Manual for Design and Detailing of Reinforced Concrete to the Code

340 Pages·2013·3.14 MB·English

by  Law Chi Wai

Manual for Design and
Detailing of Reinforced
Concrete to the Code of
Practice for Structural Use
of Concrete 2013

September 2013

Manual for Design and Detailing of Reinforced Concrete to
September 2013
the Code of Practice for Structural Use of Concrete 2013

Contents

1.0 Introduction
2.0 Some highlighted aspects in Basis of Design
3.0 Beams
4.0 Slabs
5.0 Columns
6.0 Beam-Column Joints
7.0 Walls
8.0 Corbels
9.0 Cantilevers
10.0 Transfer Structures
11.0 Footings
12.0 Pile Caps
13.0 General Detailing
14.0 Design against Robustness
15.0 Shrinkage and Creep

Appendices

Manual for Design and Detailing of Reinforced Concrete to the
September 2013
Code of Practice for Structural Use of Concrete 2013

1.0 Introduction

1.1 Promulgation of the Revised Code

The revised concrete code titled “Code of Practice for Structural Use of
Concrete 2013” was formally promulgated by the Buildings Department of
Hong Kong in end February 2013 which supersedes the former concrete code
2004. The revised Code, referred to as “the Code” hereafter in this Manual
will become mandatory by 28 February 2014, after expiry of the grace period
in which both the 2004 and 2013 versions can be used.

1.2 Overview of the Code

The Code retains most of the features of the 2004 version though there are
refinements here and there, some of which are subsequent to comments
obtained from the practitioners ever since the implementation of the 2004
version. The major revisions in relation to design and detailing of reinforced
concrete structures are outlined as follows :

(i) Introduction of the fire limit state;
(ii) A set of Young’s moduli of concrete which are “average values” is
listed in the Code, as in addition to the “characteristic values”
originally listed in the 2004 version. The average values can be used in
determination of overall building deflection. In addition, the initial
tangent in the concrete stress strain curve for design (in Figure 3.8 of
the Code) has been given a separate symbol E which is different from
d
the Young’s modulus of concrete with the symbol E as the two have
c
different formulae for determination;
(iii) The high yield bar (which is termed “ribbed steel reinforcing bar” in
the Code as in CS2:2012) is upgraded to Grade 500 to CS2:2012, i.e.
the yield strength is upgraded from f 460MPa to 500MPa;
y
(iv) The use of mechanical coupler Type 1 and Type 2;
(v) The determination of design force on the beam column joint has been
clarified, together with revision in detailing requirements in some
aspects;
(vi) The discrepancies in design provisions of cantilevers between the 2004
version and the PNAP 173 have generally been resolved in the Code;
(vii) Additional reinforcement requirements in bored piles and barrettes;
(viii) Refinement of ductility detailing in beams and columns;
(ix) Additional ductility detailing in walls.

In the aspects of design and detailing, the drafting of the Code is based on the
following national and international codes, though with modifications or
simplifications as appropriate:

(i) The British Standard BS8110 Parts 1 and 2 generally for most of its
contents;
(ii) The Eurocode EC2 on detailing as mostly contained in Chapter 8;
(iii) The New Zealand Standard NZS 3101 in the design of beam column
joint;
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Manual for Design and Detailing of Reinforced Concrete to the
September 2013
Code of Practice for Structural Use of Concrete 2013

(iv) The New Zealand Standard NZS 3101 in most of the provisions of
ductility detailing for beams and columns;
(v) The ACI Code ACI318-2011 for modifications of some of the
detailing;
(vi) The China Code GB50011-2010 in some respects of detailing
including that of wall.
(vii) The Eurocode BSEN 1536 for the detailing of bored pile and
diaphragm wall.

However, the properties of concrete including the Young’s modulus and the
stress strain relationships are based on local studies by Professor Albert K.H.
Kwan of the University of Hong Kong.

1.3 Outline of this Manual

This Practical Design and Detailing Manual intends to outline practice of
detailed design and detailing of reinforced concrete work to the Code.
Detailing of individual types of members are included in the respective
sections for the types, though the Section 13 in the Manual includes certain
aspects in detailing which are common to all types of members. The
underlying principles in some important aspects in design and detailing have
been selectively discussed. Design examples, charts are included, with
derivations of approaches and formulae as necessary.

As computer methods have been extensively used nowadays in analysis and
design, the contents as related to the current popular analysis and design
approaches by computer methods are also discussed. The background theory
of the plate bending structure involving twisting moments, shear stresses, and
design approach by the Wood Armer Equations which are extensively used by
computer methods are also included an Appendix (Appendix D) in this
Manual for design of slabs, pile caps and footings.

To make distinctions between the equations quoted from the Code and the
equations derived in this Manual, the former will be prefixed by (Ceqn) and
the latter by (Eqn).

Unless otherwise stated, the general provisions and dimensioning of steel bars
are based on ribbed steel reinforcing bars with f 500N/mm2.
y

Design charts for beams, columns and walls are based on the more rigorous
stress strain relationship of concrete comprising a rectangular and a parabolic
portion as indicated in Figure 3.8 of the Code.

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Manual for Design and Detailing of Reinforced Concrete to the
September 2013
Code of Practice for Structural Use of Concrete 2013

2.0 Some Highlighted Aspects in Basis of Design

2.1 Ultimate and Serviceability Limit states

The ultimate and serviceability limit states used in the Code carry the normal
meaning as in other codes such as BS8110. However, the Code has
incorporated an extra serviceability requirement in checking human comfort
by limiting acceleration due to wind load on high-rise buildings (in Cl. 7.3.2).
No method of analysis has been recommended in the Code though such
accelerations can be estimated by the wind tunnel laboratory if wind tunnel
tests are conducted. Nevertheless, worked examples are enclosed in Appendix
A, based on empirical approach in accordance with the Australian/New
Zealand code AS/NZS 1170.2:2011. The Australian/New Zealand code is the
code on which the current Hong Kong Wind Code has largely relied in
deriving dynamic effects of wind loads.

2.2 Design Loads

The Code has made reference generally to the “Code of Practice for Dead and
Imposed Loads for Buildings 2011” for determination of characteristic gravity
loads for design. However, the designer may need to check for the updated
loads by fire engine for design of new buildings, as required by FSD.

The Code has placed emphasize on design loads for robustness which are
similar to the requirements in BS8110 Part 2. The requirements include design
of the structure against a notional horizontal load equal to 1.5% of the
characteristic dead weight at each floor level and vehicular impact loads (Cl.
2.3.1.4). The small notional horizontal load can generally be covered by wind
loads if wind loads are applied to the structure. Identification of key elements
and designed for ultimate loads of 34 kPa, together with examination for
progress collapse in accordance with Cl. 2.2.2.3 of the Code can be exempted
if the buildings are provided with ties in accordance with Cl. 6.4.1 of the Code.
The reinforcement provided for other purpose can also act as effective ties if
continuity and adequate anchorage for rebar of ties have been provided. Fuller
discussion is included in Section 14 of this Manual.

Wind loads for design should be taken from Code of Practice on Wind Effects
in Hong Kong 2004.

It should also be noted that there are differences between Table 2.1 of the
Code that of BS8110 Part 1 in some of the partial load factors . The
f
beneficial partial load factor for wind, earth and water load is 0 and that for
dead load is 1.0 which appear more reasonable than that in BS8110 giving 1.2
for both items. However, higher partial load factor of 1.4 is used for earth and
water pressure that in BS8110 giving 1.2 and 1.0 so as to account for higher
uncertainty of soil load as experienced in Hong Kong.

2.3 Materials – Concrete

Table 3.2 of the Code has tabulated Young’s Moduli of concrete up to grade
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Manual for Design and Detailing of Reinforced Concrete to the
September 2013
Code of Practice for Structural Use of Concrete 2013

C100. The listed characteristic values in the table are based on local studies
which are generally smaller than that in BS8110 by more than 10%. In
addition, average values (with cube strength 5N/mm2 lower than the
characteristic values) are listed which are allowed to be used to check lateral
building deflections.

Table 4.2 of the Code tabulated nominal covers to reinforcements under
different exposure conditions. However, reference should also be made to the
“Code of Practice for Fire Safety in Buildings 2011”.

The stress strain relationship of concrete has been well defined for grade up to
C100. It can readily be seen that as concrete grade increases, the transition
point between the parabolic and rectangular portion at  1.34f / /E
0 cu m d
shifts so that the parabolic portion lengthens while the rectangular portion
shortens. In addition, the ultimate strain also decreases from the value 0.0035
to 0.00350.00006 f 60 when f 60as illustrated in Figure 2.1 for
cu cu
grades C35, C60, C80 and C100. These changes are due to the higher
brittleness of the concrete at higher grades which are modified from BS8110
as BS8110 does not have provisions for high grade concrete.

Concrete Stress Block for Grades C35, C60, C80 and C100
fcu = 35 fcu = 60 fcu = 80 fcu = 100
 =0.3121
50 cu
45
 =0.3171
40 cu
a) 35
P
M
Stress ( 2350
crete 20 0=0.2136 0=0.2510
n
Co 15
10 =0.2840
0
=0.1569
5 0
0
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035
Strain in Concrete

Figure 2.1 – Stress Strain Relationship of Grades C35, C60, C80 and C100

in accordance with the Code

Following the provisions in BS8110 and other codes include the Eurocode
EC2, the Code has provisions for “simplified stress block” as indicated in its
Figure 6.1 of the Code which is reproduced in Figure 2.2. The simplified stress
block is to simulate the more rigorous stress block with the aim of simplifying
design calculations. However, instead of a single stress block of 0.9 times the
neutral axis as in BS8110, the Code has different factors for concrete grades
higher than C45 and C70 to achieve higher accuracy. The equivalent factors
for force and moments of the more rigorous stress block have been worked out
as compared with that of the simplified stress block for concrete grades from
C30 to C100 as shown in Figure 2.3. It can be seen that the simplified stress
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Manual for Design and Detailing of Reinforced Concrete to the
September 2013
Code of Practice for Structural Use of Concrete 2013

block tends to over-estimate both force and moments at low concrete grades
but under-estimate at high concrete grades.
0.0035 for f 60
cu
0.00350.00006 f 60 for f 60 0.67f /
cu cu cu m
0.9x for f 45;
cu
0.8x for45 f 70;
cu
0.72x for 70 f
cu
Neutral Axis
Strain Profile Stress Profile
Figure 2.2 – Simplified stress block for ultimate reinforced concrete design

Variation of Equivalent Force and Moment Factors against Concrete Grade
Simplified Stress Block Rigorous Stress Block - Force Rigorous Stress Block - Moment
1
0.9
0.8
or
ct
a
F
0.7
0.6
0.5
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Concrete Grade

Figure 2.3 – Equivalent Factors of Rigorous Stress Blocks for Force and

Moments as Compared with the Simplified Stress Block

2.4 Ductility Requirements

As discussed in para. 1.2, an important feature of the Code is the incorporation
of ductility requirements which directly affects r.c. detailing. By ductility we
refer to the ability of a structure to undergo “plastic deformation”, which is
often significantly larger than the “elastic” deformation prior to failure. Such
ability is desirable in structures as it gives adequate warning to the user for
repair or escape before failure. Figure 2.4 illustrates how ductility is generally
quantified. In the figure, the relation of the moment of resistance of two
Beams A and B are plotted against their curvatures and their factors of
ductility are defined in the formula listed in the figure. It can be described that
Beam B having a “flat plateau” is more ductile than Beam A having a
comparatively “steep hill”. Alternatively speaking, Beam B can tolerate a
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Manual for Design and Detailing of Reinforced Concrete to the
September 2013
Code of Practice for Structural Use of Concrete 2013

larger curvature, i.e. angular rotation and subsequently deflection than Beam A
before failure.

Moment of Resistance
MmaxA Ductility Factor
0.8MmaxA AuyAA BuyBB

0.75MmaxA

MmaxB

0.8MmaxB

0.75MmaxB

yB yA uA uB Curvature
Figure 2.4 – Illustration of Plots of Ductility of Beams

The basic principles for achieving ductility by r.c. detailing as required by the
Code are highlighted as follows :

(i) Use of closer and stronger transverse reinforcements to achieve better
concrete confinement which increases concrete strengths and
subsequently enhances both ductility and strength of concrete against
compression, both in columns and beams and walls. As an illustration,
a plot of the moment of resistance against curvature of the section of a
500500 column of grade C35 with various amounts of links but with
constant axial load is shown in Figure 2.5. It can be seen that both
flexural strength and ductility increase with stronger links.

Variation of Moment of Resistance with Curvature for 500x500 Column (Grade C35) with
8T20 under Average Axial Stress = 0.6fco with Different Confinement by Transverse Bars
No Link T10@250 T10@150 T12@150 T12@100
700
600
m)
N 500
k
of Resistance ( 340000
nt
me 200
Mo
100
0
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1
Curvature x 10-3

Figure 2.5 – Demonstration of Increase of Ductility by Increase of

Transverse Reinforcement in Column

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