slag and fly ash is commonly used a cementitious material in concrete mix designs - they help control shrinkage, reduce cracking, and improves the quality of concrete.
http://en.wikipedia.org/wiki/Fly_ash
http://en.wikipedia.org/wiki/Slag
"the path of least resistance" implies that there is more than one path to go. there is only one direction for gravity, and that is down.
as to why it doesn't topple over sideways - compare it to a tree that is being cut down. you cut across the trunk of the tree, and eventually the trunk will snap and the tree will fall over.
---1) the tree's center of mass is not located at the center of rigidity (the tree trunk). the tree has a natural eccentricity (grows towards light)
---2) there is only one load path in the system. all the gravity forces have to go through the remainder of the trunk, of which most of it is gone. thus, the gravity forces are eccentric with respect to the only remaining load path in the system. this causes the remainder of the trunk to bend, it snaps, the tree falls over.
a skyscraper is a highly redundant structure, meaning there are several load paths for the structure to use in the event of failure - this is mandated by building codes, and more importantly, its just good engineering practice.
---1) you can cripple many elements and still have a working system. this is because the building elements are designed in the elastic range of stresses, which means that any structural deformations are temporary. once you start "removing" structural elements from the structure, i.e. columns, you will end up going into the plastic range of stresses, where structural deformations are permanent. once a structure develops a "hinge", which is a permanent plastic deformation, the stresses in the overall structure will re-distribute and find another load path.
---2) the center of mass of the structure is located close to the center of rigidity of the structure (i.e. the lateral force resisting system, which is essentially the ENTIRE structure). this means very little inherent eccentricity in the system. for the building to actually topple over would require several rows of structural support to be physically removed so that the center of mass is much further away from the center of rigidity.
---3) most steel structures are designed with a factor of safety of around 2 or 3. at a temperature of 1000 degrees Fahrenheit, steel only has 40% of its ultimate strength, which corresponds to a safety factor of 2.5. so at 1000 degrees Fahrenheit, your structural members are functioning with almost no factor of safety. the mode of failure would likely be buckling, followed by a progressive buckling of adjacent columns until the structure cannot redistribute the loads.
http://www.engineeringtoolbox.com/metal-temperature-strength-d_1353.html
http://en.wikipedia.org/wiki/Fly_ash
http://en.wikipedia.org/wiki/Slag
"the path of least resistance" implies that there is more than one path to go. there is only one direction for gravity, and that is down.
as to why it doesn't topple over sideways - compare it to a tree that is being cut down. you cut across the trunk of the tree, and eventually the trunk will snap and the tree will fall over.
---1) the tree's center of mass is not located at the center of rigidity (the tree trunk). the tree has a natural eccentricity (grows towards light)
---2) there is only one load path in the system. all the gravity forces have to go through the remainder of the trunk, of which most of it is gone. thus, the gravity forces are eccentric with respect to the only remaining load path in the system. this causes the remainder of the trunk to bend, it snaps, the tree falls over.
a skyscraper is a highly redundant structure, meaning there are several load paths for the structure to use in the event of failure - this is mandated by building codes, and more importantly, its just good engineering practice.
---1) you can cripple many elements and still have a working system. this is because the building elements are designed in the elastic range of stresses, which means that any structural deformations are temporary. once you start "removing" structural elements from the structure, i.e. columns, you will end up going into the plastic range of stresses, where structural deformations are permanent. once a structure develops a "hinge", which is a permanent plastic deformation, the stresses in the overall structure will re-distribute and find another load path.
---2) the center of mass of the structure is located close to the center of rigidity of the structure (i.e. the lateral force resisting system, which is essentially the ENTIRE structure). this means very little inherent eccentricity in the system. for the building to actually topple over would require several rows of structural support to be physically removed so that the center of mass is much further away from the center of rigidity.
---3) most steel structures are designed with a factor of safety of around 2 or 3. at a temperature of 1000 degrees Fahrenheit, steel only has 40% of its ultimate strength, which corresponds to a safety factor of 2.5. so at 1000 degrees Fahrenheit, your structural members are functioning with almost no factor of safety. the mode of failure would likely be buckling, followed by a progressive buckling of adjacent columns until the structure cannot redistribute the loads.
http://www.engineeringtoolbox.com/metal-temperature-strength-d_1353.html
Facebook
Twitter