# Maximum Permissible Compressive Loads in Steel Scaffolds

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1

HONG KONG INSTITUTE OF VOCATIONAL EDUCATION (TSING YI)

Department of Construction

2009/10

Sessional Examination

(Autumn Semester)

Course Name (Code)
:
Higher Diploma in Civil Engineering (51301
F
)

Higher Diploma i
n Civil Engineering (51301A)

Year of Study

:

L4

(51301F)

2 (51301A)

Mode of Study

:

F
T

Unit

:
Temporary Works (CBE3
0
3
3)

Date

:
21

J
anuary

20
10

Time

:
1:30

p
.
m
.

4:30

p
.
m
.

Time Allowed

:
THREE (3) Hours

Instructions to Candidates:

1.

ANY
FOUR (4)

questions

2.

All questions carry
EQUAL

marks.

3.

This question paper has
T
WELVE

(1
2
)

pages.

4.

This question paper contains
SIX (6)

questions.

Examination Data:

Density of Concrete

25 kN/m
3

Available from In
vigilator:

Graph papers

Page
2

Q.1

Fig.Q1
shows a
typical
panel of soffit
formwork
measuring 1800x3300
mm on plan

designed for the construction of a
4
0
0 mm thick concrete
slab

at the top
.

Use the
beam
formulas on Page
8

to c
heck the adequacy of the gi
ven

spacing of the formwork components
. Assume

the following design
and formwork material information
will be
used.

Unit weight of concrete

25 kN/m
3

2.0

kN/m
2

Deflection not to exceed

1/360 o
f span

Formwork Material Information

Formwork
Materials

Permissible
Moment of
Resistance

Permissible

Stiffness

EI

Plywood Sheeting

0.420 kN
-
m /m

7.86 kN /m

2
.50 kN
-
m
2

/m

B
earers each
1.800

metres long

1.
00 kN
-
m

6
.80 kN

7
9.14 kN
-
m
2

Joists
each 3.300 m
long

5
.39 kN
-
m

3
0
.21 kN

100
.58 kN
-
m
2

(25 marks)

(Ans. Max. Spacing Req

d : Bearer
-
@ 451 mm; Joist @ 1291
mm; Prop @ 1485 mm)

Page
3

Q,2

(a)

What are the major responsibilities of a
temporary works
provider under
C
ommon
L
aw?

(5
marks)

(b)

In open country area
s
, dewatering could be employed in lowering
ground water level

for excavation works
.
Explain with

illustration

the arrangement for a typical 2
-
stage dewatering
system indicating the ground water profile before and after
de
watering.

(
10

marks)

(c)

Briefly describe the responsibilities of a contractor who carries
out trench excavation in a carriageway according
to
the

local
Construction Site Safety Regulations.

(
10

marks)

Q.3

Fig. Q3
shows a
7 m tall, 12 m long vertical w
all formwork designed for
concrete placement in a single lift from its top. The
nonlinear
formwork consists of a single skin with a profile

comprising of straight
segments as shown in the figure
. Anticipated site conditions during
con
creting operation

wi
ll be as follows:

Concrete temperature

5

30

C

Unit weight of concrete

25 kN/m
3

Rate of concrete delivery

4 trucks each of 4.5
m
3

per hour.

Given the following formula in computing concrete pressure
acting on formwork,
determine
and

plot
the des
ign concrete
pressure distribution in kPa for the wall form
.

Hint:

R
H
K
R
D
P
0
.
1
45
.
0
0
.
1
max

in kPa, or
Dh

(
2
5 marks)

2
16
36

T
K
Page
4

Ans.

Conc. Pressure Diagram
0.00
75.00
101.19
100.61
98.87
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
0.00
50.00
100.00
150.00
Conc. Pressure (kN/m2)
Elevation above bottom (m)

Page
5

Q.4

(a)

A steel bracket anchored to an external wall is designed to
withstand a vertic
al point load of 10 kN as shown in Fig. Q4(a).
There are two anchor bolts spaced at 600 mm centre to centre to
arrest the
pull
-
out

and shear mode of failures of the bracket

bolting
. Assume a factor of safety of
2.
0 for both failures,

(i)

Determine the desig
n tension and compressive force
for the anchor bolt
s
.

(5 marks)

(Ans. Tension 25 kN; Compression 25 kN; Shear 10 kN)

(ii)

If three anchor bolts
equally spaced at 300

mm

centre
to centre
by

calculation
how
the safety margin
of the steel

bracket
could be
increased.

(10 marks)

Ans.

If top bolt fails: Safe Load W=10 kN (Pull
-
out failure); W=20 kN (Shear
Failure)

If none of bolts fails: Safe Load W=20 kN (Pull
-
out failure); W=30 kN
(Shear Failure)

(b)

Formwork is one of the most importan
t temporary works in
concrete
construction.
State the general principles in design and
construction of a
high quality formwork
.

(10 marks)

Page
6

Q.5

Fig.
Q
5

shows the elevation layout of a typical transverse row of a
birdcage scaffold structure measured
7.0
m (H) and
9.1

m (W). The
re
are 12 rows of
scaffold
framework

in longitudinal direction each
co
mprising

of mild steel tubing and fittings all complying with BS1139.
The
scaffold structure
will be erected to support the construction of a
concrete slab.

Given the following configuration of the
scaffold structure
and design

Concrete slab thickness

20
0 mm

2.0 kPa

Self weight

of Formwork &
Scaffolding

1.0 kPa

Design wind speed

30

m/s

Scaffold St
ructure
Configuration

Lift Height

1.4 m

Bay Length

1.30 m c/c in transverse
direction

1.35 m c/c in longitudinal
direction

Solidity Ratio of scaffolding
exposed to transverse wind:

5%

(a)

Determine the total wind moment acting on the
scaffold structure
.
Assume that the dynamic wind pressure

q

(in
k
Pa)

for wind speed
V
d

(in m/s)
is given by
q
V
d
4
.
40

and pressure coefficient C
f

= 1.3 and 2.0 for circular scaffolding tubing and the
edge form
respectively.

(5 marks)

(Ans.11.60 kN
-
m per row
)

(b)

Determine the total leg load in kN in each individual verticals
(standards) of the scaffold under the combined vertical and
Q
5

to check the
Page
7

Identify if there are any verticals under tension.

Assume
scaffolding tubes of

USED

condition will be used.

(13 marks)

(Ans. Max. Leg Load 14.57 kN; Safe vs Buckling; None of Verticals
are in Tension)

(c)

Assume swivel couplers
each
of 6.25 kN in capacity w
ill be used
to fix diagonal braces
.

D
etermine the minimum number of
diagonal braces required
for each row of scaffold structure

to
resist the horizontal wind load. Draw the layout of the diagonal
braces provided.

(4 marks)

(Ans. Provide ONE diagonal brac
e per row)

(d)

Determine the safety factor against overturning for the

scaffold
structure

when the soffit formwork is standing EMPTY with
out

any
and
any

kentledge
required

to meet the
minimum

safety factor of
1.2

against overturning.

(3 marks)

(
Ans. 4.08)

Hint
:

Formulas for calculating leg loads induced by
overturning
moment M

are given as follows:

Q.6

A propped cantilever steel sheet piling cofferdam is designed to resist
a
5.5

m (H) high vertical cut as detailed in Fig. Q
6
. Given the
fol
lowing soil properties

:

Active earth pressure coefficient K
a

0.
30

Passive earth pressure coefficient K
p

2.85

Unit weight of soil

18 kN/m
3

2
2
2
i
i
i
l
l
p

i
i
i
l
M
p
F

Page
8

15

kN/m
2

and the net earth pressure diagram shown in

the figure, use the
FREE
EARTH SUPPORT

method to determine the:

(i)

Depth of point of zero net earth pressure (Z);

(
2

marks)

(Ans. 0.75 m)

(ii)

Prop load if the props are
spaced to leave a working
clearance of 3.0 m on plan for equipment passing between
the gro
und level and the bottom of excavation
;

(1
0

marks)

(Ans.202.59 kN)

(iii)

Calculate the maximum bending moment and shear force
in the waling member.

(
4

marks)

(Ans. Bending Moment 60.78 kN
-
m; Shear 121.55 kN)

Hint:

Max. Bending Moment for a continuous beam und
er
UDL

2
10
1
L

Max. Shear Force

L

6
.
0

(iv)

Minimum depth of embedment (D) that the pile has to be
driven if the factor of safety for moment equilibrium is
designed to be not less than 2.0. (
Hint start D=X+Z =
2.
75

m for ite
rations
).

(
9

marks)

(Ans. 2.85 m)

-

End of Paper
-

Page
9

Continuous Beam
of more than 3

spans

Max. Bending Moment= 0.107

L
2

Max. Shear

= 0.607

L

L

L

L

L

L

Max. Deflection

=
EI
L
4
00632
.
0

2
-
span Continuous Beam

Max. Bending Moment= 0.125

L
2

Max. Shear

= 0.625

L

L

L

Max. Deflection

=
EI
L
4
00521
.
0

Max. Support Reaction = 1.25

L

Single
-
span Si
mply Supported Beam

Max. Bending Moment=
2
8
1
L

Max. Shear

= 0.5

L

Max. Deflection

=
EI
L
384
5
4

L

Where

L
=

S
pan

EI

= Stiffness of Beam

Page
10

TABLE Q
5

Maximum Permissible Compressive Loads in Steel Scaffolds

(which are manufactured in accordance with BS 1139: Section 1.1:1990
with a yield stress of 225 N/mm
2
)

Effective Length
(mm)

Permissible Axial Compressi

As “New” Tubes

As “Used” Tubes

250

76.4

70.0

500

74.5

63.3

750

70.7

60.1

1000

64.3

54.7

1250

55.3

47.0

1500

45.3

38.5

1750

36.4

30.9

2000

29.3

24.9

2250

23.9

20.3

2500

19.8

16.8

2750

16.6

14.1

3000

14.1

11.9

3250

12.1

10.3

3500

10.5

8.9

3750

9.2

7.8

4000

8.1

6.9

4250

7.2

6.1

4500

6.4

5.5

4750

5.8

4.9

5000

5.2

4.4

5250

4.7

4.0

5500

4.3

3.7

5750

4.0

3.4

6000

3.6

3.1

8000

1.3

1.1

Page
11

A

Joist

Bearer

P
lywood

Spacing

Sheeting

at
4
00 mm c/c

1100

1100

1100

Prop

Spacing

A

Joist

Bearer

900

900

ELEVATION A
-
A

(All dimensions are in mm unless otherwise stated)

Fig.

Q1

4
00 mm Thick Concrete Slab to be cast in
-
situ

Page
12

2 m

3 m

Wall Form

Profile

2.5 m

2 m

2.75 m

1 m

3 m

1 m

4 m

Fig. Q3

10 kN

750

Anchor

Steel Bracket

Bolt

600

Fig. Q4 (a)

(All dimensions are in mm unless otherwise stated)

Page
13

Edge Form for

20
0 mm
thick

Concrete

Slab

20
0

mm

5

lifts each

1.
4

m high

Wind

Ground Level

7 bays each 1.3 m wide

Fig. Q
5

(Not to Scale)

15

kPa

Prop

4.50
kPa

1 m

Waling

5.5

m

34.20

kPa

Z

D

X

Steel Sheet Piling

Fig. Q6

(Net Lateral Earth Pressure Diagram)

(Not to Sca
le)
Page
14