How Mother Nature may have her way with us

I read the item about the three-storey residential structure being built in Halifax collapsing in high winds – failing, in engineering terms.  The building’s frame fell down shortly after 1:30 p.m. last Thursday damaging a car parked below.  Workers had left the site shortly before.  See The Chronicle Herald, January 31, 2013.

High winds also caused construction scaffolding to blow down at another site.  The winds restricted access to the harbour bridges.  Gusts were clocked in excess of 85 kilometres per hour in the area, and up to 105 kilometres per hour on one occasion.

What happened?

Acts of God?  Mother Nature having her way with us?  Excessive structural loading?

Some people involved in civil litigation, and others in the insurance industry, might see such collapses as “acts of God”.  Many others might see the incidents as examples of Mother Nature’s wrath.

I see collapse of the residential building as quite possibly an example of wind loading that the structure was incapable of withstanding – the wind was just too strong.  The building structure was not designed to carry such wind loading – at least, at that unfinished stage of construction.  Or possibly the structure as designed was capable but wasn’t constructed according to the design.

When structures are “weak”

It’s not likely so well known to people, in general, that some structures are at their “weakest” when they are under construction - more susceptible to failure.  The design engineer sometimes needs to pay more attention to the construction phase than to the completed phase.  Nor is it likely well known that professional engineers are not always involved in the design and construction of residential buildings and other small or seemingly unimportant structures.

Mother Nature’s loads

The loads on a structure come from Nature.  I think an entire book could be written on the concept of “load” in engineering.  But, possibly, simply put, a load on a structure is anything that the structure must stand up to, or provide for, and still function as intended.

For example, obviously, the weight of the people using a building and the weight of the equipment in the building.  Less obviously, the weight of a structure – the structure must be able to hold itself up.  We now know about wind “load”, the pressure of the wind on a building, and, by extension, also on towers, and on traffic signs along highways.  But, what about earthquake loading – the shaking that all manner of structures in an earthquake prone area must provide for?  And frost action on retaining walls and garden pathways.  All loads from Mother Nature.

Here is more information on where loads on structures come from – sent by Mother Nature and to be dealt with by professional engineers.  They can be categorized as vertical or horizontal loads.  They might also be separated into loads above the ground, at the ground surface, and below the ground:

Vertical Loads

1. Dead loads

All materials in nature have weight, called dead weight when used to form a structure – it doesn’t move around once in place.  Materials like timber, steel, concrete, plastic, and earth.  Design engineers must ensure the structure can support itself; it’s own dead weight.  And that the foundation soil material below can support all the other materials above.  Dead weight is often the greatest weight on a structure.

2. Live loads

Live loads do not usually provide such heavy loads on a structure, but they are important because they often derive from the occupancy of a structure – people.  They can also be caused by vehicles, as in a parking garage.  Storage of materials in tanks and bins generates live loads.  These objects all have weight that can be moved around; they’re “live” loads.

3. Snow loads

In northern climes, snow is another heavy load on a structure.  This material doesn’t move around once it falls and drifts into place, usually on a roof.  The nice, light stuff is light; the wet stuff is very heavy, as we all know when we must shovel it.

4. Rain water

Rain water can impose quite a load on a roof if its removal isn’t provided for properly.  When it falls on accumulated snow on a roof the combination of snow plus rain is a considerable load on a building’s roof.

5. Frost action

When wet soil freezes, particularly saturated soil, it expands – about 9%, and imposes a very great load on any part of a structure with which it has contact.  It moves everything in its path, verticallly, horizontally, and everywhere in between.  It’s not practical to resist it, the forces are so great.  In some types of soils ice lens can form and the expansion is much greater than 9%.  Foundations below the ground, and structures at ground level, like retaining walls and highways are affected.  Design engineers provide for the load from frost action by ensuring it doesn’t develop in the first place.

6. Wind

We mentioned wind above.  We all know how wind can push things over.  Less is known about how the wind can “pull” things over – called suction pressure in engineering.  It acts in all directions.  It’s the kind of wind pressure that pulls sail boats across the water and causes air craft wings to lift.  It’s a load that is being applied every time the wind blows on a structure.  It’s certain to have been a factor in the collapse of the three-storey residential building.  Design engineers know about it and provide for it.

7. Earthquake loads

Earthquake loads are considered to act in both a vertical and a horizontal direction.  They can result in large forces on a structure.  Providing for these forces when Mother Nature sends them our way is not as well understood.  Design engineers do their best with the analytical tools that are available.

8. Temperature

Construction materials expand and contract as the temperature changes.  Provision must be made for this in design.  All bridge decks have a gap between sections of the deck to accommodate the expansion of the deck in warmer weather.  Otherwise, the bridge deck would buckle – an engineering failure.  Concrete floors in buildings have expansion joints for the same reason.

Horizontal Loads

1. Earth pressure

Earth – Mother Earth, can impose a pressure on a structure and must be allowed for in design.  An obvious example of a horizontal pressure due to the earth is the pressure on a retaining wall or a basement wall.

A less obvious example of a vertical pressure due to earth is the pressure on a sliding surface that, if too great, will result in a landslide.  It’s called overburden pressure in this situation.  Design engineers can provide for these earth pressures.

2. Water pressure

Water, one of Mother Nature’s great materials, can cause problems if not considered.  Dams forming reservoirs are an obvious instance where water pressure must be provided for when designing the dam.  Less obvious is the allowance that must be made for the pressure that results from the water that flows through an earth dam.  This happens and it’s normal.  Also less obvious, water pressure must be provided for in bridge design less it cause scour and erosion around the bridge piers.

3. Dynamic loading

I wonder how many readers know that bridge decks are designed to resist the dynamic load that results when a number of vehicles all put their brakes on at the same time?  This load is related to several factors including the weight of the vehicles – weight that is characteristic of all materials in Mother Nature’s realm.

 

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