Structural engineering theory: Difference between revisions

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==Strength==

[[Strength of materials|Strength]] depends upon material properties. The strength of a material depends on its capacity to withstand axial [[stress (mechanics)|stress]], [[shear stress]], bending, and torsion. The strength of a material is measured in force per unit area (newtons per square millimetre or N/mm², or the equivalent megapascals or MPa in the [[SI system]] and often pounds per square inch psi in the United States Customary[[customary Unitsunits]] system).

A structure fails the strength criterion when the [[stress (mechanics)|stress]] (force divided by area of material) induced by the loading is greater than the capacity of the structural material to resist the load without breaking, or when the [[Strain (materials science)|strain]] (percentage extension) is so great that the element no longer fulfills its function ([[Yield (engineering)|yield]]).

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==Stiffness==

[[Stiffness]] depends upon material properties and [[geometry]]. The stiffness of a structural element of a given material is the product of the material's [[Young's modulus]] and the element's [[second moment of area]]. Stiffness is measured in force per unit length (newtons per millimetre or N/mm), and is equivalent to the 'force constant' in [[Hooke's Lawlaw]].

The [[Deflection (engineering)|deflection]] of a structure under loading is dependent on its stiffness. The [[dynamic response]] of a structure to dynamic loads (the [[natural frequency]] of a structure) is also dependent on its stiffness.

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The safety factors for material strength vary depending on the material and the use it is being put to and on the design codes applicable in the country or region.

A more sophisticated approach of modeling structural safety is to rely on [[structural reliability]], in which both loads and resistances are modeled as probabilistic variables.<ref name=":0">{{Cite book|title=Melchers, R. E. (2002), "Structural Reliability Analysis and Prediction," 2nd Ed., John Wiley, Chichester, UK.}}</ref><ref name=":1">{{cite journal|last1=Piryonesi|first1=Sayed Madeh|last2=Tavakolan|first2=Mehdi|date=9 January 2017|title=A mathematical programming model for solving cost-safety optimization (CSO) problems in the maintenance of structures|journal=KSCE Journal of Civil Engineering|volume=21|issue=6|pages=2226–2234|doi=10.1007/s12205-017-0531-z|bibcode=2017KSJCE..21.2226P |doi-access=free}}</ref> However, using this approach requires detailed modeling of the distribution of loads and resistances. Furthermore, its calculations are more computation intensive.

===Load cases===

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{{Main|Newton's laws of motion}}

The most important natural laws for structural engineering are [[Newton's Lawslaws of Motionmotion]].

Newton's first law states that ''every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed.''

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Newton's third law states that ''all forces occur in pairs, and these two forces are equal in magnitude and opposite in direction.''

With these laws it is possible to understand the forces on a structure and how that structure will resist them. The Thirdthird Lawlaw requires that for a structure to be stable all the internal and external forces must be in [[Mechanical equilibrium|equilibrium]]. This means that the sum of all internal and external forces on a ''[[free-body diagram]]'' must be zero:

* <math> \sum \vec F = 0 </math>: the vectorial sum of the [[force]]s acting on the body equals zero. This translates to

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A structural engineer must understand the internal and external forces of a structural system consisting of structural elements and nodes at their intersections.

A statically determinate structure can be fully analysed using only consideration of equilibrium, from Newton's Lawslaws of Motionmotion.

A statically indeterminate structure has more unknowns than equilibrium considerations can supply equations for (see [[simultaneous equations]]). Such a system can be solved using consideration of equations of ''compatibility'' between geometry and deflections in addition to equilibrium equations, or by using [[virtual work]].

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==Elasticity==

{{Main|Elasticity (physics)|l1=Elasticity}}

{{See also|Hooke's Lawlaw}}

Much engineering design is based on the assumption that materials behave elastically. For most materials this assumption is incorrect, but empirical evidence has shown that design using this assumption can be safe. Materials that are elastic obey Hooke's Lawlaw, and plasticity does not occur.

For systems that obey Hooke's Lawlaw, the extension produced is directly proportional to the load:

:<math> \vec{\mathbf{F}}=k\vec{\mathbf{x}} \ </math>

where

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{{Main|Plasticity (physics)|l1=Plasticity}}

Some design is based on the assumption that materials will behave [[Plasticity (physics)|plastically]].<ref name=Heyman1>{{cite book|title=Structural Analysis: A Historical Approach|author=Heyman, Jacques|publisher=Cambridge University Press|date=1998|isbn=0-521-62249-2}}</ref> A plastic material is one which does not obey Hooke's Lawlaw, and therefore deformation is not proportional to the applied load. Plastic materials are [[ductile]] materials. Plasticity theory can be used for some reinforced concrete structures assuming they are underreinforced, meaning that the steel reinforcement fails before the concrete does.

Plasticity theory states that the point at which a structure collapses (reaches yield) lies between an upper and a lower bound on the load, defined as follows: