Transcript ME31B: CHAPTERTHREE

ME31B: CHAPTERTHREE
INTRODUCTION TO
STRUCTURAL DESIGN
3.1 DESIGN

The
process
by
which
dimensions
of
structural
members are arrived at.

It is a trial and error (iterative)
process.
3.2 STEPS IN DESIGN




a) Calculation of loads which the
structure/structural member shall carry.
b) Analyze to obtain the distribution of stress
among the various members.
c) Arrangement of structural elements eg.
beams, columns etc.
d) Design: Selection of the materials and
dimensions required for each member and
for the necessary connections between
members.
3.3 LOADS



Loads can be:
a) Dead load: Load that is not
movable or is permanently set eg. self
weight of all permanent construction
eg. roofs, walls, floors etc.
b) Live loads: Load that is movable
e.g. persons, furniture, animals,
products, wt. of stored products,
equipment, vehicles etc.
Loads: Wind Loads


c) Wind load: Live loads but
usually treated separately due
to their temporal nature and
their complexity.
It is often the most critical load
imposed
on
agricultural
buildings.
3.4 ESTIMATION OF LOADS

a) Dead loads: See Table 5.3 (FAO
Farm structures book) for estimation of
loads of building materials.

b) Wind loads: Wind load depends
on wind speed, location, size, shape,
height and construction of a building.
Estimation of Dead Loads
Estimation of Wind Loads Contd.




Where wind velocities have been
recorded, estimate expected pressures
on building walls using: q = 0.0127 V 2 K
where q is the basis velocity
pressure(Pa); V is wind velocity(m/s);
K = (h/6.1)2/7
where h is the design
height of building (m) - eave height for
low and medium roof pitches.
6.1 is the height at which wind
velocities are often recorded.
Estimation of Wind Loads Contd.



In places where wind velocities have not
been measured, estimate wind velocity using
the Beaufort scale of winds (Table 5.1, FAO
book).
Wind loads depend on whether the building
is open or completely closed.
It also
depends on whether the roof is low or high
pitched.
The following table (Table 5.2, FAO book),
gives coefficients used to determine
expected pressures for low pitched and high
pitched gable roofs and open and closed
buildings.
c) Estimation of Live or
imposed loads:

See Table 5.5 for mass of farm
products. Also see Table 5.4 for loads
on suspended floors.
3.5 CLASSIFICATION OF LOADS
BASED ON LOCATION



The classification of loads (dead,
imposed and wind) are based on the
duration of load. Loads can also be
classified according to location:
a) Concentrated load: Acts at a point
e.g. weight hanging from a ceiling.
b) Uniformly distributed load (udl):
Acts along the length of a structural
member eg. roof loads, wind loads etc.
Solution
Solution: Beam A carries the floor loads contributed by half the area between beams A
ands B i.e. the shaded area L. Beam C carries the loads contributed by the shaded
area M.
Load carried by beam A = 1 m x 4 m x 10 kN/m2 = 40 kN or 40 kN/4 = 10 kN/m
Load carried by beam C = 2.5 m x 4 m x 10 kN/m2 = 100 kN or 100 kN/4 = 25 kN/m
This loading per m can be used to calculate the cross sectional area of beams
required(i.e. design the beams). See later in the next chapter.
10 kN/m
25 kN/m
4m
Loading on Beam A
4m
Loading on Beam C
Loads Classification Based on
Location Contd.
c) Distributed loads with linear variation: The load is triangular eg. pressure of
grains on bins or of water on retaining walls or dams.
Stress
Ultimate
stress
Dam
yield stress
Strain
3.6 FACTOR OF SAFETY



Most designs are based on the ultimate
stress and a factor of safety normally
given as:
Factor of safety = Ultimate stress
Working stress
Design stress = Ultimate or yield stress
Factor of safety
Reasons For Factor of Safety



Factor of safety is used since we lack
the knowledge of all properties of the
material e.g. its weakness under use.
There is also lack of knowledge of all
the loadings.
Also equations to calculate stresses
use simple assumptions.
Factor of Safety Contd.




The safety value is chosen based on
the
accuracy
in
the
loading
assumptions,
The permanency of the loads,
The probability of casualties or big
economic loss in case of failure, the
purpose of the building,
The uniformity of the building material,
the workmanship expected from the
builder and the building cost.
Factor of Safety Concluded



Note: For materials like concrete that do
not have a well defined yield point or brittle
materials which behave in a linear manner
up to failure, the factor of safety is related to
the ultimate stress(maximum stress before
breakage).
In this case, factor of safety is taken as 3 to
5. For other materials like steel with well
defined yield point, yield stress is used. In
this case, factor of safety is 1.4 to 2.4.
For farm houses, lower values are assumed
e.g. 1.5 to 2
3.7 STRUCTURAL ELEMENTS

a) Cable:
Cables, cords, strings,
ropes, and wires are flexible because
of their lateral dimensions in relation to
their length and therefore have limited
resistance to bending.

b) Rods: Rods, bars and poles resist
tensile or compressive loads.
c) Column


Rods and bars under compression.
They are named after the material of
construction e.g. stanchions (steel),
piers (brickwork or masonry) and pillars
(wood materials e.g. timber).
Columns are used to transfer load
effects from beams, slabs and roof
trusses to the foundations. They can
be loaded axially or may be designed
to resist bending when the load is
eccentric.