I read the item about\u00a0the three-storey\u00a0residential structure\u00a0being built\u00a0in Halifax collapsing in high winds – failing, in engineering terms.\u00a0 The building’s frame fell down shortly after 1:30 p.m.\u00a0last Thursday damaging a car parked below.\u00a0 Workers had left the site shortly before.\u00a0 See The Chronicle Herald, January 31, 2013.<\/p>\n
High winds also caused construction scaffolding to blow down at another site.\u00a0 The winds restricted access to the harbour bridges.\u00a0 Gusts were clocked in excess of 85 kilometres per hour in the area, and up to 105 kilometres per hour on one occasion.<\/p>\n
Acts of God?\u00a0 Mother Nature having her way with us?\u00a0\u00a0Excessive structural loading?<\/p>\n
Some people involved in\u00a0civil litigation, and others in the insurance industry,\u00a0might see such collapses\u00a0as “acts of God”.\u00a0 Many others might see the incidents as examples of Mother Nature’s wrath.<\/p>\n
I see\u00a0collapse of the residential building as quite possibly an example of wind loading that the structure was\u00a0incapable of withstanding – the wind was just too strong.\u00a0 The building structure was not\u00a0designed to carry such wind loading – at least, at that unfinished stage of construction.\u00a0 Or possibly the structure as designed was capable but wasn’t constructed according to the design.<\/p>\n
When structures are “weak”<\/strong><\/p>\n It’s not likely so well known to people, in general, that some\u00a0structures are at their “weakest” when they are under construction –\u00a0more susceptible to failure.\u00a0 The design engineer\u00a0sometimes needs to\u00a0pay more attention to\u00a0the\u00a0construction phase than to\u00a0the completed phase.\u00a0 Nor is it likely well known that professional engineers are\u00a0not always involved in the\u00a0design and construction of residential buildings and other small or\u00a0seemingly unimportant structures.<\/p>\n Mother Nature’s loads<\/strong><\/p>\n The loads on a structure come from Nature.\u00a0 I think an entire book could be written on the concept of “load” in engineering.\u00a0 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.<\/p>\n For example, obviously, the weight of the people using\u00a0a building\u00a0and the weight of the equipment in the building.\u00a0 Less obviously, the weight of a structure – the structure must be able to hold itself up.\u00a0 We now know about wind “load”, the pressure of the wind on a building, and, by extension,\u00a0also on towers, and on traffic signs along highways.\u00a0 But, what about earthquake loading – the shaking\u00a0that all manner of structures in an earthquake prone area must provide for?\u00a0 And frost action on retaining walls and garden pathways.\u00a0 All loads from Mother Nature.<\/p>\n Here is more information on\u00a0where loads on structures come from – sent by Mother Nature\u00a0and to be dealt with by\u00a0professional\u00a0engineers.\u00a0 They can be categorized as vertical or horizontal loads.\u00a0 They might also be separated into loads above the ground, at the ground surface, and below the ground:<\/p>\n 1. Dead loads<\/span><\/strong><\/p>\n All materials in nature have weight, called dead weight when used to form a structure – it doesn’t move around once in place.\u00a0 Materials like timber, steel, concrete, plastic, and earth.\u00a0 Design engineers must ensure the structure can support itself; it’s own dead weight.\u00a0 And that the foundation soil material below can support all the other materials above.\u00a0 Dead weight is often the greatest weight on a structure.<\/p>\n 2. Live loads<\/strong><\/span><\/p>\n Live loads do not usually provide such heavy loads on a structure, but they are important because they often derive from the\u00a0occupancy of a structure – people.\u00a0 They can also be caused by vehicles, as in a parking garage.\u00a0 Storage of materials in tanks and bins generates live loads.\u00a0 These objects all have weight that can be moved around; they’re “live” loads.<\/p>\n 3.\u00a0Snow loads<\/strong><\/span><\/p>\n In northern climes, snow is another heavy load on a structure.\u00a0 This material doesn’t move around once it falls and drifts into place, usually on a roof.\u00a0 The nice, light stuff is light; the wet stuff is\u00a0very heavy, as we all know when we must shovel it.<\/p>\n 4. Rain water<\/strong><\/span><\/p>\n Rain water can impose quite a load on a roof if its removal isn’t provided for properly.\u00a0 When it falls on accumulated snow on a\u00a0roof the combination of snow plus rain is a considerable load on a building’s roof.<\/p>\n 5. Frost action<\/strong><\/span><\/p>\n When wet soil freezes, particularly saturated soil, it expands – about 9%, and\u00a0imposes a very great load on any part of a structure with which it has contact.\u00a0 It moves everything in its path, verticallly, horizontally, and everywhere in between.\u00a0 It’s not practical to resist it, the forces are so great.\u00a0 In some types of soils ice lens can form and the expansion is much\u00a0greater than 9%.\u00a0 Foundations below the ground, and structures at ground level, like retaining walls and highways are affected.\u00a0 Design engineers\u00a0provide for\u00a0the load from frost action by ensuring it\u00a0doesn’t\u00a0develop in the first place.<\/p>\n 6. Wind<\/strong><\/span><\/p>\n We mentioned wind above.\u00a0 We all know how wind can push things over.\u00a0 Less is known about how the wind can “pull” things over – called suction pressure in engineering.\u00a0 It acts in all directions.\u00a0 It’s the kind of wind pressure that pulls sail boats across the water and causes air craft wings to lift.\u00a0 It’s a load that is being applied\u00a0every time the wind blows on a structure.\u00a0 It’s certain to have been a factor in the collapse of the three-storey residential building.\u00a0 Design engineers know about it and provide for\u00a0it.<\/p>\n 7. Earthquake loads<\/strong><\/span><\/p>\n Earthquake loads are considered to act in both a vertical and a horizontal direction.\u00a0 They can result in large forces on a structure.\u00a0 Providing for these forces\u00a0when Mother Nature sends them our way is not as\u00a0well understood.\u00a0\u00a0Design engineers\u00a0do their best with the analytical tools that are available.<\/p>\n 8. Temperature<\/strong><\/span><\/p>\n Construction materials expand and contract as the temperature changes.\u00a0 Provision must be made for this\u00a0in design.\u00a0 All bridge decks\u00a0have a gap between sections of the deck to\u00a0accommodate the expansion of the deck\u00a0in warmer weather.\u00a0 Otherwise, the bridge deck would buckle – an engineering failure.\u00a0\u00a0Concrete floors in buildings have expansion joints for the same reason.<\/p>\n 1. Earth pressure<\/strong><\/span><\/p>\n Earth – Mother Earth, can impose a pressure on a structure and must be allowed for in design.\u00a0 An\u00a0obvious example of a horizontal pressure due to the earth is the pressure on a retaining wall\u00a0or a basement wall.<\/p>\n 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.\u00a0 It’s called overburden pressure in this situation.\u00a0 Design\u00a0engineers can\u00a0provide for these earth pressures.<\/p>\n 2. Water pressure<\/strong><\/span><\/p>\n Water, one of Mother Nature’s great materials, can cause problems if not considered.\u00a0 Dams forming reservoirs are an obvious instance\u00a0where water pressure must be provided for when designing the dam.\u00a0 Less obvious is the allowance that must be made for the\u00a0pressure that results from\u00a0the water that flows through\u00a0an earth dam.\u00a0 This happens and it’s normal.\u00a0 Also less obvious, water pressure must be provided for in bridge design less it cause scour and erosion around the bridge piers.<\/p>\n 3. Dynamic loading<\/strong><\/span><\/p>\n I wonder how many readers know that bridge decks\u00a0are designed to resist the\u00a0dynamic load that results\u00a0when a number\u00a0of vehicles all put their brakes on at the same time?\u00a0 This load is related to\u00a0several factors including the weight of\u00a0the vehicles – weight that is characteristic of all materials in Mother Nature’s\u00a0realm.<\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" I read the item about\u00a0the three-storey\u00a0residential structure\u00a0being built\u00a0in Halifax collapsing in high winds – failing, in engineering terms.\u00a0 The building’s frame fell down shortly after 1:30 p.m.\u00a0last Thursday damaging a car parked below.\u00a0 Workers had left the site shortly before.\u00a0 … Continue reading Vertical Loads<\/strong><\/h1>\n
Horizontal Loads<\/strong><\/h1>\n