1 Basis of Structural Design Course 4 Structural action: - prestressing - plate and shell structures Course notes are available for download at http://www.ct.upt.ro/users/AurelStratan/ Prestressing Prestressing: setting up an initial state of stress, that makes the structure work better than without it Examples: – wall plugs – spider's web – bicycle wheel Main use in structural engineering: prestressed concrete
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
Basis of Structural Design
Course 4
Structural action:
- prestressing
- plate and shell structures
Course notes are available for download athttp://www.ct.upt.ro/users/AurelStratan/
Prestressing
� Prestressing: setting up an initial state of stress, that makes the structure work better than without it
� Examples:
– wall plugs
– spider's web
– bicycle wheel
� Main use in structural engineering: prestressed concrete
2
Prestressing examples: wall plug
� A hole in the wall is filled with a wooden or plastic plug
� The screw driven into the plug squeezes the plug against the sides of the hole, generating compressive stresses in the plug and in the wall around it
� Compressive prestressing generates frictional resistance to pulling out the screw
Prestressing examples: spider's web
� Spider's web threads: high tensile, but no compressive resistance
� Spider pulls its threads tight, creating a tensile prestressing
� A load in the centre of the web produces compressive forces in the threads below it
� Without the tensile prestress, the lower part of the web would go slack, being more prone to collapse
3
Prestressing examples: bicycle wheel
� Wire spokes are strong in tension but weak in compression (due to buckling)
� Spokes must be kept in tension
� When the wheel is assembled, spokes are tightened up uniformly by the turnbuckles at the rim
� Under a downward load on the wheel, the spokes in the lower part of the wheel tend to be subjected to compression
� Tensile prestress in the spokes must be higher than the compression force to keep all the spokes in tension
Prestressing examples: bicycle wheel
� Other types of loading on the wheel: due to braking and due to taking a sharp corner
� Forces due to braking:
– could not be resisted if the spokes were arranged radiating from
the centre of the hub
– spokes are set at an angle to the radii, each pair forming a
triangulated system which is able to generate tensile and
compression forces which oppose the braking force
– tensile prestress ensures that all spokes are in tension
and active
4
Prestressing examples: bicycle wheel
� Forces due to cornering:
– force is imposed on the wheel at right
angles to its plane
– the spokes are inclined with respect to the
plane of the wheel, forming a triangulated
system, which resists the forces due to
cornering
– tensile prestress ensures that all spokes
are in tension and active
Other prestressing examples
� Pneumatic tire of cycle wheel
� Inflated membranes for storage spaces and sport halls
– air pressure inside is maintained above the atmospheric pressure
by blowers
– fabric of the membrane permanently in tension
5
Other prestressing examples
� A set of books: no tensile resistance between the volumes
� The books can be moved if a pressure is applied at the middepth:
– the row of books act as a simply
supported beam
– the pressure overcomes the tensile
stress in the lower part due to own
weight of the books, enabling them to act
as a unit
� The books can be moved with lower pressure if it is applied somewhat lower than the middepth: an upward moment is introduced, which counteracts the downward moment due to own weight of the books
Reinforced concrete beams
� Concrete: weak in tension
� When loading is applied on a simply supported beam, the concrete cracks at the tension side:
– Concrete active in compression
– Steel reinforcement active in
tension
– Only a small part of the concrete
cross-section resists the applied
loading
6
Prestressed concrete beams
� Concrete is kept in compression by cables or rods
� The whole concrete cross-section can be considered in design
� Substantial economy in material
� If prestressing is applied in the centroid of the cross-section:
– by choosing correctly the
prestressing force, the entire cross-
section can be kept in compression
– a large stress is present at the
compression side
Prestressed concrete beams
� Position of prestressing force: important
� If prestressing is applied at 1/3 of the beam depth from the bottom face:
– a negative moment due to eccentric
prestressing counteracts the
positive bending moment due to
applied moment
– the pestressing force needed to
keep the entire cross-section in
compression can be reduced
– the stress at the compression side is
reduced ⇒⇒⇒⇒ the required concrete
strength can be reduced
7
Prestressed concrete beams
� Bending moment due to dead weight in a simply supported beam: parabolic shape
� The best arrangement of the prestressing tendons?
⇒⇒⇒⇒ a parabolic shape along the beam, in order to generate bending moment M=F⋅⋅⋅⋅e counteracting the bending moment due to dead load
Prestressed concrete beams
8
Prestressed concrete
� Type of prestress:
– Posttensioning: the prestressing force is applied after concrete
has been cast and has set, through tendons located in holes left
in concrete elements. The prestress is retained due to anchorage
of steel tendons at the end of the element.
– Pretensioning: prestressing wires are stretched over a long
length and the concrete is cast around them in steel forms. The
prestress is retained due to the bond between the concrete and
the steel wires.
� Problems related to prestressing:
– When the concrete sets up, it shrinks, leading to loss of
prestressing (in the case of pretensioning)
– Concrete shortens in time (creep) after it sets up due to
compression acting on it, leading to loss of compression
– High strength steel required for prestressing, in order to reduce
the loss of prestress due to shrinkage and creep
– Higher strength concrete is needed to resist higher compression
and to reduce the contraction due to creep and shrinkage
Plates
� Plates: a flat surface element that acts in bending in order to resist out of plane loading
� The simplest plate: a flat slab spanning between two supports
� It may appear to behave like a wide beam, but it is not as simple as that
9
One-way plates
� When a narrow beam bends, the material in the lower half of the beam extends longitudinally ⇒⇒⇒⇒ it contracts in the transversal direction due to Poisson effect (µµµµ times the longitudinal strain)
� The material in the upper half of the beam contractslongitudinally ⇒⇒⇒⇒it expands in the transversal direction
� An anticlastic curvature of the beam in the transversal direction equal with µµµµ times the longitudinal curvature
One-way plates
� In plates the anticlastic curvature is suppressed due to large dimension in the transversal direction (the deflected shape is almost cylindrical, except near the free edges)
� At any point of the beam there is a transverse bending moment equal to µµµµtimes the spanwise bending moment
� Suppression of the transverse curvature induces an additional spanwise curvature
� In one-way plates reinforcement is needed in both spanwise and transverse direction
10
Two-way plates
� Two-way plates simply supported on all four sides: complicated interaction between the two ways in which a load is supported
� If a slab is more than about 4 times as long as it is wide, the bending moment at the center of the plate is almost the same as in a one-way plate supported on longer edges. Why? ⇒⇒⇒⇒
� Stiffer structural action (bending in the short direction) attracts larger forces
Stiffness in structural action
� A straight bar of length L and rectangular cross-section can support a concentrated force P in two ways:
– as a column acting in compression
– as a cantilever acting in bending
� In the column the stress σσσσ1 is axial and uniform
� In the cantilever the stress σσσσ2 has a linear variation along the bar and across the cross-section ⇒⇒⇒⇒the material is far less efficient
11
Stiffness in structural action
� Column is much stronger than the beam: σσσσ2/σσσσ1 = 6(L/h)for L/h=20 ⇒⇒⇒⇒ σσσσ2/σσσσ1 = 120
� Column is much stiffer than the beam: δδδδ2/δδδδ1 = 4(L/h)2
� If the beam and the column are used in conjunction to support the load P:
– the two members deflect by the
same ammount δδδδ
– P=k∙δδδδ ⇒⇒⇒⇒ P1=k1∙δδδδ1; P2=k2∙δδδδ2. If the deflection
is the same for the two members δδδδ1=δδδδ2 ⇒⇒⇒⇒
P1/k1 = P2/k2; P1/P2=k1/k2 = 1600
– the column carries a load of (1600/1601)P
– the beam carries a load of (1/1601)P
� Of the two alternative modes of action open to this structure, it chooses the column compression, because it is stiffer
Membrane action
� Some structures can support loads only in bending.Example: simply supported beam
� Uniform loading:
– the neutral axis becomes curved
– roller support moves slightly toward the other end of the beam
12
Membrane action
� A beam pinned at both ends
� Uniform loading:
– the neutral axis becomes curved
– horizontal movement of the support is prevented ⇒⇒⇒⇒ longitudinal
tension H develops ⇒⇒⇒⇒ the beam begins to support load as a
slightly curved cable or catenary
Membrane action
� The catenary action is much stiffer than bending
� Beam action: stiffness remains constant
� Catenary action: stiffness increases with the square of the deflection
� As the load increases, the portion of the load carried axially (w1), as catenary, increases rapidly
� It can be shown that w1/w2 = 3.33(δδδδ/h)2
w2 - the portion of the loading carried through bending.When the deflection δδδδ ammounts to twice the depth of the beam, w1/w2 = 13.33, so that the catenary action ammounts to 13.33/14.33 = 0.93 of the total resistance to load
� Membranes: surface elements in which loading is resisted through direct (axial) stresses
13
Shells
� Shells: surface elements resisting loading through bending and membrane action
� Examples:
– dome
– human skull
– turtle's armour
– bird egg
Shells
� Bird's egg: weak under a concentrated loading (breaking against a cup's rim) but strong under distributed loading (squeezing between ends with palms)
– distributed loading resisted through membrane action (stronger)
– concentrated loading resisted through bending action (weaker)
� Domes:
– used since ancient times
– capable of resisting through membrane
action a variety of distributed loading
14
Dome: structural action
� The shape of a cable changes as the shape of the applied loading changes
� The same behaviour if a set of cables are hanged around a circular perimeter
– uniform loading: "bowl" shape
– larger loading toward the supports: the
"bowl" bulges toward supports and the
bottom rises slightly
– a different shape of the cable is needed in
order to resist the applied loading
through axial action only
Dome: structural action
� If a series of circumferential cables are added, capable of resisting both tension and compression
� When the load changes, the circumferential cables prevent the dome from changing its shape:
– circumferential cables near the rim are
put into tension
– those near the bottom are put into
compression
15
Dome: structural action
� A system formed by using enough cables in order to obtain a surface approximates a thin-shelled dome
� Such a structures is capable of carrying a variety of distributed loading through membrane action (stresses which are uniformly distributed over the thickness of the shell)
� A shell is capable of resisting loads either through bending stresses or direct (membrane) stresses
� Membrane action is "preferred" by the dome, as it is much stiffer for this action
� Ideally, for a membrane action to take place in a shell, it must be thin and its shape should be similar to that assumed by a flexible membrane under the same loading
Dome: structural action
� The heaviest load in many domes is their own weight
� In a hemispherical dome of a uniform thickness,
– the stresses σσσσ1 in the direction of meridians are compressive
throughout
– the circumferential stresses σσσσ2 are tensile near the rim: tensile
reinforcement needed to resist them
16
Shells: hyperbolic paraboloid
� Rectangular area to be covered: (a) taking a portion of a sphere and arching it between supports
� Rectangular area to be covered: (b) hyperbolic paraboloid - can be obtained by taking a rectangular grid of straight lines and lifting one of the corners, so that the lines would remain straight
� A flat surface becomes a curved one, known as hyperbolic paraboloid
� Lines drawn diagonally are parabolas, humped in one direction and sagging in the other direction
Shells: hyperbolic paraboloid
� Constructional advantage that elaborate formwork is not needed
� Hyperbolic paraboloid supports loads by tension/compression, as opposed to a plate, acting in bending
� Given the opportunity, a structure will support loads by direct tension and compression rather than bending