TECHNICAL PAPER # 67
UNDERSTANDING SMALL-SCALE
BRIDGE BUILDING
By
Robert J. Commins
Technical Reviewers
Dr. Luis Prieto-Portar
Alfred Samuel
Amde M. Wolde-Tinsae
Published By
VITA
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 * Fax:
703/243-1865
Internet: pr-info@vita.org
Understanding Small-Scale Bridge Building
ISBN: 0-86619-306-5
[C] 1990,
Volunteers in Technical Assistance
PREFACE
This paper is one of a series published by Volunteers in
Technical
Assistance to provide an introudction to specific
state-of-the-art
technologies of intrest to people in developing countries.
The papers are intended to be used as guidelines to help
people chooe technologies that are suitable to their
situations.
They are not intended to provide construction or
implementation
details. People are
urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
illustrated
almost entirely by VITA Volunteer technical experts on a
purely
voluntary basis.
Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.
VITA staff included Patrice Matthews
handling production, and Margaret Crouch as project manager.
The author of the paper, Robert J. Commins, is a retired
civil
engineer who has helped VITA answer technical questions
throughout
the Third World.
The paper was reviewed by Dr. Luis Prieto-Portar, the
Director of
Public Works for the City of Miami, Alfred Samuel, a retired
civil engineer specializing in water power, and Amde M.
Wolde-Tinsae,
a professor with the Department of Civil Engineering at
the University of Maryland.
VITA is a private, nonprofit organization that supports
people
working on technical problems in developing countries.
VITA
offers information and assistance aimed at helping
individuals
and groups to select and implement technologies appropriate
to
their situations.
VITA maintains an international Inquiry Service,
a specialized documentation center, and a computerized
roster of volunteer technical consultants; manages long-term
field proejcts; and published a variety of technical manuals
and
papers.
UNDERSTANDING SMALL-SCALE BRIDGE
BUILDING
by
VITA Volunteer Robert J. Commins
INTRODUCTION
Bridges are a part of the transportation system of a region.
They
are used to span an obstacle like a stream or chasm.
Bridges make
the system more efficient either by saving travel distance
or by
enabling vehicles or pedestrians to reach places that were
previously
inaccessible.
There are four basic types of free-standing bridges: beam,
arch,
truss, and suspension.
In addition, pontoon bridges, which actually
float on the surface of the water, are used in some
situations.
While all bridges are built from the basic structural
units of bending, tension, and compression members, the
design of
suspension and pontoon bridges is highly specialized and
their
construction is usually too costly for small-scale
applications.
This paper, then, limits its discussion to beam, arch, and
truss
bridges (Figure 1):
17p01.gif (270x540)
o The beam bridge is
composed of members that flex or bend where
transverse forces
are applied. The first bridge was
probably
this type of
structure: a tree that fell across a stream was
used to cross on
foot.
o The arch bridge
was developed next, first appearing in Mesopotamia
about 4000
B.C. The arch bridge is primarily a
compression
member, subject to
forces that tend to diminish its
length.
This type of structure built of masonry was
widely used
by the Greeks and
later by the Romans. Arches continue to
be
built, but now
reinforced concrete or steel is used.
o The truss bridge
is composed of both tension and compression
members.
A tension member is subject to forces that
tend to
increase its
length. The truss bridge was first
built in the
16th century A.D.
of wood; many of the covered bridges of the
world are still
built this way. The development of
iron, and
later of steel,
made truss bridges very popular for intermediate
spans (12 to 30
meters). At the same time, the
construction
of beam bridges
became less costly for spans under 12 m.
Eventually they,
also, were used for very heavy, longer spans.
DESIGN CRITERIA
The site for the bridge should be selected on the basis of
minimal
cost and maximal convenience for users.
Most bridge locations
are dictated by such obvious factors as shortest crossing
between
banks of a river or gulley, the need to join roads of a
town, and
replacement of an older structure or one that cannot be
crossed
during floods. Just
as there are no low-cost materials of standard
quality, there is no such thing as low-cost bridge
construction.
If funds are insufficient, a smaller structure should be
built.
Several questions must be answered before choosing the type
of
bridge to build:
o Why is a bridge
needed? The local people must answer this,
since they will
not only be the primary users but probably the
financers,
builders, and maintainers of the bridge.
Local
involvement is
vital in planning this kind of project.
o What type of
traffic will the bridge carry? The type of
traffic--pedestrians
or vehicles or both--determines the design
loads for the
structure. Figure 2 shows design loads
used in
17p03.gif (600x600)
the United
States. Local roads authorities should
be consulted
for loading
requirements. If a structure is for
vehicles,
consideration should be given to future
growth of the region
and to traffic
that may be generated by a more efficient crossing.
o What volume of
traffic will the bridge carry? The volume and
type of traffic
will determine the width of the bridge.
For a
bridge used for
pedestrians, a width of two or three meters is
adequate.
Vehicular traffic however requires at least
one lane
of 3 to 4 meters,
plus an additional width for pedestrians.
If
the bridge is to
be used by motorized vehicles, a raised sidewalk
or curbing should
be used to separate vehicular and pedestrian
traffic.
If the bridge is one way, adequate warning
signs
for motorized
vehicles should be provided.
o What span is
required? If the obstacle spanned is a ravine, the
answer is simply
the width of the gap. In the case of a
river
the answer is more
complex.
A bridge crossing a river should be above the high-water
elevation
to prevent the bridge from being washed out.
It must also
provide an adequate underclearance for boats or other river
traffic. The needed
high-water elevation can usually be determined
by examining the river bank and by asking local people the
highest water they have observed.
Figure 3a illustrates a typical
17p04a.gif (486x486)
river crossing.
Figure 3b illustrates the case of a wide
17p04b.gif (486x486)
floodplain. In this
instance a hydraulic study is necessary,
since the size of the floodplain is reduced and the waterway
narrowed by the combined widths of the bridge piers.
This condition
can result in flooding upstream and increased water velocity
under the bridge.
The increase in velocity can cause severe
erosion damage at the bridge site.
a. Ideal situation:
maintaining existing waterway area will not
affect drive flow
in flood stage.
b. The floodstage
waterway area is reduced by the crosshatched
areas, causing
the high water elevation to increase.
This increase
could cause
flooding upstream and erosion at bridge
site.
After establishing the need, design loads, width, and length
of
the bridge, the services of an engineer are required to
design
the foundations and superstructure.
A discussion of types of
foundations and superstructure follows, including the
information
that must be supplied to the engineer.
SUPERSTRUCTURES
The superstructure of a bridge includes the roadway, the
footpaths,
the railings, and the supporting structural members used
to span the required opening.
Figures 4 through 8 illustrate
17p050.gif (540x540)
types of superstructure.
Wood Beams
Wood beams (Figure 4) require structural grade timber.
Since the
17p05.gif (540x540)
strength of various types of wood varies widely, a source of
a
structural grade timber of known strength characteristics
must be
established before considering this type of structure.
The wood
must be treated with preservatives to prevent rotting.
A wood structure can be built by people with ordinary
carpentry
skills and tools.
The only special equipment that might be needed
is some type of lifting device if the bridge beams are of
excessive
weight.
Concrete Beams
Concrete superstructures can be of the flat slab or of the
beam
and slab type (both shown in Figure 5).
Selection of the type to
17p06.gif (600x600)
be used depends on the load and span requirements of the
structure.
The materials required are wood for building forms, cement,
sand and gravel, clean (potable) water, and reinforcing
steel.
Construction of the forms for this type of structure can be
complex, because they must be capable of supporting the
weight
of the concrete until it is cured.
The dimensions shown in Figure 5 are based on the following
properties
of construction materials:
o Wood:
Allowable stress = 100 kilograms per square
centimeter;
allowable shear
parallel to grain = 10 to 15 kg/sq cm
o Concrete:
Allowable compressive stress = 200 kg/sq cm
o Reinforcing steel:
Allowable stress = 1400 kg/sq cm
o Structural steel
: Allowable tensile and compressive
stress in
bending = 1400
kg/sq cm
These properties are listed to help in estimating how much
material
may be needed. They
may be used for preliminary design.
Building the forms requires ordinary carpentry skills.
Placing
the reinforcing steel and placing and finishing the concrete
can
be done with unskilled labor, provided that the mixture is
properly
vibrated to eliminate air spaces.
Technical skills are
required to design the formwork and determine the
appropriate
mixtures for the concrete.
Required equipment includes carpentry tools, a concrete
mixer,
shovels, wheelbarrows, and concrete-finishing tools
(trowels,
floats, straight-edge, etc.)
To avoid the need to build complex forms, sections of the
structure
can be precast on the ground near the site and then lifted
into place after curing.
The weight of these members may make it
necessary to use a lifting device to set them in place and
means
must be provided to hold them in place after erection.
Precasting
and lifting are more complex and dangerous than pouring the
concrete into forms that have been built in place.
In this case,
the hazards arise from removing the forms before the
concrete has
cured sufficiently to bear its own weight.
Steel
Two types of steel bridges are shown: a truss (Figure 6) and
a
17p07a.gif (600x600)
beam (Figure 7) system.
17p07b.gif (600x600)
The truss type of structure requires
smaller steel members but needs
extensive fabrication by a local
specialist. Because
the needed skills
are not common, truss construction
may not be an available option.
The steel beam type of structure with a wood or concrete
traffic
surface can be built locally.
Carpentry skills are required for
laying the wood deck, or for building forms for the concrete
deck. It requires
the same skills to build a concrete deck as to
build a concrete bridge, but the forming is much simpler.
The needed equipment includes a lifting device to set the
steel
beams or trusses in place, and ordinary carpentry tools for
laying a wood deck.
A concrete mixer, wheelbarrows, and shovels
are needed to construct a concrete deck, in addition to hand
tools and wire that are needed to place and support
reinforcing
rods.
Arches
A masonry or concrete arch type of structure (shown in
Figure 8)
17p08.gif (540x540)
may be considered for short span lengths of 3 to 12
meters. This
type of structure, if built of masonry, requires skilled
masons
and a local quarry for a supply of stone.
The forming for an
arch is quite complex because curved forms are required to
support
the weight of the masonry or concrete.
The tools and skills required to build a concrete arch
bridge are
the same as those needed to build a concrete beam
bridge. Carpentry
and masonry skills and tools are required if a masonry arch
is chosen.
Table 1 gives guidelines for selecting the type of structure
to
be used for vehicular traffic.
The span lengths noted are a
general guide for bridges from 3 to 25 meters; they vary
depending
on design loads.
TABLE I
GUIDELINES FOR SELECTING TYPE OF BRIDGE
TO BE USED FOR VEHICULAR TRAFFIC
MATERIAL
SPAN LENGTH, SKILLS
TOOLS
COMMENTS
m
Beam Bridge
Wood 3
to 15 Ordinary
Carpentry
Wood of known strength
carpentry
tool
characteristics and use
of wood preservatives are
needed.
Concrete 3
to 10 Ordinary
Carpentry
Reinforcing steel of
(flat slab)
carpentry
tools, known strength
character
skills for
a concrete
istics is needed.
Regular
forming;
mixer,
inspection of steel and
design
wheelbarrow
concrete should be made.
concrete
and shovels
mixes of
desired
strength.
Concrete 3
to 15 As under Con-
As under
As under Concrete
(beam)
crete (flat
Concrete
(flat slab)
slab)
(flat slab)
Steel 3
to 25 Ordinary
Carpentry
Steel of known strength
carpentry
tools.
See characterics.
skills for
also con-
forming or
crete above
placing the
if concrete
deck.
deck is used.
Lifting device.
Truss Bridge
Wood 15
to 25 Carpentry
Carpentry
Structural grade timber
tools
and is required and skilled
a lifting
carpenters for fitting
device
and joining are needed.
Steel 15
to 25 Steel fab-
Drills,
Truss is made up of
rication
wrenches,
angles or channels, and
cutting
and skill in fabrication is
/or
welding needed.
equipment for
steel, and a
Lifting
device.
Arch Bridge
Concrete 3
to 10 See Con-
See Concrete
See Concrete (flat
crete (flat
(flat slab)
slab). In addition,
slab)
skilled carpenters
are required to build
curved forms.
Masonry 3
to 10 Carpentry
Carpentry
Skilled masons and
and masonry
and masonry
carpenters are required
to build curves and the
forms to support the
structure during con-
struction.
The maximal wheel loadings and the minimal spacing between
vehicles
should be established by the community or the authority
requiring the bridge.
For this purpose, an impact figure should
be added to information obtained from vehicle manufacturers.
Sidewalk (footpath) flooring and supports should be designed
for
a uniform load of 400 kg/sq m, unless a load concentration
is
expected.
The cost of the structure is not covered in this
discussion: it
depends on material and labor costs, and these vary widely
from
region to region.
Maintenance
These types of superstructure require minimal maintenance:
o Wood structures
require periodic reapplication of wood preservative.
o Steel structures
require periodic painting to avoid excessive
corrosion.
o Concrete
structures require patching of spalled (flaked or
chipped) areas with
cement grout if they occur.
Reinforced concrete structures can be difficult to maintain
and
often impossible to repair.
The best defense against the need
for maintenance is extreme care in proportioning, mixing,
and
placing the concrete.
Careful placement of reinforcing is equally
important.
Broken and spalled concrete areas should be patched; worn
roadway
surfaces should be given a suitable wearing and paving coat
for
protection. Cracks
should be sealed with a commercial compound
recommended for this purpose.
FOUNDATIONS
The foundations of a bridge include those structural units
that
transmit the loads from the superstructure to the underlying
soil. There are two
types: piers and abutments.
Piers are the
intermediate supports for multispan structures.
Abutments are
the end supports.
The types of piers and abutments to be discussed
are shown in Figures 9 and 10.
Piers and abutments are
17p110.gif (600x600)
supported by foundations, which are of two types:
spread footings
and piles.
A spread footing (Figure 9) is a shallow foundation and is
the
17p11.gif (600x600)
more economical of the two.
It can generally be used for small-span
bridges (less than 12 meters), provided that the soil can
bear the weight (at least 10 T/sq m. Piles (Figure 10) are
required
17p12.gif (600x600)
only if soft surface material is found to be incapable of
carrying shallow footing loads.
Piling is then used to carry the
footing loads to a deeper and firmer stratum.
The use of piling requires someone skilled in soil
evaluation and
boring procedures.
This person performs a soil evaluation at the
site to determine what kind of piling would be the most
economical
and what equipment would be required to install the piling.
Abutments
Abutments carry vertical loads from the superstructure and
lateral loads from the retained earth on one side (Fig.
10a).
17p12a.gif (540x540)
Abutments are of two types:
gravity or cantilever. A gravity
abutment carries its load through compression, and a
cantilever
abutment through a combination of bending and
compression. Since
a gravity abutment is subject to compressive loads only, it
can
be constructed of masonry or unreinforced concrete.
The cantilever
abutment requires the use of reinforced concrete to
withstand
the stress caused by bending.
Piers
Piers carry spans between abutments in order to shorten the
deck
lengths; they are subject to the following forces:
vertical
loads from the structure and from the traffic upon it;
lateral
forces due to the expansion and contraction of the
superstructure
and to the braking of vehicles on the bridge; lateral forces
from
water or ice due to stream flow; and lateral forces due to
wind
loads on the superstructure and to traffic loads.
In the case of
small-span bridges these forces are negligible except for
the
vertical loads from the superstructure and the ice pressures
in
deep rivers of cold-climate areas.
If we disregard all forces
except the vertical loads from the superstructure, the pier
can
be considered a compression member and can be built of
masonry or
unreinforced concrete.
If unreinforced concrete abutments or piers are used, a
square
mesh of 1.25 cm-diameter reinforcing rods should be placed
at
30-cm horizontal and vertical intervals to help control
shrinkage
and surface cracking.
Should a crack develop due to settlement or
temperature stresses, the mesh will keep the faces of the
crack
in contact.
Maintenance
Maintenance of substructure units is normally minimal,
consisting
of patching of spalled concrete or masonry.
Major maintenance
occurs only if erosion undermines abutments or piers.
In this
case filling in the eroded area and placing rock protection
to
prevent further erosion are required.
As prevention, substructure
units should be inspected yearly for erosion damage or
immediately
after unusual run-off.
BIBLIOGRAPHY
1. Gidlow, B. Design
of Suspension Footbridge, College Camp
IN-CE-90
2. Strung, N. Your
Own T (R)oll Bridge, December. 1990
3. Weatherfrod,
G.E., Bridge construction using Logs,
Timbers, Stones
and Soil, VITA Case No. 31977, 1980
4. Small
Footbridges: Design and Construction,
GPO, 1972
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