unit 7 ( design details of beams )

REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/

UNIT 7

DESIGN DETAILS OF BEAMS

GENERAL OBJECTIVE

To understand the principles in providing the design details of reinforced

concrete beams according to BS 8110 requirements.

At the end of this unit you will be able to;

1. determine the amount of nominal cover to reinforcement.

2. check whether the minimum and maximum areas of reinforcement are

within limits.

3. determine the distance between bars.

4. calculate anchorage lengths.

5. calculate lap lengths.

6. calculate the cut-off distance for curtailments.

1

OBJECTIVES

SPECIFIC OBJECTIVES

INPUT 1


7.1 Introduction

Reinforced concrete structural elements are designed so that they meet the

requirements of Ultimate Limit State (ULS) and Serviceability Limit State

(SLS). In addition to these requirements, they should also satisfy other

requirements such as the concrete mix, nominal cover to reinforcement and

detailing of the reinforcements. These requirements if fulfilled will ensure that

the elements are durable; this is in construction, economical and cost-effective.

7.2 Concrete Cover

Nominal cover is the thickness of concrete which protects all reinforcements

including links from the surrounding environment in which they are exposed

to. Adequate cover should be provided in order to protect the reinforcement

from fire and corrosion. The amount of cover to be provided is given in Table

3.4 and 3.5, BS 8110. It can be seen that the nominal cover to be provided

depends on various affecting factors. They are as follows;

1) exposure conditions

2) grade of concrete and

3) fire resistance.

2


The limits on water/cement ratio and cement content will automatically be

assured by specifying the minimum grades of concrete indicated in the

Table3.3, BS8110. In actual structure, the nominal cover dimension shown in

the drawings can never be maintained at 100%. Therefore, some tolerances

must be provided. The actual cover to all reinforcements should never be less

than the nominal cover minus 5 mm. The nominal cover should also comply

with the recommendations for bar size, aggregate size and concrete cast

against uneven surfaces. These are elaborated as follows;

a) The nominal cover to a main bar should not be less than the size of the

main bar if it is a single bar, or the equivalent size if they are in pairs.

b) The nominal cover should not be less than the nominal maximum size

of aggregates. This is to facilitate the compaction of fresh concrete.

c) Concrete cast directly against earth as in foundations, the nominal

cover should not be less than 75 mm. If blinding is provided, the

nominal cover should not be less than 40 mm.

In choosing the appropriate cover for a particular structural element, we

should take the largest value derived from the following factors;

a) bar size

b) environmental conditions

c) fire resistance

3


7.3 Minimum and maximum area requirements

BS 8110 recommends that the minimum crack width should not exceed

0.3mm. This is to avoid corrosion of the reinforcements. Observing the

detailing rules regarding the minimum areas of reinforcement and also the

maximum spacing of the bars will satisfy this requirement. Minimum areas of

reinforcement are given in Table 3.27 of the code. Reference should also to be

made to clause 3.12.5.3. The area of reinforcement provided must exceed the

minimum given in Table 3.27 to ensure that the structural element will not be

under reinforced.

The maximum area of reinforcement requirement is to ensure that the

structural element will not be congested with too much steel. Clause 3.12.6 of

the code states that the maximum area of reinforcement in a beam should not

exceed 4% of the gross concrete sectional area.

4

ACTIVITY 7a


TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE TO THE

NEXT INPUT!

7.1. From the given information below, determine the nominal cover

required for simply supported beam.

Exposure condition: moderate

Concrete grade: 35

Fire resistance: 1 .5 hour

Maximum aggregate size: 20 mm

For the given beam section below, calculate the minimum and

maximum areas of reinforcement and then decide whether the

steel provided is satisfactory.

5

4T20

b = 250

h = 700

Figure 7.1: Typical section of beam


Now, let’s check your answers.

3.1 From Table 3.4, BS8110:

Nominal cover = 35 mm

From Table 3.5, nominal cover = 20 mm

Maximum aggregate size = 20 mm

Therefore, the nominal cover is the greatest value derived from all the

three conditions, i.e. 35 mm .

1. From Table 3.25, BS 8110 :

Minimum % of reinforcement = 0.13 %

This implies that,

= 228 mm 2

The minimum area of reinforcement is 228 mm 2 .

6

FEEDBACK 7a

INPUT 2


Maximum area of reinforcement

Thus, for 4T20 (As = 1260 mm2), since 0.13% < As < 4% bh, therefore, the

area of reinforcement provided is all right.

`

7

Oh, you are clever!

You can do it. When you

study and understand it,

everything is not difficult for

you. Come on. We will do

the next part.


7.4 Spacing of reinforcement

BS 8110 specifies the minimum and maximum distances between tension

reinforcement. The minimum distance is based on the need to achieve good

compaction of the concrete around the reinforcement. The limit on the

maximum distance is to ensure that the maximum crack width is under control

to prevent corrosion of the embedded bars. For singly reinforced simply

supported beams, the clear horizontal distance between tension bars, denoted

as must lie within the following limits;

a) For fy = 250 N/mm2 ;

+ 5 mm or bar size ≤ sb ≤ 300 mm

b) For fy = 460 N/mm2 ,

+ 5 mm or bar size ≤ sb ≤ 160 mm

Where hagg is the maximum size of coarse aggregate used in the concrete mix.

8


If the beam is provided with more than one layer of reinforcement, the clear

vertical distance between bars should not be less than . This is shown

diagrammatically in Figure 7.2 below;

9

hagg + 5 mm

aggh3

2

Fig 7.2: Minimum Clear Distance

ACTIVITY 7b


For the given beam section below, determine the clear distance between bars

and then decide whether the spacing requirements are satisfactory. Assume

that the cover is 40 mm and maximum aggregate size is 25 mm.

10

T16 T16 T20

325 mm

FEEDBACK 7b

INPUT 3


Let the clear distance between bars is .

Then, + 2 (40 + 16 + 20 + 20) = 325 mm

Therefore, = 133 mm

From Table 3.30, BS8110,

For fy = 460 N/mm2 and assuming no redistribution of moment,

Clear distance between bars = 160 mm

hagg + 5 mm = 25 + 5 = 30 mm

Since hagg + 5 mm or bar size ≤ ≤ 160 mm, the spacing requirement is

all right.

11

ba ba

16 20 16

40 40


7.5 Anchorage of bars

Reinforcement bars subjected to direct tensile force must be adequately

anchored so that they will not slip-out of the concrete. Bars subjected to

bending should also be provided with adequate anchorage to ensure that the

design stress (0.87 fy for mid-span) will not be reduced. Bars are anchored in

the concrete in order to develop this value. Figure 7.3 gives some clarification

on this matter.

Anchorage is normally achieved by extending bars beyond the point at which

they are theoretically no longer required. This length is equal to the greater of

the:

12

LoadBar slip when anchorage is not adequate

SupportSupport

Bar slip when anchorage is not adequate

Fig 7.3 Anchorage Failure


a) effective depth of the member

b) 12 times the bar size

In order to be effective, anchorage is also dependent upon the bond strength

between bar, concrete and surface area in contact. Anchorage length is

calculated using the following equation:

Where = bar size and = bonding coefficient from Table 3.28 of the code.

Sometimes it is possible to use straight bars when available space is limited. In

this case, anchorage is provided using hooks or bends. The anchorage values

of hooks and bends are shown in Figure 7.4a and 7.4b that’s given in the next

page.

13

4

r

REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 14

4

r

Figure 7.4b: Anchorage length for hook

Note that the anchorage length for 900 bend = 4r but it is not

greater than 12

Note: Anchorage length for hook = 8r but it is not greater than 24

Figure 7.4a: Anchorage length for 900 bend


It should be noted that the radius of the bend is greater for high yield bars than

for more ductile mild steel bars. Bending is to be done in accordance to

BS4466.

The reductions, which are made for bends and hooks, are formed in

accordance with the standards for bar bending. Refer to Figure 7.4a and 7.4b.

15

Note that the anchorage length for 90 bend = 4r but it is not greater

than 12

Note: Anchorage length for hook = 8r but it is not greater than 24

For mild steel bars minimum radius, r = 2

For high yield bars minimum radius, r = 3 or 4 for sizes 25 mm

and above.

ACTIVITY 7c


Calculate the anchorage length that is required for the bar shown below;

7.2

7.3

16

2T20

?

d = 550mm

?

3T25

Anchorage length = _______________________


Anchorage length = _______________________

7.4 Given, fy = 460 N/mm2, fcu = 30 N/mm2, bar size is 16 mm of Deformed Bar

Type 2 High Yield Steel as tension reinforcement. Calculate the anchorage

length for this bar.

Compare your answers with the calculations that are given below. Please refer

to your lecturer if you get any confusion. Enjoy it!

7.2 Anchorage length = 12

= 12 x 20 mm

= 240 mm

This is to be extended beyond the centre line of support.

7.3 Anchorage length measured from the face of support;

17

FEEDBACK 7c

INPUT 4


7.4 From Table 3.28 of BS8110,

Anchorage length,

7.6 Curtailment of reinforcement

Steel reinforcement can be cut-off or their numbers can be reduced at suitable

places in the beams for various reasons. They are as follows;

a) Bending moment decreases on either side at mid-span. The

corresponding area of bending reinforcement will be reduced because

smaller bending moments require a smaller number and hence smaller

areas of reinforcement.

18

ACTIVITY 7d


b) The reduction in number of bars helps in reducing cost and they are

easier to handle too.

c) Congestion of bars can be avoided thus enhancing the compaction of

fresh concrete.

Simplified rules for curtailment of bars are given in Clause 3.12.10.2 of

BS8110. These are shown diagrammatically in Figure 3.24 of the code for

simply supported, continuous and cantilever beams.

The simplified rules are used when the following conditions are met;

a) The beams carry predominantly uniformly distributed loads.

b) In the case of continuous beams, the spans are approximately equal.

Calculate the curtailment distance indicated in the given figures below;

7.5 Simply supported beam

19


Distance=______________________________

7.6 Cantilever

Distance = ____________________________________

Let’s check the answers together.

7.5 0.08 = 0.08 x 10,000 mm

20

10.0m

?

FEEDBACK 7d

?4 T20

INPUT 5


= 800 mm

7.6

45 = 45 x 20

= 900 mm

Since , the curtailment distance is taken as 1500 mm.

7.7 Laps in reinforcement

Lapping of bars is necessary in order to transfer stresses from a bar to another

bar. Bars that have been joined in this way acts as a single length bar. The lap

should be sufficiently long. The minimum lap length should not be less than

21

“Are your answers correct? ‘Yes!’ Congratulations if you have got the right answers. ‘No?’ Never mind if your answers are wrong. Please correct them and make sure you do not repeat it.

ACTIVITY 7e


15 times the bar diameter or 300 mm, whichever is the greater, as stated in

Clause 3.12.8.11.of BS8110.

For tension laps, it is normally equal to the tension anchorage length, but will

often need to be increased as outlined in clause 3.12.8.13 of the code. The

anchorage length, L, is calculated using the following equation;

Where is the diameter of the smaller bar and LA is to be obtained from

Table 3.29 of the code.

For compression laps, the lap length should be at least 1.25 times the

compression anchorage length.

Note that a longer lap length is required at the top and corner of the beam

section. This is because at the top of the section, fresh concrete is less compact

and contain more water than at anywhere else in the section, while at the

corner of the section, bars are less restrained.

Fill in the blanks.

7.7 Lapping of bars is required to __________ stresses from one bar to the

other.

7.8 Lap length to be provided must not be less than ______________.

22

L = LA

Cover = 30 m

m


7.9 Longer lap lengths are required at __________.

7.10 For T20 Deformed Type 1 bars, tension lap length is equal to

__________ mm if concrete of grade 25 is used.

7.11 For T16 deformed Type 2 bars, the compression lap length is equal to

______ mm when concrete of grade 30 is used.

7.12 The minimum lap length for bars in question 5 is equal to

__________mm.

7.13 The compression anchorage length, L for bar in question 4 is equal to

______mm.

7.14 Calculate the lap length for bars given in question i to iv.

i)

ii).

23

Cover = 40mm


iii)

iv)

7.7 Transfer

7.8 15 or 300 mm

24

65 mm

FEEDBACK 7e

Cover = 40 mm

60 mm


7.9 top and corner of section.

7.10

= 1020 mm

7.11 7Φ = 37 x 16

= 592 mm

7.12 1.25 x 29 Φ = 1.25 x 29 x 16 = 580 mm

7.13 41 Φ = 41 x 20 = 820 mm

7.14 i) Lap length = 1.4 x tension anchorage length

= 1.4 x 37 Φ

= 1.4 x.37 x 20

= 1036 mm

ii) Lap length = 1.4 x 37 Φ = 1.4 x 20 = 1036 mm

iii) Lap length = 1.4 x 37 Φ = 1.4 x 20 = 1036 mm

iv) Lap length = 2.0 x tension anchorage length

= 2.0 x 37 Φ

= 2.0 x 37 x 20

= 1480 mm.

25


This unit should give you some understanding of the design details of

reinforced concrete beams. The design details requirements that we have

discussed are as follows;

1. Minimum concrete cover

This is given by clause 3.3.1 of BS 8110 and is dependent on:

a) diameter of bar

26

SUMMARY


b) type of structural member

c) exposure conditions of concrete

d) type of cement used

e) type of aggregate used

f) fire resistance required

2. Laps

These may be required…

a) to reduce bar handling length

b) at construction joints

c) in confined spaces

The minimum length of bars is given by…

a) concrete mix

b) type of bar

c) type of stress

The position of laps must be:

a) at points of minimum stress

b) staggered in adjacent bars

3. Hooks and Bends

The minimum internal radius must be:

a) twice the bar diameter for mild steel

b) thrice the bar diameter for high yield steel

c) the radius of the anchoring bar which is to be bend round

27


Hooks and bends must:

a) have a minimum straight length of four times the bar diameter

b) be marked hook up or hook down when at right angles to the

plan of the detail drawing

c) not be positioned in tensile zones

d) not foul with other bars

4. Minimum and maximum areas of reinforcement

For rectangular beams with overall dimensions b and h, the area of

tension reinforcement, As should lie within the following limits;

0.24% ≤ As ≤ 4 % bh when fy = 250 N/mm2

0.13 % bh ≤ As ≤ 4% bh when fy = 460 N/mm2

5. Arrangement of bars

a) a minimum cover must satisfy clause 3.3.1 of BS 8110

b) horizontal and vertical spacing must satisfy clause 3.12.11 of

BS 8110

c) if different diameters, then the larger diameter bars must be ;

i. in a single row placed outside

ii. for two or more rows placed in the lower row

iii. placed at the upper row on top over supports

28


6. Curtailment of bars

a) theoretical cut-off position determined from bending moment

diagram

b) actual position allowed for bond length

c) cut-off bars in pairs symmetrical about beam centre line

d) 50% of bottom bars to be carried through to the support

e) simplified curtailment rules must satisfy clause 3.121.10.2 of

BS8110

Answer all the questions given in this section. Award 1 mark for every correct

answer. There are TEN questions (100 % marks). Select your answer by

encircling the alphabet of your choice (A to D).

Questions:

29

SELF-ASSESSMENT


1. Which of the following beam sections correctly explain the meaning of

nominal cover, c?

2. The nominal cover should not be less than the following values

EXCEPT …

A. the amount shown in Table 3.4 of BS8110.

B. the amount shown in Table 3.2 of BS8110.

C. (for main bars) the bar size.

D. the nominal maximum size of aggregate.

30

Cover

Cover

Cover Cover

A. B.

C. D.


3. For the given beam section, what is the minimum area of

reinforcement, assuming high yield steel is used?

A. 468 mm2

B. 324 mm2

C. 234 mm2

D. 432 mm2

4. What is the maximum area of reinforcement for the beam section in

Question 3?

A. 7200 mm2

B. 10800 mm2

C. 14400 mm2

D. 18000 mm2

5. If T16 bars are used, what is the minimum lap length required?

31

600 mm

300 mm


A. 220 mm

B. 250mm

C. 300 mm

D. 320 mm

6. For the beam and reinforcement shown below, what is the minimum

horizontal clear distance between bars if the maximum aggregate size ,

hagg = 20 mm ?

A. 20 mm

B. 25 mm

C. 30 mm

D. 32 mm

7. What is the minimum vertical clear distance for bars in beam section

shown below? (Use hagg = 25 mm).

32

T32 T25 T32


A. 10 mm

B. 17 mm

C. 24 mm

D. 31 mm

8. The beam section given below is taken at mid-span of a simply

supported rectangular beam. If Type 1 deformed bars are used, the

bond coefficient, β is equal to …

A. 0.28

33

?

2 T20


B. 0.35

C. 0.40

D. 0.50

9. For the given continuous beam, what is the length of curtailment for

the reinforcement shown?

A. 975 mm

B. 1625 mm

C. 650 mm

D. 520 mm

10. For the given continuous beam of equal span of 6.5 m, what is the

curtailment distance for top steel at support indicated in the figure

where 40% of reinforcement will be cut-off ? (Assume d = 600 mm).

34

?


A. 975 mm

B. 1625 mm

C. 300 mm

D. 529 mm

Now check your answers with the answers given below.

1. D

2. B

3. C

4. A

5. D

NOW TOTAL UP YOUR MARKS.

Calculate your score as shown below:

35

?

6.5 m

FEEDBACK ON SELF-ASSESSMENT

6. D

7. B

8. C

9. C

10. A


Score = total marks obtained x 100% 10

36

unit 7 ( design details of beams )

Documents

concrete cover nominal

nominal cover dimension

actual cover

minimum area of reinforcement

appropriate cover

adequate cover

thickness of concrete

concrete mix