Concrete is arguably the most important building material, playing a part in all building structures. Its virtue is its versatility, i.e. its ability to be moulded to take up the shapes required for the various structural forms. It is also very durable and fire resistant when specification and construction procedures are correct.
Concrete can be used for all standard buildings both single storey and multistorey and for containment and retaining structures and bridges. Some of the common building structures are shown in Fig. 1.1 and are as follows:
- The single-storey portal supported on isolated footings;
- The medium-rise framed structure which may be braced by shear walls or unbraced. The building may be supported on isolated footings, strip foundations or a raft;
- The tall multistorey frame and core structure where the core and rigid frames together resist wind loads. The building is usually supported on a raft which in turn may bear directly on the ground or be carried on piles or caissons. These buildings usually include a basement.
Complete designs for types 1 and 2 are given. The analysis and design for type 3 is discussed. The design of all building elements and isolated foundations is described.
Structural Elements And Frames
The complete building structure can be broken down into the following elements:
- Beams horizontal members carrying lateral loads
- Slabs horizontal plate elements carrying lateral loads
- Columns vertical members carrying primarily axial load but generally subjected to axial load and moment
- Walls vertical plate elements resisting vertical, lateral or in-plane loads
- Bases and foundations pads or strips supported directly on the ground that spread the loads from columns or walls so that they can be supported by the ground without
To learn concrete design it is necessary to start by carrying out the design of separate elements. However, it is important to recognize the function of the element in the complete structure and that the complete structure or part of it needs to be analysed to obtain actions for design. The elements listed above are illustrated in Fig. 1.2 which shows typical cast-in-situ concrete building construction.
A cast-in-situ framed reinforced concrete building and the rigid frames and elements into which it is idealized for analysis and design are shown in Fig. 1.3. The design with regard to this building will cover
1. one-way continuous slabs
2. transverse and longitudinal rigid frames
3. foundations
Various types of floor are considered, two of which are shown in Fig. 1.4. A one-way floor slab supported on primary reinforced concrete frames and secondary continuous flanged beams is shown in Fig. 1.4(a). In Fig. 1.4(b) only primary reinforced concrete frames are constructed and the slab spans two ways. Flat slab construction, where the slab is supported by the columns without beams, is also described. Structural design for isolated pad, strip and combined and piled foundations and retaining walls (Fig. 1.5) is covered in this article.
Structural Design
The first function in design is the planning carried out by the architect to determine the arrangement and layout of the building to meet the client’s requirements. The structural engineer then determines the best structural system or forms to bring the architect’s concept into being. Construction in different materials and with different arrangements and systems may require investigation to determine the most economical answer. Architect and engineer should work together at this conceptual design stage.
Once the building form and structural arrangement have been finalized the design problem consists of the following:
- Idealization of the structure into loadbearing frames and elements for analysis and design
- Estimation of loads
- Analysis to determine the maximum moments, thrusts and shears for design
- Design of sections and reinforcement arrangements for slabs, beams, columns and walls using the results from 3
- Production of arrangement and detail drawings and bar schedules
Fig. 1.2 (a) Part elevation of reinforced concrete building; (b) section AA, T-beam; (c) section BB, column; (d) continuous slab; (e) wall; (f) column base.
Fig. 1.3 (a) Plan roof and floor; (b) section CC, T-beam; (c) section DD, column; (d) side elevation, longitudinal frame; (e) section AA, transverse frame; (f) continuous one- way slab.
Fig. 1.4 (a) One-way floor slab; (b) two-way floor slab.
Fig. 1.5 (a) Isolated base; (b) wall footing; (c) combined base; (d) pile foundation; (e) retaining wall.
Calculation, Design Aids and Computing
Calculations form the major part of the design process. They are needed to determine the loading on the elements and structure and to carry out the analysis and design of the elements. Design office calculations should be presented in accordance with Model Procedure for the Presentation of Calculations, Concrete Society Technical Report No. 5 [1].
The examples in the book do not precisely follow this procedure because they are set out to explain in detail the steps in design. The need for orderly and concise presentation of calculations cannot be emphasized too strongly.
Design aids in the form of charts and tables are an important part of the designer’s equipment. These aids make exact design methods easier to apply, shorten design time and lessen the possibility of making errors. Part 3 of BS8110 consists of design charts for beams and columns, and the construction of charts is set out in this book, together with representative examples.
The use of computers for the analysis and design of structures is standard practice. In analysis exact and approximate manual methods are set out but computer analysis is used where appropriate. Computer programs for element design are included in the book. However, it is essential that students understand the design principles involved and are able to make manual design calculations before using computer programs.
Detailing
The general arrangement drawings give the overall layout and principal dimensions of the structure. The structural requirements for the individual elements are presented in the detail drawings. The output of the design calculations are sketches giving sizes of members and the sizes, arrangement, spacing and cut-off points for reinforcing bars at various sections of the structure. Detailing translates this information into a suitable pattern of reinforcement for the structure as a whole. Detailing is presented in accordance with the Joint Committee Report on Standard Method of Detailing Structural Concrete [2].
It is essential for the student to know the conventions for making reinforced concrete drawings such as scales, methods for specifying bars, links, fabric, cut-off points etc. The main particulars for detailing are given for most of the worked exercises in the book. The bar schedule can be prepared on completion of the detail drawings. The form of the schedule and shape code for the bars are to conform to BS4466:1981: Specification of Bending Dimensions and Scheduling of Bars for the Reinforcement for Concrete
It is essential that the student carry out practical work in detailing and preparation of bar schedules prior to and/or during his design course in reinforced concrete.
Computer detailing suites are now in general use in design offices. Students should be given practice in using the software during their degree course.
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