actions_01 permanent ed1-2004.ppt

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Structural ActionsPermanent Actions



Design Elements

Structural Design

Consider different service scenarios

possible during the life of the structure

Structural design aims to select systems and

members that will perform satisfactorily for

a given environment (including actions)

Scenarios

Actions

Check all scenarios

Evaluate actions on structure Action Effects

Detail

Determine design action effects on

elements

Check for all action effectcombinations

Design elements for adequate

performance at the loads for variouslimit states

Check for all action effectcombinations

Detail for construction

Designers must make realistic,

but conservative assumptions



Actions and their Effects

Action• any set of displacements or forces acting on

an element or structure • eg weight, wind pressure, earthquake, ground movement

Action Effect

• internal structural effect caused by actions

• eg bending moment, axial force, shear force

Design Actions

• Values of actions estimated for evaluation

of performance prior to construction



STRUCTURAL ACTIONS

Origin

• permanent (G)

• imposed (Q)

• wind (W )• earthquake ( E )

• snow (F sn)

• other

Characteristics of actions

Confidence• known

• estimated

Distribution• distributed

• concentrated

Duration

• long-term

• short-term

Return period

• frequent event

• rare event



Origin of action Consider anticipated use of the

structure – what events will apply actions tothe structure during its lifetime?

• Permanent (G)

• Imposed (Q)

• Wind (W )

• Earthquake ( E )• Snow (F sn)

• Other

Structure will be subjected to a combination of

different actions over its lifetime - must design for all

possible scenarios



Distribution

Distributed -

• forces applied over large area

• eg. Crowds, wind pressure...

• distributed

• concentrated

Assume concentrated loads act in the worst position

Often put many concentrated loads together as a distributed loading

Concentrated -

• forces applied over specific,

localised area

• eg load-bearing wall or column,feet of machinery or furniture...



Confidence / certainty

Reflects how accurately we can predict the actionsthat will be applied to the structure during itslifetime.

Known - well defined

• data obtained from manufacturer• usually imposed actions eg machinery,

filing cabinets, shelving, hoists or jacksused in construction

• known

• estimated

Estimated -

• environmental actions eg. wind, earthquake actions

• occupancy imposed actions eg. crowds

When in doubt, make conservative assumptions



Duration

Important for composite materials

(concrete, fibreglass, timber) that may creep

• longer-term

• shorter-term

Shorter-term < 5 months, wind, instantaneous

• Peak events tend to be of shorter duration

eg: crowds, wind gusts, earthquakes

• Instantaneous gives elastic response only

Longer-term 5 months, permanent

• eg: self-weight of structure, semi/permanent

installed items, longer-term storage,

furniture

• Can cause increase in deformations

due to creep - serviceability issues

• For some materials may cause decrease in strength over time



Frequent events

• may be of longer duration

• lower intensity

• low return period

Rare events• often shorter duration

• high intensity

• large return period

Return period

Variable loads may have a range of magnitudes – imposed, wind, snow, earthquake actions

• frequent event

• rare event

Intensity

Return period

1 10 100 1000



PERMANENT ACTIONS

Forces caused by weight of structure itself including:

Permanent stored

materials

Prestressing forces

< AS/NZS 1170.1 Section 2 >

All structural members,

permanent cladding

Permanent equipment - fixtures and fittings

• machinery, wiring, airconditioning

Self-weight of structure

Things that don’t change in the life of the structure

Permanent partitions



Characteristics of Permanent Actions Known

• Most manufactured buildingmaterials

• AS/NZS 1170.1 tabulatesdensities and unit weights forcommon building materials

Estimated

• If densities, dimensions,

moisture content vary

Permanent

• longer duration

Distributed - floors and roofs

• except under columns



Permanent Actions - Design summary

• G1 Analyse structure

• find tributary areas

• G2 Convert area to weight force using

• unit weight per area <AS/NZS 1170.1 Table A2>

• density <AS/NZS 1170.1 Table A1>

• G3 Calculate action effect due to permanent action

using analysis of structural form

• axial tension• axial compression

• bending moment

• shear force

• reaction force

These effects are

used in design of

specific members and

connections



Tributary AreasCan use approximate analyses to determine contributory areas

Floor carrying udl (kPa)

BearerFloor joistsBearer spacing

Lines midway between supports divide

systems into areas contributing to supports

Half bearer

spacing

Tributary area –

central bearer



Tributary AreasMore complicated shapes can also be addressed – eg hip rafter in a roof

Hip rafter

Eaves lineRafters spanning

eaves to hip

Rafters spanning

eaves to ridge

Ridge beam

Loading diagram

Bending moment diagram

Tributary

area

Line connecting mid points

of rafters spanning

eaves to hip



Load Paths

Tracking forces through structure• gravity loads on floor

• give axial force in

columns

joist

bearer

column



Load Path & Tributary Area

Tributary area of bearer

Line midway

between bearers




Tributary Area of Upper Column

Line midway

between columns

Line midway

between columns




Tributary Area of Lower Column

Line midway

between columns

Line midway

between columns

Line midwaybetween columns Line midway

between columns



Permanent Actions - worked example

Example 2.2 (HB108-1998)

Permanent actions on internal bearerThe tributary area for the internal floor bearer is a strip 2.4 m

wide that runs the length of the bearer.

Find the permanent actions on the internal bearer, and the moment

and shear force that the permanent actions induce in the members.Additional information

• Flooring – 19mm particleboard (Unit weight = 0.13kN/m2)

• Joists – 140 x 45 F8 seasoned radiata pine (estimate) Check

• Bearers – 245 x 65 glulam radiata pine (estimate) CheckNo walls and partitions fall within the tributary area Check



Worked example

Particle board flooring

External bearer

Internal bearer

Joists

3.6 m

450 2.4 m



Worked example

Flooring Area contributing in one metre length of bearer is m2

Weight of flooring in tributary area kN/m

(Table 2.3 HB108)

2 4 1 0 2 4. . .

2 4 0 13 0 31. . .

The bearer is a linear member and load per metre is used to find the

bending moment and shear force. The calculations will thereforedetermine the load per linear metre on the bearer. G1 Tributary area for the bearer

The tributary area ran the full length of the bearer ( 3.6 m) and

took in 1.2 m on both sides of the bearer centre-line.

G2 Weight of structure in the tributary area

Each of the calculations will be performed for a 1 m length of bearer.



Worked example

Volume of joist contributing m3 /m

<AS 1720.1 Table 2.2> gives density of seasoned radiata pine as 550 kg/m3

Mass of joist contributing kg/m

Weight of joist contributing N/m kN/m

5 33 0 14 0 045 33 6 103

. . . .

550 33 6 10 18 53 . .

18 5 9 81. . 0181.

Joists Length of joist contributing in one metre length of bearer

m33.545.0

0.14.2



Bearer

Volume of bearer in 1 m m3 /m

<AS 1720.1 Table 2.2> gives density of seasoned radiata pine as 550 kg/m3

Mass of bearer contributing kg/m

Weight of bearer contributing N/m kN/m

Worked example

1 0 0 245 0 065 15 9 10 3. . . .

550 15 9 10 8 753

. .

8 75 9 81. . 086.0

Total permanent action on the bearer kN/m

(a uniformly distributed line load)The permanent action is by nature permanent - a longer

duration loading

0 31 0181 0 086 0 58. . . .



Worked example

G3 Moment and shear force due to permanent action

The permanent action was a uniformly distributed line load over thefull length of the bearer. Assuming simply supported at the ends of the

bearer.

Bending moment due to permanent action

M w L

2 2

8

0 58 3 6

80 940

. .. kNm

Shear force due to permanent action

V w L

2

0 58 3 6

2104

. .. kNm

•Both of these action effects are permanent in duration

•No load factors have been included in the actions or

action effects

actions_01 permanent ed1-2004.ppt

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