02 Loads and Forces

Institution of Structural Engineers Part 3 Exam

Paul Martin

Loads and Forces

Load Buildup Basics 1.

Have clear purpose, so you only do it once

2.

Organise it neatly so you find loads easily later when in a panic

Load Buildup Sources of information 1.

Basic Material densities:

BS 648

Get a “ feel” for approx densities Steel:

78 kN/m 3

Concrete:

24 kN/m 3

Clay Brick:

20 kN/m 3

Block:

10 kN/m 3 Standard:

Light:

Glass:

27 kN/m 3

Plaster:

14 kN/m 3

Plasterboard:

9 kN/m 3

Softwood:

5 kN/m 3

18 kN/m 3

Load Buildup Sources of information 2.

Systems:

Trade Literature

 Again, Get a “ feel” :

kN/m 2 on elev .

Curtain Walling:

0.5

Windows:

0.3

Steel wall framing:

0.3

Cladding and rails:

0.5

Load Buildup Sources of information 3.

Live Loads:

BS 6399

Keep on getting the “ feel” : 1 person

=

75 kg =

1 person + 1 desk =

0.75 kN

?

0.75 kN+ say 0.5 kN in say 1.25 m x 1.0 m =1.0 kN/m 2 1 Car

=

1000 kg =

10kN

Car park loading = 2.5 kN/m 2 = 1 car/4m 2

Combining loads sensibly Convert all loads into same units Normally udl - kN/m 2 on plan. •e.g.1

Beams: (weight/m) / spacing

= udl

Beams in two directions: (sum of both weights/m)  Average spacing e.g.2

Sloping roofs:

= udl

Convert to udl on plan

•DON’T START ADDING BEAM REACTIONS!!

Purpose 1.

Slab design No beam self-weight

2.

Beam design

3.

Column design

4.

Transfer structures and Foundations design

Purpose 1.

Keep loads un-factored for ground bearing check

2.

Keep Dead and Live loads separate. E.g.

to apply live load reduction factor  Reduce live load even further for settlement check?

3.

 Apply load factors only when calculating effects

Continuity effect • How much is the shift in point of zero shear? • End spans:~10% • Internal span: Negligible

Continuity effect

Worst case: Propped cantilever: Changes reactions by 12.5%

Calculation of Effects 1.

Moments

2.

Shear Forces

3.

Axial forces

4.

Deflections

5.

Torsion moments (only if you really can’t avoid them!)

Calculation of Effects 1.

Avoid fancy analysis

2.

Use either: • Simple design aids Steel Designers Manual Reynolds

• Statically determinate calcs. • Simple moment distribution

Statically determinate hand calcs. 1.

Find one reaction

2.

Find point of zero shear [max moment] (remembering load = slope of shear force diagram)

3.

Find max moment.

Which will be the critical effect?

1.

Long, lightly loaded spans:

Deflection

Roofs, slabs, footbridges

2.

Medium spans:

Bending

Floor beams

3.

Short, heavily loaded spans: Foundations, transfer beams

Shear 

Ductility 1.

Use it when you have it •Can redistribute effects •Can rely more on estimated effects (But beware of equilibrium)

2.

Beware when you don’t have it!! •Non-plastic (class 1) steel sections •“ Brittle” connections

Simple hand calcs • Overall moments and shears are independent of the internal shape

Moment Distribution

•  At its simplest (and most common?): to find column moment in braced concrete frame

Moment Distribution

Sway frames

•  Assume points of contraflexure at column mid-height