10-04-2012, 04:23 PM
Reinforced Concrete
reinforced concrete design theory & examples_T.J.MacGINLEY_2003_8.27M shihexjtu.pdf.pdf (Size: 8.4 MB / Downloads: 370)
Introduction
1.1 REINFORCED CONCRETE STRUCTURES
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.
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.
DESIGN STANDARDS
In the UK, design is generally to limit state theory in accordance with
BS8110:1985: Structural Use of Concrete
Part 1: Code of Practice for Design and Construction
The design of sections for strength is according to plastic theory based on behaviour
at ultimate loads. Elastic analysis of sections is also covered because this is used in
calculations for deflections and crack width in accordance with
CALCULATIONS, 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.
Simply supported and continuous beams
The aim in this chapter is to put together the design procedures developed in Chapters
4, 5 and 6 to make a complete design of a reinforced concrete beam. Beams carry
lateral loads in roofs, floors etc. and resist the loading in bending, shear and bond. The
design must comply with the ultimate and serviceability limit states.
Further problems in beam design are treated in the chapter as they arise. These
include arrangement of loads for maximum moments and shear forces, analysis of
continuous beams, redistribution of moments, maximum moment and shear envelopes,
curtailment of reinforcement and end anchorage.
SIMPLY SUPPORTED BEAMS
Simply supported beams do not occur as frequently as continuous beams in in situ
concrete construction. They are an important element in precast concrete construction.
The effective span of a simply supported beam is defined in BS8110: Part 1, clause
3.4.1.2. This should be taken as the smaller of
1. the distance between centres of bearings or
2. the clear distance between supports plus the effective depth
Design of ties
Steel reinforcement provided for a tie can be designed to act at its characteristic
strength. Reinforcement provided for other purposes may form the whole or part of
the ties. Ties must be properly anchored and a tie is considered anchored to another at
right angles if it extends 12 diameters or an equivalent anchorage length beyond the
bar forming the other tie.
Internal ties
Internal ties are to be provided at the roof and all floors in two directions at right
angles. They are to be continuous throughout their length and anchored to peripheral
ties. The ties may be spread evenly in slabs or be grouped in beams or walls at
spacings not greater than 1.5lr where lr is defined below. Ties in walls are to be within
0.5 m of the top or bottom of the floor slab.
Horizontal ties to columns and walls
Each external column and, if a peripheral tie is not located within the wall, every
metre length of external wall carrying vertical load should be tied horizontally into the
structure at each floor and at roof level. The tie capacity is to be the greater of 2Ft or
(ls/2.5)Ft if less or 3% of the ultimate vertical load carried by the column or wall
where ls is the floor-to-ceiling height in metres.