actions_01 permanent ed1-2004.ppt
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7/29/2019 Actions_01 Permanent Ed1-2004.ppt
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Structural ActionsPermanent Actions
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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
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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
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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
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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
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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...
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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
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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
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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
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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
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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
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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
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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
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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
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Load Paths
Tracking forces through structure• gravity loads on floor
• give axial force in
columns
joist
bearer
column
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Load Path & Tributary Area
Tributary area of bearer
Line midway
between bearers
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Load Path & Tributary Area
Tributary Area of Upper Column
Line midway
between columns
Line midway
between columns
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Load Path & Tributary Area
Tributary Area of Lower Column
Line midway
between columns
Line midway
between columns
Line midwaybetween columns Line midway
between columns
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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
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Worked example
Particle board flooring
External bearer
Internal bearer
Joists
3.6 m
450 2.4 m
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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.
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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
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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. . . .
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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
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