Types of Beams and Support Reactions

8/10/2019 Types of Beams and Support Reactions

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UNITI Basic Concepts of Engineering Mechanics

Lecture 6: Typesof members and supports and support reactions.

Beams

A beam is generally considered to be any member subjected to principally to

transverse gravity or vertical loading.

The term transverse loading is taken to include end moments.

There are many types of beams that are classified according to their size,

manner in which they are supported, and their location in any given structural

system.

Types of Beams Beams are generally classified according to their geometry and the manner in which

they are supported

Geometrical classification includes such features as the shape of the cross-section,

whether the beam is

Straight or

Curved

Or whether the beam is

Tapered, or

Has a constant cross section

Beams can also be classified according to the manner in which they are supported.

Figure 6.1 Types of Beams Based on the

Manner in Which They are Supported.


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Fig 8.2 Simply Supported Beams in Buildings

Fig 8.3 Continuous Beams in Cable Stayed Bridge


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Beams used in Buildings and Bridges

Girders

Usually the most important beams, which are frequently at wide spacing.

JoistUsually less important beams, which are closely spaced, frequently

with truss-type webs.

Stringers

Longitudinal bridge beams spanning between floor beams.

Purlins

Roof beams spanning between trusses.

Girts

Horizontal wall beams serving principally to resist bending due to wind

on the side of an industrial building.

Lintels

Members supporting a wall over window or door openings.

Loading on Beams

Fig 8.4 Loading on beams, (a) axial (b) moment (c) concentrated and uniformly distributed

with moment

Beam can be subjected to concentratedloads or

distributedloads or combination of both.


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Supports and Support Reactions:

The three common types o f connections which join a built structure to its foundation

are- roller, p innedandfixed. A fourth type, not often found in building structures, is known

as a simple support. This is often idealized as a frictionless surface). All of these supports

can be located anywhere along a structural element. They are found at the ends, at

midpoints, or at any other intermediate points. The type of support connection determines

the type of load that the support can resist. The support type also has a great effect on the

load bearing capacity of each element, and therefore the system.


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If a support prevents translation or rotationof a body in a given direction, a force is developed on

the body in that direction.

ROLLER SUPPORTS

Roller supports are free to rotate and translate along the surface upon which the roller rests.

The surface can be horizontal, vertical, or sloped at any angle. The resulting reaction force is

always a single force that is perpendicular to, and away from, the surface. Roller supports are

commonly located at one end of long bridges. This allows the bridge structure to expand andcontract with temperature changes. The expansion forces could fracture the supports at the

banks if the bridge structure was locked in place. Rollersupports can also take the form of

rubber bearing, rockers or a set of gears which are designed to allow a limited amount of

lateral movements.

PINNED SUPPORTS

A pinned support can resist both vertical and horizontal forces but not a moment.

They will allow the structural member to rotate, but not to translate in any direction. Many

connections are assumed to be pinned connections even though they might resist a smallamount of moment in reality. It is also true that a pinned connection could allow rotation in

only one direction; providing resistance to rotation in any other direction. The representation

of a pinned support includes both horizontal and vertical forces.

FIXED SUPPORTS

Fixed supports can resist vertical and horizontal forces as well as a moment. Since they

restrain both rotation and translation, they are also known as rigid supports. This means that

a structure only needs one fixed support in order to be stable. All three equations of

equilibrium can be satisfied. The representation of fixed supports always includes two forces(horizontal and vertical) and a moment.


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SIMPLE SUPPORTS

Simple supports are idealized by some to be frictionless surface supports. This is correct in as

much as the resulting reaction is always a single force that is perpendicular to, and away

from, the surface. They are also similar to roller supports in this. They are dissimilar in that

a simple support cannot resist lateral loads of any magnitude. The built reality often depends

upon gravity and frict ion to develop a minimum amount o f frictional resistance to

moderate lateral loading.

Problem 6.1

Find the reaction at the supports of the following beam as shown in figure.


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Problem 6.2

Determine the horizontal and vertical component of reactions at the fixed supports. Neglect

the thickness of the beam.

Solution:


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Problem 6.3

Determine the horizontal and vertical components of reactions of member AB, caused by a

roller at A and a pin at B.

Solution:

Free Body Diagram

Types of Beams and Support Reactions

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