TECHNICAL PAPER # 67
Robert J. Commins
Dr. Luis Prieto-Portar
Amde M. Wolde-Tinsae
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
Tel: 703/276-1800 * Fax:
Understanding Small-Scale Bridge Building
Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introudction to specific
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
They are not intended to provide construction or
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
almost entirely by VITA Volunteer technical experts on a
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
engineer who has helped VITA answer technical questions
the Third World.
The paper was reviewed by Dr. Luis Prieto-Portar, the
Public Works for the City of Miami, Alfred Samuel, a retired
civil engineer specializing in water power, and Amde M.
a professor with the Department of Civil Engineering at
the University of Maryland.
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries.
offers information and assistance aimed at helping
and groups to select and implement technologies appropriate
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
UNDERSTANDING SMALL-SCALE BRIDGE
VITA Volunteer Robert J. Commins
Bridges are a part of the transportation system of a region.
are used to span an obstacle like a stream or chasm.
the system more efficient either by saving travel distance
enabling vehicles or pedestrians to reach places that were
There are four basic types of free-standing bridges: beam,
truss, and suspension.
In addition, pontoon bridges, which actually
float on the surface of the water, are used in some
While all bridges are built from the basic structural
units of bending, tension, and compression members, the
suspension and pontoon bridges is highly specialized and
construction is usually too costly for small-scale
This paper, then, limits its discussion to beam, arch, and
bridges (Figure 1):
o The beam bridge is
composed of members that flex or bend where
are applied. The first bridge was
this type of
structure: a tree that fell across a stream was
used to cross on
o The arch bridge
was developed next, first appearing in Mesopotamia
B.C. The arch bridge is primarily a
member, subject to
forces that tend to diminish its
This type of structure built of masonry was
by the Greeks and
later by the Romans. Arches continue to
built, but now
reinforced concrete or steel is used.
o The truss bridge
is composed of both tension and compression
A tension member is subject to forces that
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
later of steel,
made truss bridges very popular for intermediate
spans (12 to 30
meters). At the same time, the
of beam bridges
became less costly for spans under 12 m.
also, were used for very heavy, longer spans.
The site for the bridge should be selected on the basis of
cost and maximal convenience for users.
Most bridge locations
are dictated by such obvious factors as shortest crossing
banks of a river or gulley, the need to join roads of a
replacement of an older structure or one that cannot be
during floods. Just
as there are no low-cost materials of standard
quality, there is no such thing as low-cost bridge
If funds are insufficient, a smaller structure should be
Several questions must be answered before choosing the type
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
builders, and maintainers of the bridge.
vital in planning this kind of project.
o What type of
traffic will the bridge carry? The type of
or vehicles or both--determines the design
loads for the
structure. Figure 2 shows design loads
States. Local roads authorities should
requirements. If a structure is for
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.
bridge used for
pedestrians, a width of two or three meters is
Vehicular traffic however requires at least
of 3 to 4 meters,
plus an additional width for pedestrians.
the bridge is to
be used by motorized vehicles, a raised sidewalk
or curbing should
be used to separate vehicular and pedestrian
If the bridge is one way, adequate warning
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
the answer is more
A bridge crossing a river should be above the high-water
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
Figure 3b illustrates the case of a wide
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.
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
the high water elevation to increase.
flooding upstream and erosion at bridge
After establishing the need, design loads, width, and length
the bridge, the services of an engineer are required to
the foundations and superstructure.
A discussion of types of
foundations and superstructure follows, including the
that must be supplied to the engineer.
The superstructure of a bridge includes the roadway, the
the railings, and the supporting structural members used
to span the required opening.
Figures 4 through 8 illustrate
types of superstructure.
Wood beams (Figure 4) require structural grade timber.
strength of various types of wood varies widely, a source of
structural grade timber of known strength characteristics
established before considering this type of structure.
must be treated with preservatives to prevent rotting.
A wood structure can be built by people with ordinary
skills and tools.
The only special equipment that might be needed
is some type of lifting device if the bridge beams are of
Concrete superstructures can be of the flat slab or of the
and slab type (both shown in Figure 5).
Selection of the type to
be used depends on the load and span requirements of the
The materials required are wood for building forms, cement,
sand and gravel, clean (potable) water, and reinforcing
Construction of the forms for this type of structure can be
complex, because they must be capable of supporting the
of the concrete until it is cured.
The dimensions shown in Figure 5 are based on the following
of construction materials:
Allowable stress = 100 kilograms per square
parallel to grain = 10 to 15 kg/sq cm
Allowable compressive stress = 200 kg/sq cm
o Reinforcing steel:
Allowable stress = 1400 kg/sq cm
o Structural steel
: Allowable tensile and compressive
bending = 1400
These properties are listed to help in estimating how much
may be needed. They
may be used for preliminary design.
Building the forms requires ordinary carpentry skills.
the reinforcing steel and placing and finishing the concrete
be done with unskilled labor, provided that the mixture is
vibrated to eliminate air spaces.
Technical skills are
required to design the formwork and determine the
mixtures for the concrete.
Required equipment includes carpentry tools, a concrete
shovels, wheelbarrows, and concrete-finishing tools
floats, straight-edge, etc.)
To avoid the need to build complex forms, sections of the
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
must be provided to hold them in place after erection.
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
cured sufficiently to bear its own weight.
Two types of steel bridges are shown: a truss (Figure 6) and
beam (Figure 7) system.
The truss type of structure requires
smaller steel members but needs
extensive fabrication by a local
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
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
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
A masonry or concrete arch type of structure (shown in
may be considered for short span lengths of 3 to 12
type of structure, if built of masonry, requires skilled
and a local quarry for a supply of stone.
The forming for an
arch is quite complex because curved forms are required to
the weight of the masonry or concrete.
The tools and skills required to build a concrete arch
the same as those needed to build a concrete beam
and masonry skills and tools are required if a masonry arch
Table 1 gives guidelines for selecting the type of structure
be used for vehicular traffic.
The span lengths noted are a
general guide for bridges from 3 to 25 meters; they vary
on design loads.
GUIDELINES FOR SELECTING TYPE OF BRIDGE
TO BE USED FOR VEHICULAR TRAFFIC
SPAN LENGTH, SKILLS
to 15 Ordinary
Wood of known strength
characteristics and use
of wood preservatives are
to 10 Ordinary
Reinforcing steel of
tools, known strength
istics is needed.
inspection of steel and
concrete should be made.
to 15 As under Con-
As under Concrete
to 25 Ordinary
Steel of known strength
deck is used.
to 25 Carpentry
Structural grade timber
and is required and skilled
carpenters for fitting
and joining are needed.
to 25 Steel fab-
Truss is made up of
angles or channels, and
and skill in fabrication is
steel, and a
to 10 See Con-
See Concrete (flat
slab). In addition,
are required to build
to 10 Carpentry
Skilled masons and
carpenters are required
to build curves and the
forms to support the
structure during con-
The maximal wheel loadings and the minimal spacing between
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
a uniform load of 400 kg/sq m, unless a load concentration
The cost of the structure is not covered in this
depends on material and labor costs, and these vary widely
region to region.
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
structures require patching of spalled (flaked or
chipped) areas with
cement grout if they occur.
Reinforced concrete structures can be difficult to maintain
often impossible to repair.
The best defense against the need
for maintenance is extreme care in proportioning, mixing,
placing the concrete.
Careful placement of reinforcing is equally
Broken and spalled concrete areas should be patched; worn
surfaces should be given a suitable wearing and paving coat
should be sealed with a commercial compound
recommended for this purpose.
The foundations of a bridge include those structural units
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.
the end supports.
The types of piers and abutments to be discussed
are shown in Figures 9 and 10.
Piers and abutments are
supported by foundations, which are of two types:
A spread footing (Figure 9) is a shallow foundation and is
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
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
This person performs a soil evaluation at the
site to determine what kind of piling would be the most
and what equipment would be required to install the piling.
Abutments carry vertical loads from the superstructure and
lateral loads from the retained earth on one side (Fig.
Abutments are of two types:
gravity or cantilever. A gravity
abutment carries its load through compression, and a
abutment through a combination of bending and
a gravity abutment is subject to compressive loads only, it
be constructed of masonry or unreinforced concrete.
abutment requires the use of reinforced concrete to
the stress caused by bending.
Piers carry spans between abutments in order to shorten the
lengths; they are subject to the following forces:
loads from the structure and from the traffic upon it;
forces due to the expansion and contraction of the
and to the braking of vehicles on the bridge; lateral forces
water or ice due to stream flow; and lateral forces due to
loads on the superstructure and to traffic loads.
In the case of
small-span bridges these forces are negligible except for
vertical loads from the superstructure and the ice pressures
deep rivers of cold-climate areas.
If we disregard all forces
except the vertical loads from the superstructure, the pier
be considered a compression member and can be built of
If unreinforced concrete abutments or piers are used, a
mesh of 1.25 cm-diameter reinforcing rods should be placed
30-cm horizontal and vertical intervals to help control
and surface cracking.
Should a crack develop due to settlement or
temperature stresses, the mesh will keep the faces of the
Maintenance of substructure units is normally minimal,
of patching of spalled concrete or masonry.
occurs only if erosion undermines abutments or piers.
case filling in the eroded area and placing rock protection
prevent further erosion are required.
As prevention, substructure
units should be inspected yearly for erosion damage or
after unusual run-off.
1. Gidlow, B. Design
of Suspension Footbridge, College Camp
2. Strung, N. Your
Own T (R)oll Bridge, December. 1990
G.E., Bridge construction using Logs,
and Soil, VITA Case No. 31977, 1980
Footbridges: Design and Construction,