BCN 240
5C
: Con
struction Mechanics II (Strength of Materials).
Instructor: Dr Ian Flood
1
DESIGN
OF
BEAM
S
Notes
:
The main considerations when designing a beam are:
Ensuring that the allowable bending stresses are not exceeded.
Ensuring that the allowable shear stresses are not exceeded.
Ensuring that the allowable deflection is not
exceeded.
Ensuring that the beam has adequate lateral support to prevent buckling.
Selecting a design that will keep costs as low as possible.
Building codes and standards
provide guidelines for designers on:
How strengths should be determined.
What t
he minimum safety factors should be.
The types and magnitudes of loads and forces that should be considered.
Structural standards currently
allow two design methods:
ASD

allowable stress design.
LRFD

Load and resistance factor design.
All concepts
involved in beam design have really already been covered in earlier lectures.
The overall process can be summarized as follows:
1.
Determine the superimposed loading system
,
including the spans and
the
support
conditions
.
Also, e
stablish all design constr
aints such as allowable stresses and
deflections.
2.
Draw a load diagram and calculate the reactions.
3.
Determine the maximum shear force and maximum bending moment.
4.
Select a
trial cross section
that:
i.
has sufficient strength to resist the maximum applied moment
ii.
has sufficient stiffness to limit deflection to within the specified limit.
iii.
is economic (usually least weight).
5.
Add the beam weight to the applied loads and repeat steps 2 and 3. Then check
that the beam is still satisfactory for
bending
moment and deflec
tion.
6.
Check for shear (shear rarely critical for steel beams but may be in timber).
Revise the design if necessary.
Design is usually an iterative process in that we choose a design and then refine it by
moving towards the cheapest design that satisfies
all other requirements.
BCN 240
5C
: Con
struction Mechanics II (Strength of Materials).
Instructor: Dr Ian Flood
2
Note: unless adequately restrained, the compression side of a beam will have a tendency
to buckle (deflect laterally).
You can think of this as if the beam is trying to revert to its
original (unstrained) length:
the compression
side moving sideways results in it
regaining some of the length it lost to compression; the tensile
side, on the other hand, is
trying to shorten back so this side is pulled straight. Buckling resistance
is often provided
by the fl
oor that the beam is s
upporting, on other occasions
restraint must be
added.
Reading
:
Section
1
6

1
to 1
6

3
Worked Examples
:
Starting page
4
4
8
:
1
6

1
to 1
6

3
.
Problems
:
Starting page
4
58
:
1
6

1
to
1
6

1
1
(
odd numbered problems only,
answ
ers provided in
back of book).
buckling
(
compression
side)
concrete floor slab provides buckling restraint for “
I
” beams
compression
flange
of “
I
” beams
BCN 240
5C
: Con
struction Mechanics II (Strength of Materials).
Instructor: Dr Ian Flood
3
Class Problem
s
:
Prob. 16

1
: Select the lightest W shape to support a uniformly distributed load of 2.1
kips/ft
on a simple span of 24
ft
.
Prob. 16

1
: Design a timber beam of hem

fir (S4S) to support a uniformly distributed
line load of 600
lb/ft
on a simply supported span of 15
ft
. Deflection is not to exceed
span/360.
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