(2)2013 lecture 1b - loading

7/21/2019 (2)2013 Lecture 1b - Loading

1/54

L im it States Des ign

AS1170.0-2002


2/54

Struc tural Performance

Structures must comply with BCA:Includes level of safety

Structures must perform reliably

under all expected actions

withstand extreme or frequent actionsClients have an expectation of satisfactory

performance to fulfill design function


3/54

L im it State Des ign

Limit states different design situations where the

structure is at its limit of satisfactoryperformance Targets performance at all stages of the design life of

structure.

Each limit state has different:

load combinations

behaviour models

performance limits


4/54

L im it States

Limit states ensure structures perform to

satisfaction of client. Includes: Serviceability

avoiding excessive cracking or bending

penalty - inconvenience, offensive appearance

Ultimate

avoiding failure, breakage

penalty - risk to life

Strength

avoiding failure, buckling

Stability

avoiding detachment,

collapse

Others

Fire

Safe evacuation of

occupants

Safety for fire-fighters

Fatigue

Avoiding growth of

dangerous cracks under

repeated loadings


5/54

Serviceabi l i ty

Functionality appearance or visual impact

unsightly deflection

misalignment at corners

discomfort vibration - or noise

inconvenience inoperation jamming doors or windows

slope of drainage lines damage to partitions or brittle building

elements due to deflection ofmembers under or over


6/54

Serv iceab i l ity -def lect ion l imi ts

Must be associated with a given load combination

not given in design codes - subject to agreement between

client and designer

Item Controlling Load scenario Limit Design MoE

Columns Side sway Ws H/500 E mean

Rafters / Trusses Sag G+s Q L/300E mean

Floor joists (UDL) Sag G+ Q L/300 E mean

Floor bearers

Bearers over

partition

Sag

Broken

partition

G+ Q

G+Q

L/300

Clearance

(>12mm)

E mean

E 0.05

some guidance given inAS/NZS 1170.0 App D


7/54

Serviceabi l i ty

Limits absolute eg 15mm clearance over partition

relative eg span/350 for appearance, comfort

Dynamic frequency, damping, comfort

Result

Estimate of structural response under likelyserviceability conditions

Model

Elastic/plastic deformation models


8/54

Serviceabi l i ty Modulus of Elasticity E

characteristic E - close to mean used for appearance or non-criticalapplications

Section properties use design dimensions

seasoned timber - nominal dimensions unseasoned timber - nominal dimensions

minus 3 mm

Loads use serviceability loads

close to normal working loads

Span Use design span (see Design Codes for steel, concrete,

timber)


9/54

Make conservative assumptions

- Imagine the worst scenarios

Includes Strength and Stability LimitStates

Both relate to safety - seriousconsequences

Failure unacceptable to society

minimise risk - (5x10-10)

Ult imate Lim it States

Failure where

load exceeds strength(strength limit state)

destabilising effectsexceedstabilising effects(stability limit state)


10/54

Ult imate L im it State

Load factors larger load factor gives lower probability of exceeding

pr

load

Long-term serviceability imposed action

load factor is a function of variability of the load

lower variability for permanent actions - lower load factor

higher variability for imposed actions - higher load factor

Short-term serviceability imposed action

Strength Limit State

imposed action

Nominal Code imposed action


11/54

Actions (Loads)


12/54

Structural design actions

Part 1: Permanent, imposed

and other

actions

AS/NZS 1170.1:2002

AS/NZS 1170.2:2002

Structural design actions

Part 2: Wind actions

AS/NZS 1170.3:2003

Structural design actionsPart 3: Snow and ice actions


13/54

Permanent Ac t ions

Weight of the structure itself Estimated - permanent loads

serviceability G

strength (must use G in every combination)

1.25 G with gravity loads or

0.8 G with uplift loads


14/54

Imposed Ac t ions

Weight of occupancy, use, incidental loads Construction, maintenance, normal function

known loads - machinery, installed equipment(often permanent duration)

estimated loads - storage, people, stock, materials etc.

(shorter term & permanent duration) permanent duration loads - small percentage of nominal load

(default - use (y cQ)

shorter-term load - close to nominal (includes perm Q)

serviceability - y s Q (shorter) y l Q (longer)

strength - 1.5 Q (shorter), 1.5(y c Q) (longer)y c Q with wind


15/54

Forces caused by pressures induced by wind passing

over structure

Wind Act ions

Flow lines

Pressure on

Windward wall

Suction on Roof

Suction on

Leeward wall

Suction

Air moving away from surfac

Windward

wall

Leeward

wall


16/54

)( ,, tscatzdRsit MMMMVV =

Site wind Speed Vsit, b

F ind for each Cardinal Di rection N, NE, E, SE, SW, W, NW

Regional 3 sec gust

wind speed (m/s)Wind directional

multiplier

Terrain and structure

height multiplierShielding multiplier

Topographic multiplier


17/54

Md The variation of wind speed at a location with compass

direction depending on the type of commonly occurring wind

events and the weather patterns in which they arise.

Mz,cat Variation of wind speed with height (related to the

speed for terrain category 2 at 10 m height). This includes

consideration of the type of wind event, e.g., cyclones or

synoptic winds such as thunderstorms.

MtModification of wind caused by topographic features

such as mountains and hills. Again the factor may vary with

direction at a site.

MsShielding caused by adjacent structures.


18/54

Wind Loads

Force = wind pressure tributary area Vector sum of internal and external forces

Evaluated separately for each member

For a given member spacing, wind forces are

expressed in kN/m

Must consider all combinations of wind

direction and openings

Strength limit state uses long return period

(eg. V500)

Serviceability limit state uses V25


19/54


20/54

AS/NZS4055


21/54

REGION CLASSIFICATION


22/54

TOPOGRAPHIC MULTIPLER


23/54

TOPOGRAPHIC DEFINITIONS


24/54

SHIELDING MULTIPLER

FULLY SHIELDED FS

PARTIALLY SHIELDED PS

NO SHIELDING - NS


25/54

TERRIAN CATEGORY

TC 1 EXPOSED OPEN TERRIAN, FEWOBSTRUCTIONS

TC 2 OPEN TERRAIN GRASS LAND, AIRFIELD

TC 2.5 TERRAIN WITH FEW TREES

TC 3 TERRAIN WITH NUMEROUS CLOSELY

SPACED OBSTRUCTIONS


26/54


27/54

TERRAIN CATEGORY 2.0


28/54



29/54

TERRAIN CATERGORY 3.0


30/54

TERRAIN CATERGORY 3.0


31/54



32/54


33/54

DESIGN GUST WIND SPEED


34/54

[ ] dynfigdesair CCVfp2

,)5.0(, =

Design w ind pressure

Related to shape of roof or building, and thestructures response to fluctuations in wind

Constant 1.2 kg/m3

Already evaluated from site wind speed

Related to shape of building and

aerodynamics. Different expressions for:

internal pressure Cfig = Cp,iKc

External pressure Cfig = Cp,eKa KcKlKp

Frictional drag Cfig = CfKc

= 1 for normal structures

important for wind-

sensitive structures


35/54

DESIGN GUST PRESSURE

WIND LOAD P = qu x Cp x LOAD WIDTH


36/54

Forces caused by pressures induced by wind passing

over structure

Wind Act ions

Flow lines

Pressure on

Windward wall

Suction on Roof

Suction on

Leeward wall

Suction

Air moving away from surfac

Windward

wall

Leeward

wall


37/54

PRESSURE COEFFICIENTS FOR WIND CLASSES N1 TO N6

(REGIONS A AND B FOR ULTIMATE STRENGTH AND SERVICEABILITY)

Housing

component

Factored

external

pressure

coefficient

(Cp,eKl)

Internal

pressure

coefficient

(Cp,i)

Net pressure

coefficient

(Cp,n)

Roof

Trusses and

rafters

-0.9 +0.2 -1.1

+0.4 -0.3 +0.7

Roof

Cladding

andfasteners

-1.8 +0.2 -2.0


38/54

Housing

component

Factored

external

pressure

coefficient

(Cp,eKl)

Internal

pressure

coefficient

(Cp,i)

Net pressure

coefficient

(Cp,n)

Wall

Studs

+0.7 -0.3 +1.0

-0.65 +0.2 -0.85

Wall

Cladding

and

Fasteners

-1.3 +0.2 -1.5

PRESSURE COEFFICIENTS FOR WIND CLASSES N1 TO N6

(REGIONS A AND B FOR ULTIMATE STRENGTH AND SERVICEABILITY)


39/54

Wind Loads

Force = wind pressure tributary area Vector sum of internal and external forces

Evaluated separately for each member

For a given member spacing, wind forces are

expressed in kN/m Must consider all combinations of wind

direction and openings

Strength limit state uses long return period

(eg. V500)

Serviceability limit state uses V25


40/54

DESIGN WIND FORCEWwl = qu x Cp x LOAD WIDTH

LOAD WIDTH IS THE TRIBUTARY AREA THAT

IMPARTS LOAD TO A SUPPORTING MEMBER


41/54

EXAMPLE

A NEW RESIDENTIAL HOUSE IS BEING

CONSTRUCTED IN BALLARAT

FARMING DISTRICT.

DETERMINE THE WIND LOAD FOR

STRENGTH DESIGN ON THE RAFTERS

WHICH ARE SPACED AT 750MM

CENTERS.


42/54

SOLUTION

DETERMINE


TOPOGRAPHIC MULTIPLER

SHEILDING MULTIPLER TERRAIN CATEGORY MULTIPLER


43/54



44/54

SOLUTION

DETERMINE


REGION A


45/54

TOPOGRAPGHIC MULTIPLER


46/54

SOLUTION

DETERMINE


REGION A

TOPOGRAPHIC MULTIPLER TC = 1


47/54

NO SHEILDING FROM ADJOINING

BUIDLINGS

Ms = 1.0

SHEILDING MULTIPLER


48/54

SOLUTION

DETERMINE


REGION A


SHEILDING MULTIPLER

NS


49/54

TERRIAN CATEGORY

TC 1 EXPOSED OPEN TERRIAN, FEWOBSTRUCTIONS

TC 2 OPEN TERRAIN GRASS LAND, AIRFIELD

TC 2.5 TERRAIN WITH FEW TREES

TC 3 TERRAIN WITH NUMEROUS CLOSELY

SPACED OBSTRUCTIONS


50/54

SOLUTION

DETERMINE


REGION A


SHEILDING MULTIPLER

Ms = NS

TERRAIN CATEGORY MULTIPLER Mz,cat = 2.0


51/54


52/54

SOLUTION

WIND CLASSIFICATION N2

Vu,site = 40m/s qu = 0.96 kPa

Vs,site = 26m/s qs = 0.41 kPa

Cp,n = -1.1 or +0.7


53/54

LOADING

Strength

Wwl = qu x Cp,n x Load width

= 0.96 x -1.1 x 0.75 = -0.79 kN/m

= 0.96 x 0.7 x 0.75 = +0.50 kN/m


54/54

(2)2013 lecture 1b - loading

Documents