the base prior to painting on site and the high cost of site labour and access, together with the reduction in cost and the genera) availability of blast cleaning and auto­ matic painting olant at the works, has in recent years made painting at the works a viable economic choice. One finishing coat applied at site, or preferably just touching up damaged or previously unpainted areas is all that is necessary on site.

The provision of large steel frames to indoor sub­ *stationsthefrequencyrevealedofthe need to reduce

maintenance painting, and grit blasting followed by a good paint sequence is one method of obtaining this finish. One of the best specifications is for a normal paint treatment using a special primer on a galvanised or other form of zinc, or aluminium-dipped or spray coating. Up to 20 years protection before repainting is necessary can be anticipated with this treatment.

In certain areas the steelwork is entirely encased in brickwork or reinforced concrete; this is mostly in offices and amenity areas and provides fire protection to the steel frame and makes it easier to build filler walls and apply decorations.

13Reinforced concrete

13.1General

The principles including mixing, transporting, placing and vibration of concrete in foundations, described in Section 5.1 of this chapter, are equally applicable to concrete used in superstructures. A larger proportion of high quality concrete is used,on superstructures as most of the work involves columns, beams, slabs, walls and similar structural members, which are more highly stressed than foundation concrete.

Chapter 3

13.2Formwork

In addition to timber and steel panels as used in foundations, shutters lined with plastic and glass-fibre may be used to obtain a better finish to the concrete.

13.3 Reinforcement

Although mild steel reinforcement is used extensively, high tensile steel reinforcing bars and mesh fabric may fie used. Due to the higher stresses which may be developed, the saving of steel in a structure using high tensile reinforcement may be quite large. Additionally, the reduced diameter or smaller number of bars of this type may help to simplify complicated reinforcement arrangements and case the placing of concrete. The design of reinforced concrete.allows for the transfer of stress by bond from steel to the concrete and vice-versa throughout the length of the bars. Shrinkage of the concrete, whilst setting, develops the bond between the circular outer lace of mild steel bats ami the concrete.

High tensile steel bars have a deformed outer face which permits them to develop higher bond stresses. In addition to bond requirements the ends of bars may require hooks or bends to ensure sufficient anchorage.

A product certification scheme for reinforcing steel known as the Certification Authority for Reinforcing Steels (CARES) is in operation and the use of Cz\ RES material is mandatory in all civil engineering contracts. A typical arrangement of reinforcing bars in a slab, beam and column is shown in Fig 3.42.

Corrosion of steel reinforcement in well-designed reinforced concrete structures is normally prevented by the high alkalinity of the concrete surround. Particu­ larly in very exposed conditions or in acid environ­ ments, however, the designer must consider possible reinforcement. Increasing the concrete cover has been the traditional way of protecting such reinforcement, but recently, plastic-coated reinforcement has been introduced for such situations.

1^.4 Design of reinforced concrete

The design activity splits naturally into the two distinct spheres of non-seisinic-resistant and seismic-resistant aseismic analysis and detailing.

In the UK conventional (i.e., coal, oil, etc.) stations are not designed to resist earthquakes. Therefore the reinforced concrete may be engineered according to normal practice and in particular British Standard Codes of Practice. Nuclear stations are designed to resist an extreme seismic event (with 1 in 10 000 prob­ ability per annum), but it should be noted that not all buildings need to be seismic resistant, only those that are safety related require special treatment. Aseismic design is outside the range of normal UK practice and reference is usually made to US Standards and Codes of Practice.

requires internal stiffening by the use of diaphragm plates and/or rolled-steel sections. The diaphragm plates must have manhole access for steelwork erection and maintenance purposes. Manhole access is also provided in the web/flange plates at ground floor level and at a top floor level.

Although steel to BS4360 [17] grade 43 (mild steel) is used extensively, grade 50 (high yield strength) steels are available. The higher working stresses permitted with this material enable lighter sections to be used for the same purpose with their attendant saving in costs, although larger deflections need to be tolerable.

12.2 Design of members

The design of a building frame can be based on simple, semi-rigid or rigid design methods. If the structural engineer uses simple design the distribution of forces may be determined assuming that members intersecting at a joint are pin connected, i.e., freely supported at their ends. In this case the building frame should be adequately braced against the tendency to sway under applied horizontal loads. In most cases considerable saving in weight of steel can be effected by the applica­ tion of rigid or semi-rigid design or a combination of both methods. These allow for varying degrees of redistribution of primary bending moments between beams or columns, under given loading conditions, throughout the structure.

The total loading on a frame is the result of the imposed loads and dead loads of the structure. As stated earlier it is possible at an early stage in the design calculations to ascertain the imposed loads on the frame. Although experienced structural engineers can make close approximations for the dead loads of the structure it is only after the members have been designed that first assumptions can be checked to ensure that the dead loads have not been under­ estimated.

The design of steel frames to buildings is carried out in accordance with the requirements of BS5950: Part 1 [18]. The general principles and design methods are based on the limit state concept which requires con­ sideration of the limit states at which structures would become unfit for their intended use by applying appro­ priate factors for the ultimate limit state and service­ ability limit state. Examples of limit states relevant to steel structures are given in Section 2 of the standard. The standard comprises nine parts, Part 1 was pub­ lished in 1985 and its use by the profession is in its early stages. It is intended that design guides will be issued from time to time. To date the British Constructional Steelwork Association has issued a set of design examples entitled ‘Course Notes for BS5950, Structural Use of Steelwork in Buildings’ which includes examples of beam and column design. For information regarding properties of steel sections and other useful design

Steel frames

information reference should be made to the publica­ tion of the constructional Steel Research and Develop­ ment Organisation (Constrado) entitled ‘Steelwork Design Guide to BS5950: Part 1: 1985 — Volume 1, Section Properties and Member Capacities’ [19].

12.3 Connections

When using rigid or semi-rigid design methods of construction the connections of the beams to the columns must be capable of transferring the bending moments from the beam to the column, which in turn must have sufficient stiffness to withstand the bending moments induced by the beams. A- typical detail of a rigid joint between a beam and a box column is shown in Fig 3.41. This type of joint is normally described as a moment connection. Friction grip bolts are generally used to transfer the bending moment from the end of the beam into the column. These bolts are tightened up to a specified shank tension such that the shear forces are transferred to the column by friction between the plates and the bending moment is developed by the bolt groups. Slip of the connection is therefore prevented and the bolts are not subjected to shear or bearing forces. The correct shank tension to induce the neces­ sary friction is measured by means of torque wrenches applied to the nuts of the bolts. Alternatively, load­ indicating washers or bolts can be used to perform the same function of ensuring the correct shank tension. Load-indicating washers are crimped and the heads of load-indicating bolts are shaped in a manner such that deformation of the washer or bolt head takes place under the load applied when the nut is tightened to induce the specified, shank tension. The amount of deformation is related to the induced shank tension. The deformation is measured by a feeler gauge or by observation by skilled steelwork erectors. Another type of friction grip bolt is designed such that the top section of the bolt shears when the required shank tension is reached.

In connections which are required to resist shear and bearing forces, grades 4.4 or 8.8 bolts are employed. The latter grade permits higher stresses to be used.

Welding of connections is permitted if carried out under controlled conditions in the fabrication shop where it is subjected to rigorous non-destructive test­ ing. Site welding is only permitted in exceptional circumstances and requires the permission of the engineer. As a rule all main site connections are bolted by the methods described.

12.4 Protection of steelwork

A disadvantage with steel is its lack of resistance to corrosion. Although non-ferrous metals, mainly of the aluminium alloy groups, have been used successfully in certain instances for structural frames, their high cost

Civil engineering and building works

A ULS is reached if the structure becomes unfit for use by reason of collapse, overturning or buckling, it is expected that this condition could be triggered by unexpected loads greater than the characteristic loads, inaccurate assessment of load effects, unforeseen stress redistribution and variations in dimensional accuracy. The characteristic loads are therefore multiplied by an appropriate partial safety factor and the strength of the structural member is checked against the stresses calcu­ lated for the enhanced load condition. When assessing the strength of the structural member the characteristic strengths of materials are divided by appropriate partial safety factors to take account of differences between actual and laboratory values, local weaknesses and inaccuracies in assessment of the resistance of struc­ tural sections.

Thus typically a design load combination could be:

1.2 dead + 1.2 imposed + 1.2 wind

and (in checking the section design strength required to -resist these loads) partial safety factors for strengths of materials could be:

The usual design path is to complete the ULS analysts first and then check the SLS. Generally the SLS can be ensured by attention to deemed-to-satisfy provisions such as limiting span-to-depth ratios, minimum steel percentages and reinforcement bar spacing during the ULS analysis. If a more rigorous analysis is necessary (e.g., deflections and crackwidths) then BS8110: Part 2: Section 3 [3] contains detailed analysis techniques.

The loading conditions during erection and con­ struction should be considered in design and should be such that the structure’s subsequent compliance with the more permanent limit state requirements is not impaired.

Requirements for durability and fire resistance are secondary to the primary limit states in the sense that they require a smaller part of the effort required to develop a design. However, in order to achieve the intended objective (of satisfactory lifetime perform­ ance) they are no less important than the ULS and SLS.

The stress analysis can be split into two stages — global and local:

Global Is analysis of the complete structure to obtain a set of member internal forces and moments that are in equilibrium with the design loads for both the ULS and SLS.

Local Is analysis of the component sections for ULS where inelastic material behaviour is appropriate; analysis of sections for SLS where linear-elastic material behaviour may be used to check the perform­ ance expected under the characteristic loads, with 'appropriate allowance for creep and shrinkage.

It should be noted that a linear elastic analysis is commonly made as a basis for both ULS and SLS, then

Chapter 3

for the ULS forces ami moments to be redistributed with due regard to the ductility of the members concerned. Alternatively an inelastic analysis using yield line theory or Hillerborg’s strip method for slabs, and the plastic hinge concept for beams, may be made.

The state of development of digital computers and commercial software packages is now such that the global stress analysis is often best done using a linearelastic frame or finite element structural analysis pro­ gram. Inelastic yield line and plastic hinge approaches have also been computerised, but the applicability of these is limited when compared with classical linearelastic analysis. Such programs vary tremendously in size, capability and cost. They can be run on the full range of micro (personal), mini and mainframe com­ puters. For large analyses, and particularly when finite elements are employed, mesh generation capabilities and graphical output are necessities, while interactive analysis capability can also be very useful for smaller jobs.

Tens, if not hundreds, of programs are available of which the Structural Design Language — STRUDL — family of programs is notable. The original version operated as a subsystem of the Integrated Civil Engineering System (ICES) developed by Massa­ chusetts Institute of Technology, USA. It uses the Problem Oriented Language (POL) for free-format data input and commands, making it much more user friendly than fixed format programs. Many versions and other compatible programs have been developed for use on all types of computer. Early enhanced versions which included the more advanced finite element, dynamic and geometrically non-linear analysis capabilities were unreliable, but now the most compre­ hensive packages, from IBM and McDonnell Douglas amongst others, are robust and very convenient tools.

13.4.2Seismic-resistant design

In the UK, the magnox nuclear stations were not specifically designed to resist an earthquake, nor was the first tranche of AGR nuclear power plants. The second generation AGR stations at Heysham 2 and Torness were designed to have an elastic response up to a design basis earthquake (DBE), and to be capable of resisting events up to safe shutdown earthquake (SSE) without prohibiting safe shutdown of the facility. The SSE (which has a probability of occurrence of 1 in 10 000 per annum) has a free-field ground motion twice as intense as the corresponding DBE. This design approach is particular to these stations and is unlikely to be repeated. An overall description of the structural design of these plants, with particular emphasis on the design of the reinforced concrete shear walls surround­ ing the prestressed concrete reactor pressure vessel, can be found in ‘The Aseismic Design of a Reactor Building for the Advanced Gas Cooled Reactor Power Plant’ [20].

MAIN TENSION BARS IN

BOTTOM OF SLAB

DISTRIBUTION BARS TO

SLAB REINFORCEMENT

MAIN BARS IN SuAB TURNED

UPTO TOP TO RESIST TENSION

IN SLAB OVER P.r/.M

MAIN COLUMN

REINFORCING BARS

BARS CONTINUED TO MAINTAIN REINFORCEMENT CAGE AND ACT AS COMPRESSION BARS IF REQUIRED

TENSION BARS

IN BEAM

SHEAR STIRRUPS TO ASSIST

IN RESISTING SHEAR FORCE ANO

PREVENT DISPLACEMENT OF

REINFORCEMENT WHEN

PLACING CONCRETE

13.4.1Non-seismic-resistant design

For non-seismic-resistant design it is most convenient to use BS8110 [3] which deals with the structural use of concrete and is published in three parts:

Part 1 — Code of practice for design and construction

Part 2 — Code of practice for special circumstances

Part 3 — Design charts for singly-reinforced beams, doubly-reinforced beams and rectangular columns

This Standard uses the philosophy of limit state design in order to achieve an acceptable probability that the structures being designed will perform satisfactorily

during their intended life (about 40 years), sustaining with an adequate degree of safety all the loads of normal construction and use, having adequate dura­ bility, and also being adequately resistant to the effects of misuse and fire.

For the purposes of structural design two primary limit states can be identified, the ultimate limit state (ULS) and serviceability limit state (SLS).

An SLS is reached if the structure becomes unfit for use by reason of deformation, cracking, vibration, etc., when sustaining loads no greater than those reasonably expected to occur during the design lifetime; These are the characteristic loads (refer to Section 11 of this chapter).