TOPIC 1.3

RESPONSE OF CIVIL ENGINEERING

PROJECT

FORCE IN CIVIL ENGINEERING PROJECT

A) Identify the system response: ● Sway ● Deflection and

● Vibration

B) Identify the component response: ● Buckling ● Torsion ● Shortening ● Elongation, ● Shearing and ● Bending

Learning Outcomes

System Response

i. Swayii. Deflection iii. Vibration

(i) Sway

Definition :

move back and forth, swing ,lean in a certain direction; be inclined toward.

to move or swing to and from, as something fixed at one end or resting on a support.

Example of sway :

Impact of wind load Tacoma Narrows Bridge collapse

Definition :

is a term that is used to describe the degree to which a structural element is displaced under a load.

Deviation, diversion, turning aside

(ii) Deflection

Deflection of Beams

• Excessive deflections are unacceptable in building construction; can cause cracking of plaster in ceilings and can result in jamming of doors and windows.

• Most building codes limit the amount of allowable deflection as a proportion of the member's length, i.e. 1/180, 1/240 or 1/360 of the length.

(ii) Deflection

Example of deflection :

Definition : mechanical oscillations about an equilibrium

point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.

More often, vibration is undesirable, wasting energy and creating unwanted sound – noise.

(ii) Vibration

Such vibrations can be caused by imbalances in the rotating parts, uneven friction, the meshing of gear teeth, etc. Careful designs usually minimize unwanted vibrations.

(ii) Vibration

Types of vibration

• Free vibration - occurs when a mechanical system is set off

with an initial input and then allowed to vibrate freely.

- e.g ; pulling a child back on a swing and then letting go or hitting a tuning fork and letting it ring.

(ii) Vibration

Types of vibration

• Forced vibration - occur when an alternating force or motion is applied to a mechanical system. - e.g ; vibration of a building during an earthquake. - In forced vibration the frequency of the vibration is the frequency of the force or motion applied, with order of magnitude being dependent on the actual mechanical system.

(ii) Vibration

Buckling Torsion Shortening Elongation Shearing Bending

Component Response

(1) BUCKLING(1) BUCKLING• Definition :

In engineering, buckling is a failure mode characterized by a sudden failure of a structural member subjected to high compressive stresses,

where the actual compressive stresses at failure are smaller than the ultimate compressive stresses that the material is capable of withstanding.

This mode of failure is also described as failure due to elastic instability.

Column buckling Buckling test

(2) TORSION(2) TORSION• Definition :twisting, winding, bending, distortion, contortion

In solid mechanics, torsion is the twisting of an object due to an applied torque. In circular sections, the resultant shearing stress is perpendicular to the radius.

Torsion in beam

Torsion

(3) SHORTENING (3) SHORTENING

• Definition :make shorter, abbreviate, cut, truncate;

become shorterCause of shortening

(i) elastic stresses(ii) shrinkage(iii) creep

The shortening of the column if its initial height is 5m. and Force 2000kN

Example of shortening

(4) ELONGATION(4) ELONGATION

• Definition :lengthening, extension, making longeran addition to the length of somethingWhen a material is tested for tensile strength

it elongates a certain amount before fracture takes place. The function of this test is to measure the ductility of steel.

(5) SHEARING(5) SHEARING

Definition :• is defined as a stress which is applied parallel

or tangential to a face of a material, as opposed to a normal stress which is applied perpendicularly

• Shear stress is the outcome of sliding one part over the other in opposite directions. The rivets and bolts of an aircraft experience both shear and tension stresses

The formula to calculate average shear stress is:

whereτ = the shear stress; F = the force applied; A = the cross sectional area.

Example of shearing

Failure of short column by oblique shear

(6) BENDING(6) BENDING

Definition :• crouching, stooping; curving, twisting;

contortion, distortion.• In engineering mechanics, bending (also

known as flexure) characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to an axis of the element.

Bending in beam

100 lb

Bending