Footbridge Design Report
Footbridge Design Report
Footbridge Design Report
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
Rural Zimbabwe has a net work of local streams and rivers which during
the rainy season become impassable due to high flood. The rivers in
most cases become barriers to the service points. Cases of school
children either missing school or being swept away by flooded rivers are
a common occurrence. It is important Therefore that suitable, safe and
affordable communication options be provided for these rural areas.
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
The client requested that the designs cover inter alia, the following major
aspects:-
(i) the designs will include three possible foundations conditions for the
substructures. The conditions being rock, good gravel (or sand) and poor
soil (especially active clays). The detailed ground treatment methods for
each case will be given as accompanying notes on the working drawings.
(ii) a sample of the calculations for the longest span (160m) will be
presented to the client first before a detailed design of the rest of the
bridges is carried out. The client has to approve in writing the design
concept and working drawing layouts first before the rest of the designs
are done
(iii) a sample of the calculations with the variables will be submitted so
that the client can in future carry out calculations without going through a
rigorous calculations
(iv) the design project was to take a total of twelve (12No.) weeks to full
reports presentation from written commission
Page 5 of Chapters 1 to 5
DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
The client requested for footbridges to cater for low lying areas. This was
taken to mean that the river channel crossings might not be very deep
such the footbridges might need to be elevated to gain enough freeboard.
The suspension bridge has the deck supported from the main cables
which are in-turn supported on high rise towers. The towers enable the
requisite freeboard to be taken care of from the bridge rather than from
the river. The main disadvantage of the suspension bridge is the high
towers which may give construction problems since the erection of the
towers and cables might need the use of specialised contractors. The
community participation aspect in the bridge construction will therefore
fall away.
The bridge members will be selected such that the elements can be
manufactured or fabricated easily in a workshop without the need for
complex machinery. The bridge elements should be transported easily to
site and be erected with ease by manual means or by simple lifting or
handling equipment.
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
The design was meant to accommodate the various types of bridge sites
to be expected. The deep and well defined river channels have no
problems in supports and clearance whereas for flat banked rivers, there
is need to gain artificial height clearance for the superstructures. These
two scenarios are represented schematically on the working drawings.
The gain in height can be achieved by raising the approaches by
construction of steps or sloping approach ramp.
In general the bridge supports might not be on the same level thus the
supervising engineer should carry out a design check to cater for the
localised conditions. This might entail combining more than one different
standard drawings.
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
3.1 General
Production of standard designs for use on different sites reduces the
design work load and ensures quality of drawings and construction by
using tested and tried methods. The standard drawings, however, need
to be modified to suit the prevailing site conditions. The site conditions
normally considered are:- accessibility to site, availability of local
resources (i.e materials and labour), the site geological, topographical,
geotechncial and hydrological conditions. These factors need to be
investigated for each site before adopting any particular standard design.
There exists special cases where the suspension bridge might have a
single tower at one end and a gravity foundation at the other end. For
long spans (spans more than sixty metres), the use of windguys (side
sway cables) to limit the sidesway of the bridge is a must for safety
reasons. The main cables are clamped at the top of the towers. There is
a gap between the walkway and towers (tower foundations) such that the
deck is free to 'swing' in all directions.
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
VERTICAL
HANGERS
MAIN
CABLES RC PORTAL
SAG = d
FREEBOARD
HFL
TOWER
MAIN
WALK WAY
CABLES
FLOW
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
3.3.1 Foundations
The foundations will be designed to suit the site conditions. In this case
the foundations will be Gravity Anchored Foundations. Gravity
Foundations derive their stability from the foundation self weight. The
forces to be considered are the loads from the suprstructure,the wind
load acting directly on the footings, lateral earth pressure as well as the
foundation self weight. Loads on top of the foundations will be considered
only where the loads are guaranteed. The earth load on top of the
footings is therefore neglected in this design since in cases where the
load exists, the effect on the footing is stabilising and will be an added
advantage.
TOWER FOOTINGS
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
For spans less than eighty metres (80M), direct windguys might be
provided. These are discussed more in subsequent detailed designs.
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
4. LOADING
4.1 General
The main load cases to be considered for this bridge are:- live load, dead
loads, wind loads and temperature effects. Special and unexpected loads
will also be considered.
This crowd loading is 82% of the calculated ultimate load from literature.
It is therefore assumed safer and within design limits to design for the
worst load case of live load from literature. The crowd load will only be
taken as the design load in cases where the assumed live load becomes
less that the crowd loading.
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
The dead load will be calculated from all basic components of the bridge
structure. The effect of eccentric loading on the bridge during
construction will be considered. The bridge will be loaded non uniformly
during construction from one end or from both ends thus the stability will
be investigated.
4.4 Wind Loads
Wind loads will be considered only on long spans. Only the horizontal
component of the wind will be considered. The wind load is taken as a
uniformly distributed load along the bridge.
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
5. MATERIALS SPECIFICATIONS
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DESIGN CALCULATIONS - STANDARD RURAL SUSPENSION FOOTBRIDGES
The cable terminals for the main cables are anchored into mass concrete
by means of round mild steel pipes which are embedded into the
concrete. The cables form a loop around the steel pipes. The cables
might have bull dog grips or thimbles between joints. The bolts and nuts
should be hot-dip galvanised.
5.3 Concrete
All structural concrete to be grade C25 or better. A proper mix design has
to be made and trial mixes done before actual construction. The density
of reinforced concrete should be taken as 24 KN/m3
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