Support Types

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Support and Connection Types

Structural systems transfer theirloading through a series of elements tothe ground. This is accomplished bydesigning the joining of the elementsat their intersections. Each connectionis designed so that it can transfer, orsupport, a specific type of load orloading condition. In order to be able

to analyze a structure, it is firstnecessary to be clear about the forcesthat can be resisted, and transfered, ateach level of support throughout thestructure. The actual behaviour of asupport or connection can be quitecomplicated. So much so, that if all ofthe various conditions were considered, the design of each support would be a terribly lengthy process.And yet, the conditions at each of the supports greatly influence the behaviour of the elements whichmake up each structural system.

Structural steel systems have either welded or bolted connections. Precast reinforced concrete systemscan be mechanically connected in many ways, while cast-in-place systems normally have monolithicconnections. Timber systems are connected by nails, bolts, glue or by engineered connectors. No matterthe material, the connection must be designed to have a specific rigidity. Rigid, stiff or fixed connectionslie at one extreme limit of this spectrum and hinged or pinned connections bound the other. The stiffconnection maintins the relative angle between the connected members while the hinged connectionallows a relative rotation. There are also connections in steel and reinforced concrete structural systemsin which a partial rigidity is a desired design feature.

SUPPORT TYPESThe three common types of connectionswhich join a built structure to its foundationare; roller, pinned and fixed. A fourth type,not often found in building structures, isknown as a simple support. This is oftenidealized as a frictionless surface). All ofthese supports can be located anywherealong a structural element. They are found atthe ends, at midpoints, or at any otherintermediate points. The type of support

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connection determines the type of load thatthe support can resist. The support type also has a great effect on the load bearing capacity of eachelement, and therefore the system.

The diagram illustrates the various ways in which each type of support is represented. A single unifiedgraphical method to represent each of these support types does not exist. Chances are that one of theserepresentations will be similar to local common practice. However, no matter what the representation,

the forces that the type can resist is indeed standardized.

REACTIONSIt is usually necessary to idealize thebehaviour of a support in order tofacilitate an analysis. An approach is takenthat is similar to the massless, frictionlesspulley in a physics homework problem.

Even though these pulleys do not exist,they are useful to enable learning aboutcertain issues. Thus, friction and mass areoften ignored in the consideration of thebehavior of a connection or support. It isimportant to realize that all of thegraphical representations of supports are idealizations of an actual physical connection. Effort should bemade to search out and compare the reality with the grpahical and/or numerical model. It is often veryeasy to forget that the assumed idealization can be strikingly different than reality!

The diagram to the right indicates the forces and/or moments which are "available" or active at each typeof support. It is expected that these representative forces and moments, if properly calculated, will bring

about equilibrium in each structural element.

ROLLER SUPPORTSRoller supports are free to rotate and translate along the surface upon which the roller rests. The surfacecan be horizontal, vertical, or sloped at any angle. The resulting reaction force is always a single forcethat is perpendicular to, and away from, the surface. Roller supports are commonly located at one end oflong bridges. This allows the bridge structure to expand and contract with temperature changes. Theexpansion forces could fracture the supports at the banks if the bridge structure was "locked" in place.Roller supports can also take the form of rubber bearings, rockers, or a set of gears which are designedto allow a limited amount of lateral movement.

A roller support cannot provide resistance to a lateral forces. Imagine a structure (perhaps a person) onroller skates. It would remain in place as long as the structure must only support itself and perhaps aperfectly vertical load. As soon as a lateral load of any kind pushes on the structure it will roll away inreponse to the force. The lateral load could be a shove, a gust of wind or an earthquake. Since moststructures are subjected to lateral loads it follows that a building must have other types of support inaddition to roller supports.




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PINNED SUPPORTSA pinned support can resist both vertical and horizontal forces butnot a moment. They will allow the structural member to rotate, butnot to translate in any direction. Many connections are assumed tobe pinned connections even though they might resist a smallamount of moment in reality. It is also true that a pinnedconnection could allow rotation in only one direction; providing

resistance to rotation in any other direction. The knee can beidealized as a connection which allows rotation in only onedirection and provides resistance to lateral movement. The designof a pinned connection is a good example of the idealization of thereality. A single pinned connection is usually not sufficient tomake a structure stable. Another support must be provided at somepoint to prevent rotation of the structure. The representation of a pinned support includes both horizontaland vertical forces.

PINNED CONNECTIONSIn contrast to roller supports, a designer can often utilize pinnedconnections in a structural system. These are the typical connectionfound in almost all trusses. They can be articulated or hidden fromview; they can be very expressive or subtle.

There is an illustration of one of the elements at the Olympic Stadiumin Munich below. It is a cast steel connector that acts as a node toresolve a number of tensile forces. Upon closer examination one cannotice that the connection is made of a number of parts. Each cable isconnected to the node by an end "bracket" which is connected to a

large pin. This is quite literally a "pinned connection." Due to thenature of the geometry of the bracket and pin, a certain amount ofrotational movement would be permitted around the axis of each pin.

One of the connections from the pyramid of I.M. Pei's Loiuvreaddition follows below. Notice how it too utilized pinned connections.




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Pinned connections are confronted daily. Every time a hinged door is pushed open a pinned connectionhas allowed rotation around a distinct axis; and prevented translation in two. The door hinge preventsvertical and horizontal translation. As a matter of fact, if a sufficient moment is not generated to createrotation the door will not move at all.

Have you ever calculated how much moment is required to open a specific door? Why is one door easierto open than the another?




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FIXED SUPPORTSFixed supports can resist vertical and horizontal

forces as well as a moment. Since they restrainboth rotation and translation, they are also knownas rigid supports. This means that a structure onlyneeds one fixed support in order to be stable. Allthree equations of equilibrium can be satisfied. Aflagpole set into a concrete base is a good exampleof this kind of support. The representation of fixedsupports always includes two forces (horizontaland vertical) and a moment.

FIXED CONNECTIONS

Fixed connections are very common. Steelstructures of many sizes are composed of elementswhich are welded together. A cast-in-placeconcrete structure is automatically monolithic andit becomes a series of rigid connections with theproper placement of the reinforcing steel. Fixedconnections demand greater attention duringconstruction and are often the source of buildingfailures.

Let this small chair illustrate the way in which two types of "fixed" connections can be generated. One iswelded and the other is comprised to two screws. Both are considered to be fixed connections due to the

fact that both of them can resist vertical and lateral loads as well as develop a resistance to moment.Thus, it it found that not all fixed connections must be welded or monolithic in nature. Let the hinges atlocations A and B be examined in closer detail.




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SIMPLE SUPPORTS




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Simple supports are idealized by some to be frictionless surface supports. This is correct in as much asthe resulting reaction is always a single force that is perpendicular to, and away from, the surface.However, are also similar to roller supports in this. They are dissimilar in that a simple support cannotresist lateral loads of any magnitude. The built reality often depends upon gravity and friction to developa minimal amount of frictional resistance to moderate lateral loading. For example, if a plank is laidacross gap to provide a bridge, it is assumed that the plank will remain in its place. It will do so until a

foot kicks it or moves it. At that moment the plank will move because the simple connection cannotdevelop any resistance to the lateral loal. A simple support can be found as a type of support for longbridges or roof span. Simple supports are often found in zones of frequent seismic activity.

IMPLICATIONSThe following movies illustrate the implications of the type of support condition on the deflectionbehavior and on the location of maximum bending stresses of a beam supported at its ends.

Simple Beams that are hinged on the left and roller supported on the right.




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Simple Beams that are hinged on the left and fixed on the right.

Simple beams that are fixed at both ends.

Questions for Thought

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Homework Problems

Additional Reading

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Copyright 1995 by Chris H. Luebkeman and Donald Peting

Copyright 1996, 1997, 1998by Chris H. Luebkeman


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