Last edited 15 Sep 2017

Hurricane design considerations

Hurricane-over-the-usa.jpg

Contents

[edit] Introduction

In early September 2017, tropical storm Irma (category 5 at its peak) covered an area the size of France, wreaking a trail of devastation in the Caribbean and Florida. It was the strongest Hurricane recorded for ten years. During that time a great deal has been learnt about hurricane proof construction reflected in American building regulation and control.

Residential buildings in the Caribbean islands were not so strictly controlled and many older structures preceded research and improvements in hurricane design. This article raises some of the issues that should be addressed when planning replacement of temporary housing solutions being erected as an emergency provision.

[edit] Storm surge and increase of sea levels

Historically many towns and cities have grown around port and estuary facilities. It does not take a lot of imagination to visualise the combination of heavy rain storm water and a tidal wave/ raising of sea levels to cause catastrophic flooding to such coastal areas.

To combat this risk reconstruction should be sited:

  • On raised ground say 6 m above sea level.
  • Or on elevated anchored foundations:
    • Offering minimum resistance to sea storm surge.
    • Shaped to avoid sea facing flat surfaces and anchored to rock or on sunken piles. Circular or V shaped columns deflect on coming surface wave pressure.

[edit] Wind loadings

There are four major considerations to wind speeds that can gust as much as 200 mph:

  • Resistance to horizontal wind loads (and rain) with particular reference to door and window openings.
  • Uplift caused by wind suction and differential pressures with particular reference to roofs
  • Damage caused by flying projectiles and debris.
  • External construction and chattels such as sheds, screens and benches that in themselves could become projectiles and cause damage.

Roof design should be such that the roof structure is anchored back to the foundations either:

  • By straps or bolted rods.
  • Or via the wall construction if it is tensioned with steel or cable reinforcement.
  • Roof sheathing should be fixed with special hurricane resistant nails and cedar tiles glued to surfaces as proscribed in local codes in hurricane zones.
  • Roof overhanging eaves should be minimal and special attention afforded to the intrusion of driving horizontal rain that can penetrate attics causing ceiling to collapse.
  • Roofs can be shaped and designed to produce improved aerodynamics and susceptibility to wind damage. For instance, a curved dome design and an absence of valley gutters lowers risk of wind damage or water ingress. Corrugated surfaces also offer more wind resistance by spreading surface area and dissipating impact.

With warning of an impended hurricane, unanchored garden structures should be removed or made safe. Aluminium swimming pool sliding covers appear to be particularly susceptible to destruction. Garden sheds should be anchored to their bases and their roofs strapped down.

Hurricane1.jpg

[edit] External openings

Windows, doors, roof lights and garage shutters are all vulnerable weak points against hurricane forces. Broken glass can cause injury and once penetrated wind forces provide different pressures inside a property to the outside.

Subsequently, damage and possibly collapse is inevitable. A whole industry has emerged in the USA and Hong Kong specialising in hurricane protective products not least:

  • Steel, aluminium or polypropolyne hurricane proof shutters. These can be:
    • Permanent and hinged fitting within and flush with an opening.
    • Slatted roller shutters manually operated in case of power failure and securely fixed in deep channel tracks.
    • Temporary bolted panels stored when not in use. Again, corrugation to exposed surface reduces impact.
  • Glazing should be thicker than normal and be internally lined with shatter proof film.
  • Window fixings and waterproofing should be of the highest integrity.
  • Garage up and over doors in the case of an impending storm should be reinforced externally with a strong woven fabric screen.

[edit] Refuge room

It make a lot of sense in a hurricane zone to designate a secure strong room that can be occupied in an emergency situation. This might include tornados, intruder attack or nuclear threat although hurricanes are the most likely threat. A basement area is a safe location, providing that the location is not susceptible to flooding.

The construction needs to be very solid both in walls and ceiling and have a secure steel doors. There should be two means of escape in case of blockage to the exit and they should open inwards. The refuge room should be well ventilated and equipped with the following:

  • A standby generator of min 1,500 KV with an exhaust pipe to the building exterior.
  • Enough generator fuel for a month’s intermittent use.
  • Temporary lighting and power provision.
  • A tool kit inc electric drill and saw.
  • Water proof bags and container.
  • A month’s supply of bottled water.
  • A month’s supply of ready meals and tinned food (inc. pet food if needed).
  • Bedding and waterproof clothing/footwear.
  • A whistle and sound signalling device.
  • A wind up hand cranked flashlight.
  • Wireless communication equipment.
  • An inflatable raft and paddles.
  • Medical pack and first aid manual.
  • Medical and vetinary records and any other essential records.
  • Cash.

[edit] Isolation of services

Just prior to the storm as a precaution it is worth considering isolating the incoming services of water, electricity and gas as well as removing plugs to computers and TV sets susceptible to lightning strikes.

Care needs to be taken in restoring the services in case plumbing joints have fractured and water has not penetrated the electrics.

[edit] Rebuilding devastated homes

The state of Florida has developed a set of building codes that have undoubtedly saved lives and destruction of property over a period of fifteen years refinement. The International Hurricane Research Centre has fed its research into the regulations as have insurance companies.

Probably the strongest form of structure is a monolithic thin shelled concrete dome. Anyone who has stood on an ostrich egg will witness the strength of its shape. The ferro- concrete form of construction of swimming pools and even boat hulls has been around for some time so it is not a new concept. It is an economic way of producing curved shapes with significant structural integrity.

A force 5 hurricane of 150 mph will exert 100 lbs per square inch on a 10 m high dome structure which would hardly be felt inside. It is also suitable protection against tornadoes which are stronger than hurricanes and earthquakes. Germany used this design for bomb proof purposes in the Second World War. Any puncture caused by flying debris can easily be repaired.

While the author advocates such design as an engineering solution he concedes it probably fails to tick the box aesthetically.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

[edit] Bibliography

  • Wikipedia – Hurricane proof design
  • Florida State Building Codes – High wind standards
  • Florida Building Commission Website
  • Monolithic Dome Institute website
  • Monolithic Dome Benefits/strength Dr Arnold Williams 13-3-2009