Paul Martin

Institution of Structural EngineersPart 3 Exam

Loads and Forces

Load BuildupBasics

1. Have clear purpose, so you only do it once

2. Organise it neatlyso you find loads easily later when in a panic

Load BuildupSources of information

1. Basic Material densities: BS 648Get a feel for approx densities

Steel: 78 kN/m3

Concrete: 24 kN/m3

Clay Brick: 20 kN/m3

Block: Light: 10 kN/m3 Standard: 18 kN/m3

Glass: 27 kN/m3

Plaster: 14 kN/m3

Plasterboard: 9 kN/m3

Softwood: 5 kN/m3

Load BuildupSources of information

2. Systems: Trade LiteratureAgain, Get a feel: kN/m2 on elev.

Curtain Walling: 0.5

Windows: 0.3

Steel wall framing: 0.3

Cladding and rails: 0.5

Load BuildupSources of information

3. Live Loads: BS 6399Keep on getting the feel:

1 person = 75 kg = 0.75 kN1 person + 1 desk = ?

0.75 kN+ say 0.5 kN in say 1.25 m x 1.0 m

=1.0 kN/m2

1 Car = 1000 kg = 10kN

Car park loading = 2.5 kN/m2 = 1 car/4m2

Combining loads sensibly

Convert all loads into same units

Normally udl - kN/m2 on plan.

e.g.1 Beams: (weight/m) / spacing = udl

Beams in two directions:

(sum of both weights/m) = udlAverage spacing

e.g.2 Sloping roofs: Convert to udl on plan

DONT 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 cant avoid them!)

Calculation of Effects

1. Avoid fancy analysis

2. Use either: Simple design aids

Steel Designers ManualReynolds

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: DeflectionRoofs, slabs, footbridges

2. Medium spans: BendingFloor beams

3. Short, heavily loaded spans: ShearFoundations, transfer beams

Ductility

1. Use it when you have itCan redistribute effectsCan rely more on estimated effects

(But beware of equilibrium)

2. Beware when you dont 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 contraflexureat column mid-height

Load BuildupBasicsLoad BuildupSources of informationLoad BuildupSources of informationLoad BuildupSources of informationCombining loads sensiblyPurposePurposeContinuity effectContinuity effectCalculation of EffectsCalculation of EffectsStatically determinate hand calcs.Which will be the critical effect?DuctilitySimple hand calcsMoment DistributionMoment DistributionSway frames