Basics of trusses and truss analysis

2. TRUSSES
Presented to: Mr. Ahmad Naveed
Presented by:
2017-ME-51 Shahriyar Kashif
2017-ME-53 Muhammad Huraira
2017-ME-57 Usama Javed
2017-ME-67 Salman Irfan
2017-ME-77 Hamza Imtiaz
Mechanics of materials presentation

3. WHAT IS A TRUSS?
In engineering, a truss is a structure that consists of two force members only.
We assume each member of truss to be a straight member that can only have
forces applied to the ends of that member. A truss is usually designed from
straight pieces of metal or timber to form a series of triangles lying in a single
plane. Trusses are used in bridges, buildings, railway tracks and many other
structures.

4. GEOMETRY OF TRUSSES
A truss is basically a series of triangles lying in a plane. The geometry of
trusses enables them to carry much larger loads that cause tension and
compression.
Triangles are the strongest shape because any added force is evenly spread
through all three sides.
Example, a Warren truss is characterized by its construction from
equilateral triangles. Each member of the equilateral triangle is subjected
to either a compressive or a tensile force. Warren trusses are mostly used
for walkway trusses, crossovers and bridges.

5. TRUSSES USED IN BUILDINGS
Two basic categories of trusses used in buildings are:
• Common truss or Pitched truss
• Parallel chord truss or Flat truss
Each member of a truss under loading either experiences a tensile force or a
compressive force. Thus,
• The member of a truss that has a tensile force acting on it is called a tie.
• The member of a truss that has a compressive force acting on it is called a
strut.

6. TYPES OF TRUSSES
Different types of trusses are used in buildings, bridges and other structures. Some of
the most prominent truss types include
• Pratt truss
• Howe truss
• Bowstring truss
• K-truss
• Warren truss
• Lenticular truss
• Vierendeel truss
• King post truss
• Baltimore truss
For example, Pratt truss is one of the most common type of trusses.
Designed in 1844 by Thomas and Caleb Pratt. It is mainly used for rail
bridges with a simple and strong design. Its real life example is the
Governor’s Bridge in United States (Patuxent River).

7. Some terminologies in mechanics of trusses
Dead Weight
Collective self weight of all the components of the structure. May include weight of beams in bridge, weight of
supporting elements etc.
Zero Force Member
It is that member of a truss which, under the application of some load arrangement, is at rest i.e. neither in
tension nor in compression. In trusses it is most commonly found at pins (where no external force acts)
Ultimate Stress
It is the maximum stress that a body or a structural element can withstand before failure point/breakage.

8. Factor of Safety
It is a term used to express how much a system is stronger than it is needed to be for an intended load
application.
Mathematically calculated as:
𝐴𝑝𝑝𝑙𝑖𝑒𝑑 𝐿𝑜𝑎𝑑
𝑈𝑙𝑡𝑖𝑚𝑎𝑡𝑒 𝐿𝑜𝑎𝑑
Cyclic Load
Cyclic load is defined as the continued and repeated application of a load on a material or a structural
component that deteriorates the material and ultimately leads to failure and fatigue.

9. Live Load
It refers to the loads which can and do change with time. That is, they are variable such as people walking on a
bridge etc. They play a very important role in stress analysis of different structures.
Impact Load
It is the load experienced by the body when some moving object collides with the body. A moving object
posses kinetic energy due its motion, and during collision this energy is transferred to the structure as impact
load. The stress produced as a result of an impact load is called impact stress.

10. Analysis of trusses
There are two methods to solve trusses
• Node method or method of joints
• Section method
Method of joints
In node method fee-body diagram of each joint is drawn and forces sum in x and y direction is put to the
zero. Best way is to start from two-member joint because we cannot find force in case of more than two
unknown forces.
⅀ Fx = 0 ⅀ Fx = 0

11. Section method
In section method truss is sectioned/divided and each section is treated as rigid body and analyzed accordingly. We
cut the section involving member in which we want to calculate force. We pass a cutting plane through truss and
each cut section is shown as equivalent force in the member. We apply three equilibrium conditions, two of force
and one for moment. So, there in cutting at mot there can be three members. these equilibrium conditions are as
follows:
⅀ Fx = 0 ⅀ Fx = 0 ⅀ M0 = 0

12. Which method to use for truss analysis ?
There is no rule for selecting method of joints or method of sections. It is a matter of preference depending upon
type of problem assigned. Both methods can be used in any problem but sometimes one will be more convenient.
Figure 1 Figure 2

13. Warren Truss
The Warren truss consists of longitudinal members joined
by angled cross-members, forming alternately
inverted equilateral triangle-shaped spaces along its length.

14. History of warren truss bridge
 James Warren
 Willoughby Theobald
Design
 Usage of equilateral triangles.
 Subdivided Warren truss bridge

15. Forces acting on warren truss:
 Point load
 Uniformly distributed load.
Advantages
Disadvantages

16. Application of Trusses
• Roof of Factory Shade
• Ware House
• Railway Platform
• Garbage Shed
• Transmission Towers
• Crane Truss
• Bridge Truss
• Sports Stadium Truss
Bridge Truss
In Bridges trusses are very commonly used. As
trusses are lighter than beam so it is better to use truss
instead of Beam.

17. Truss in Roofs
It goes without saying that one of the most important aspects of a building is its roof. Without a roof, buildings
would be exposed to all kinds of elements, rendering them completely useless.
A roof should also be built to last the lifespan of the building, which involves careful planning of the structure
and materials used. The roof must be able to support a heavy load over a long period of time, yet also leave as
much free room as possible in order to allow for an airy and spacious building.
Truss in Ware House
Warehouse is very large in size. Also we need to cover
warehouse also so we make truss roofs so that it will
cost us very expensive.

18. Crane Truss
Crane has to lift up and down.
Therefore we cannot have beams
instead of trusses. As beam will be
too heavy to lift up and down
Transmission Towers
Transmission Towers have also made up of
trusses. In transmission towers power is
transmitted from. These towers are very
tall so they have been based on truss for
material saving.

19. THANK YOU!