Cam Leitch

Shane Holborn

Designing structures to manage high wind loads

Presentation summary• Background to the project• Cyclones• Designing to withstand extreme wind• Risk levels and wind speed• Cyclone Yasi – overview of field data and

observations• Critical factors for structural performance• Recommended good practice details

Mission is to:• Conduct quality research and testingConduct quality research and testing

(Leads to (Leads to better materials andbetter materials and building practices) building practices)• Community education –a research & community education Community education –a research & community education

program to improve the resilience of housing to severe winds.program to improve the resilience of housing to severe winds.• Damage Investigations following severe wind events – Damage Investigations following severe wind events –

essential to investigate both undamaged buildings that essential to investigate both undamaged buildings that performed well, buildings that suffered damage (how & why?) performed well, buildings that suffered damage (how & why?) and to see if measures are needed for improvements or and to see if measures are needed for improvements or changes to construction details.changes to construction details.

Vision - To Vision - To minimise loss and suffering as a result of minimise loss and suffering as a result of severe wind eventssevere wind events

Cyclone Testing Station – James Cook Uni

• Static or cyclic loadingStatic or cyclic loading of of roofing, wall cladding, roofing, wall cladding, structural elements, joints, complete structuresstructural elements, joints, complete structures

• UUplift, racking, flexure, pressure, plift, racking, flexure, pressure, wind driven wind driven debris debris impactimpact

• Wind tunnel studiesWind tunnel studies

• Risk assessmentRisk assessment

Testing and consulting for industry;

Cyclone Testing Station

Previous Cyclones in Queensland

(Winifred 1986)

Cyclone Larry – 2006

• Larry made landfall close to Innisfail on 20 March 2006

• Just prior to landfall, reported as Just prior to landfall, reported as a Category 5 by BoMa Category 5 by BoM

• Wind gusts reaching 240 kilometres per hour

• Traveling W –NW at ~ 30 kphTraveling W –NW at ~ 30 kph• Estimted $1.5 billion damage.• The costliest tropical cyclone to

ever impact Australia surpassing Cyclone Tracy (1974).

Cyclone Larry - 2006

Cyclone Ului - 2010•Fluctuated in intensity•Weakened to category 2 before regaining category 3 strength.•21st March crossed the outlying Whitsunday Islands and made landfall at Airlie Beach, Queensland.•Damage $20 million and agricultural losses reached A$60 million.

Cyclone Yasi - 2011

•Made landfall in northern Queensland, Australia in the early hours of 3 February, 2011, causing severe damage to affected areas •The eye crossed the coastline at Mission Beach just before midnight and passed over Tully soon after.•The storm caused an estimated US$3.6 billion in damage, making it the costliest tropical cyclone to hit Australia on record

Cyclone Yasi - 2011

Wind speed estimated from numerical models, street sign data and the occasional Anemometer

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15

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20

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C a i r n s

C a r d w e l l

T o w n s v i l l e

C l u m p P o i n t J e t t y

L u c i n d a

a i r n s A M O

I n g h a m

T o w n s v i l l e A M O

G r e e n I s

C a i r n s A P

A I M S O R P H E U S I S

F l y i n g F i s h P o i n t

E t t y B a y

W o n g a l i n g B e a c h

S o u t h M i s s i o n B e a c h

T u l l y H e a d s

S o u t h J o h n s t o n e

B a b i n d a

I n n i s f a i l

M o u r i l y a n

K u r r i m i n e B e a c h

B i n g i l B a y

E l A r i s h

S i l k w o o d

T u l l y

M o u r i l y a n H a r b o u r

C o w l e y B e a c h

M i s s i o n B e a c h

B r a m s t o n B e a c h

M i r r i w i n n i

G o r d o n v a l e

Recording wind speed in tropical cyclones

• Max upper bound gust speed ~240 km/h ~67 m/s• Design wind speed houses 250 km/h ~ 69 m/s• Max upper bound gust ~95% design speed

Cardwell, Tully Heads, South Mission Beach • Max upper bound gust ~85% design speed

Tully, Kurrimine Beach• Low Cat 4 wind speeds (mainland) - issue with

what the affected community thinks went through – complacency in builders and home owners for possible future events

Yasi – Estimated Wind Speeds

The Gap ‘Super Cell’ Brisbane 2008

More frequent, intense events - climate change predictions…..?

Not just cyclones

Townsville mini-tornado 2012

Nursery structures particularly susceptible

Designing to resist extreme wind events

AS/NZS1170.2-2011 Wind regions

Region C 69 m/sRegion C 69 m/s

Region A 45 m/sRegion A 45 m/s

Region D 88 m/sRegion D 88 m/sRegion B 57 m/sRegion B 57 m/s

Regional Wind SpeedsRegional Wind Speeds

Tropical cyclone categories

Cyclone Cyclone CategoryCategory

Maximum Gust SpeedMaximum Gust SpeedCentral Central

Pressure Pressure (hPa)(hPa)

Damage PotentialDamage Potential(m/s)(m/s) (km/h)(km/h)

1 25 – 3525 – 35 90 – 12590 – 125 >990>990 Negligible house damageNegligible house damage22 35 – 4735 – 47 125 – 170125 – 170 970 970 – – 985985 Minor house damageMinor house damage33 47 47 – – 6363 170 170 –– 225 225 950 950 – – 965965 Some roof & structural damageSome roof & structural damage44 63 – 78 63 – 78

6969225 – 280225 – 280

250250930 930 – – 945945 Significant roof and structural Significant roof and structural

damagedamage

5 >78>78 >280>280 <925<925 Extremely dangerous with Extremely dangerous with widespread destructionwidespread destruction

Region C Design Wind Speed in open terrain at 10m height for “standard” structures (houses)

Critical Wind Loading ParametersModify this wind speed to account for the following:

1. Topographic Effects (Wind speeds-up on hills)

2. Height of Structure (Taller structures have higher wind speeds)

3. Shielding by other buildings or terrain (normally trees are not counted, as the leaves are assumed to be stripped in a severe cyclone. However, at lower wind speeds, trees can act as a wind break).

onhills

increasing

over theocean

in thesuburbs

near thecoast

decreasing

onhills

increasing

over theocean

in thesuburbs

near thecoast

decreasing

Designing to Resist Wind EventsDesign Wind Action < Actual Capacity of the Structure

•Design Wind Action - Pressures (and forces) applied depends on the actual wind speed of the cyclone that impacts the – NO control over this

•Actual Capacity of Structure - The structure should be designed to resist a selected “Design Wind Speed”. This is chosen so the risk of this design wind speed being exceeded is relatively low – We choose (VD) to manage the risk of failure

•Design wind speed means the design gust wind speed for the area where the building is located, calculated in accordance with AS/NZS

Risk Levels and Wind SpeedThe probability (risk) of having a cyclone with at least a certain wind speed reduces as the wind speed increases.

Wind engineers specify the risk of occurrence in any one year as being the “Annual Probability of Exceedance”, expressed as a ratio (e.g. 1:250, equiv to 1/250 or 0.4%)

The risk of a cyclone occurring in each year is independent of what happened the year(s) before.

A structure that will be used for a longer time (ie a larger Design Life), will have an increased risk of exposure to being impacted by a cyclone with at least the selected design wind speed, over the longer time frame.

Risk Levels for Different Design Life

from AS/NZS1170.0 (2002)

Design Working Life

Importance Level

Annual Probability of Exceedance for Wind Design

Cyclonic Non- Cyclonic

Less than 6 months

1 1/250 1/25

2 1/250 1/100

5 Years1 1/250 1/25

2 1/250 1/250

25 Years1 1/250 1/50

2 1/250 1/250

50 Years1 1/250 1/100

2 1/500 1/500

Importance Levels for Different Structures

Consequences of Failure

Description Importance Level

Comment

LowLow consequence for loss of human life, or small or moderate economic, social or environmental consequences

1Minor structures (failure not likely to endanger human life)

OrdinaryMedium consequence for loss of human life, or considerable economic, social or environmental consequences

2Normal structures and structures not falling into other levels

HighHigh consequence for loss of human life, or very great economic, social or environmental consequences

3 Major structures (affecting crowds)

4Post-disaster structures (post-disaster functions or dangerous activities)

from AS/NZS1170.0 (2002)

Yasi Damage to Nursery Structures

Loss of covers to Igloos

Yasi Damage to Nursery Structures

Shade house footing damage

Yasi Damage to Nursery Structures

Destroyed potting shed and shade house damage

Yasi Damage to Nursery Structures

Unrepaired damage remains in an almost totally devastated business

Review of Field PracticeCable Guyed Structures

View of a typical cable-guyed structure

Shade cloth blown off cables after TC Yasi

Review of Field PracticeCable Guyed Structures

Cable Guyed Structures

Recommended plans and bay sizes/dimensions

Cable to Column Top Details

Cable Guyed Structures

Horizontal cables clamped to round bar welded to top of interior column

Cable Guyed Structures

External column

Cable Guyed Structures - footings

Footing failures to a Cable Guyed Structure

Some general points about foundations:

•Each type of footing for a particular structure should be purpose designed (do not copy footing sizes from other areas where factors such as the design loads and soil type are likely to be different).•In general terms, footings in sand will usually be larger than those in clay.•The sides of footings should be close to vertical, wherever feasible. •Extend the tops of footings at least 75 mm above finished ground level and provide a slope to the top surface.

Footing NOT extended far enough above ground

Cable Guyed Structures - footings

Cable Guyed Structures - footings

Horizontal cables clamped to round bar welded to top of interior column

Cable-guyed structure with winder to roll up cloth

Review of Field PracticeCable Guyed Structure with roll-back cloth

Review of Field PracticeCantilever Post Structures

General view of cantilever post structure, fitted with top rails

Cantilever Post Structures

Details for typical cantilever post structure

Install extra bolt to pipe clamp

Cantilever Post Structures

Review of Field PracticeTemporary Structures

Temporary- Low Cost Structure

Bent columns to cantilever post structure

Review of Field PracticeCantilever Post Structures

Review of Field PracticeHoop or Igloo Structures

Hoop and “Igloo” structures

Damaged structures

Review of Field PracticeHoop or Igloo Structures

Hoop Structures

Hoop Structures

Hoop Structures – Avoid Mullion Failure

Good Practice to use a bolt through clamps to end mullions

Igloo Structures

Typical “Igloo” Structure

End Wall and Roof Bracing to Typical “Igloo” Structure

Good Practice to use Roof Bracing to provide support at the top of every Mullion (End Wall Column)

Top of Mullion should be supported by Roof Bracing

Igloo Structures

Roof Bracing – Should support Mullions

Diagonal pipe wall braceDiagonal pipe wall brace

Crossed tension wire bracingCrossed tension wire bracing

Top of Mullion NOT supported

Good Practice to provide support to top of mullions

Igloo Structures - Provide Support to Mullions

Igloo Structures – Struts to Roof Bracing

Buckling to struts between structural frames

Struts between frames failed by buckling

Critical wind loading factor for shade houses

Choice of shade cloth

Choice of Shade Cloth•Shade cloth normally chosen for its “Shade Factor”

•However wind load on cloth controlled by its porosity.

•Some evidence that wind uplift (load perpendicular to the surface) is significantly reduced on porous cloth

•Anecdotal evidence suggests that shade cloth of no more than 30% shade appeared to fare better in severe wind events.

Critical Factor for Shade housesChoice of Shade Cloth

70% and 30% shade cloth

16mm Quad Net cloth

Critical Factor for Shade housesChoice of Shade Cloth

Recommended Good Practice DetailsClips to Shade Cloth

Clips - connect cloth to support cables

Good Practice to provide adequate connections from cloth to support wires

Critical Factors for Performance 

1.Design - select an appropriate Design Wind Speed (usually set by regulation) and commission an engineer to perform a structural design to ensure that all of the members and connections can withstand the calculated wind loads

2.Construction quality and detailing - build the structure to comply with the design

3.Regular inspection and maintenance - set up regular inspection and maintenance schedules for structures.

4.Planning and preparation of your response well prior to an event

The wind finds the weakest link

• Building resilience – long term factors to consider for your business• Notes for new nurseries – design, layout and construction• Before the cyclone – how to prepare in advance of a cyclone or storm• During the cyclone – what to do during the event• After the cyclone – what to do directly after• After the disaster checklist – a tool to assist in the assessment of damage for insurance purposes• Contacts and resources – a list of references for further information and specialist support• Emergency contacts card – which you can fill in and give to your staff.

Extension pack

•Develop plans to prepare, respond and recover from

disasters whether they be storms, cyclones, floods

•Think about your property its layout and individual

considerations like location and topography

•Make sure everyone knows the plan and their role

within it

•Allow for the significant value of the plants to be

protected by the structure

•Think about broader disruptions – no power or water

•Talk to your insurer – know what is and is not covered

Building resilience