1 2 STATICALLY DETERMINATE PLANE TRUSS OBJECTIVES: This chapter starts with the definition of a truss and briefly explains various types of plane truss. The determinacy and stability of a truss also will be discussed. The procedures of analyzing statically determinate trusses will be developed using the method of joints and the method of sections. For advance, the alternative computation using joint equilibrium method will be included in this topic. 2.1 INTRODUCTION A truss is defined as a structure composed of slender elements joined together at their end points. The members commonly used in construction consist of wooden struts, metal bars, angles or channels. Plane or planar truss composed of members that lie in the same plane and frequently used for bridge and roof support. Loads that cause the entire truss to bend are converted into tensile and compressive forces in the members. compression tension Fig. 2-1 Loading causes bending of truss, which is develops compression in top members, tension in bottom members.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
2 STATICALLY DETERMINATE
PLANE TRUSS
OBJECTIVES:
This chapter starts with the definition of a truss and briefly explains various
types of plane truss. The determinacy and stability of a truss also will be
discussed. The procedures of analyzing statically determinate trusses will be
developed using the method of joints and the method of sections. For
advance, the alternative computation using joint equilibrium method will be
included in this topic.
2.1 INTRODUCTION
A truss is defined as a structure composed of slender elements joined together
at their end points. The members commonly used in construction consist of
wooden struts, metal bars, angles or channels.
Plane or planar truss composed of members that lie in the same plane and
frequently used for bridge and roof support.
Loads that cause the entire truss to bend are converted into tensile and
compressive forces in the members.
compression
tension
Fig. 2-1
Loading causes bending of
truss, which is develops
compression in top
members, tension in
bottom members.
2
2.2 TYPES OF TRUSSES AND ITS APPLICATION
2.2.1 Roof trusses
Often used as part of an industrial building frame as shown in Fig. 2-2.
Fig. 2-2
The roof load is transmitted to the truss at the joints by series of purlins.
Generally, the roof trusses are supported either by columns of wood, steel or
reinforced concrete.
The roof truss along with its supporting columns called as a bent.
To keep the bent rigid and capable of resisting horizontal wind forces, knee
braces are sometimes used at the supporting columns.
The space between adjacent bents is called a bay. Bays are often tied
together using diagonal bracing to maintain rigidity of the structure.
The gusset plate is the connections which formed by bolting or welding at
the ends of the members to a common plate.
Fig. 2-3 : Gusset plate
Trusses used to support roofs are selected on the basis of the span, the slope
and the roof material.
Some of common types of trusses are displayed in the Fig. 2-4.
3
Fig. 2-4: Common types of plane trusses
2.2.2 Bridge trusses
The main structural elements of a typical bridge truss are shown in Fig. 2-5.
Fig. 2-5 : The structure elements of typical bridge truss
The load on the deck is first transmitted to stringers then to floor beam and
finally to the joints of the two supporting side trusses.
4
The top and bottom cords are connected by top and bottom lateral bracing
which serves to resist the lateral forces caused by wind and the sidesway
caused by moving vehicles.
Additional stability is provided by the portal and sway bracing.
The typical forms of bridge trusses currently used for single span are shown
in Fig. 2-6.
Fig. 2-6 : The forms of bridge trusses
2.3 ASSUMPTIONS IN ANALYSIS
The assumptions are necessary to determine the force developed in each
member when the truss is subjected to a given loading.
Fig. 2-7 : Truss member
C A to C,
C to B,
B to A
the connection
between them
is called as
truss member
5
the assumptions are;
i) All members are connected at both ends by smooth frictionless pins.
ii) All loads are applied at joints (member weight is negligible).
ASK STUDENTS:
Why the analysis assume that the smooth pin is frictionless?
Why the analysis assume that the member weight is negligible?
Because of these two assumptions, each truss acts as an axial force member
and the forces acting at the ends of the member must be directed along axis of
the member.
If the force tends to elongate the member, it is tensile force (T) as shown in
Fig. 2-8.
If the force tends to shorten the member, it is compressive force (C).
In analysis it is important to state whether the force is tensile or compressive.
The forces inside the member and outside should opposite
Fig. 2-8 : The tensile and compressive force
2.4 STABILITY AND DETERMINACY
2.4.1 General
The structures mechanics involves determination of unknown forces on the
structures.
Some of these structures can be completely analyzed by using the equations
of equilibrium.
ΣFx = 0 (F = F)
ΣFy = 0 (F = F )
ΣMz =0 (M = M)
On the other hand, if there exist extra redundant reaction components, then
the structure is said to be statically indeterminate.
T T
C C
Smooth pin
The structures are known as statically determinate.
6
2.4.2 Determinacy Criteria for Structures
To be in a state of static equilibrium, a structure must meet the requirements
of stability.
A statically indeterminate structure is a structure that had more unknown
forces.
Eg; ( to determine whether the truss is determinate or indeterminate)
For a pinned joint frame (trusses);
i) m+r = 2j ……just stiff (statically determinate)
ii) m+r < 2j ……under stiff (form a mechanism)…unstable
(it will collapse since there will be an insufficient number of bars @