Support and Connection Types
Structural systems transfer their loading through a series of elementsto the ground. This is accomplished by designing the joining of the elementsat their intersections. Each connection is designed so that it can transfer,or support, a specific type of load or loading condition. In order to beable to analyze a structure, it is first necessary to be clear about theforces that can be resisted, and transfered, at each level of support throughoutthe structure. The actual behaviour of a support or connection can be quitecomplicated. So much so, that if all of the 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 ofthe elements which make up each structural system.
Structural steel systems have either welded or bolted connections. Precastreinforced concrete systems can be mechanically connected in many ways,while cast-in-place systems normally have monolithic connections. Timbersystems are connected by nails, bolts, glue or by engineered connectors.No matter the material, the connection must be designed to have a specificrigidity. Rigid, stiff or fixed connections lie at one extreme limit ofthis spectrum and hinged or pinned connections bound the other. The stiffconnection maintins the relative angle between the connected members whilethe hinged connection allows a relative rotation. There are also connectionsin steel and reinforced concrete structural systems in which a partial rigidityis a desired design feature.
SUPPORT TYPES
The three common types of connections which join a built structure to itsfoundation are; roller, pinned and fixed. A fourthtype, not often found in building structures, is known as a simplesupport. This is often idealized as a frictionless surface). All of thesesupports can be located anywhere along a structural element. They are foundat the ends, at midpoints, or at any other intermediate points. The typeof support connection determines the type of load that the support can resist.The support type also has a great effect on the load bearing capacity ofeach element, and therefore the system.
The diagram illustrates the various ways in which each type of supportis represented. A single unified graphical method to represent each of thesesupport types does not exist. Chances are that one of these representationswill be similar to local common practice. However, no matter what the representation,the forces that the type can resist is indeed standardized.
REACTIONS
It is usually necessary to idealize the behaviour of a support in orderto facilitate an analysis. An approach is taken that is similar to the massless,frictionless pulley in a physics homework problem. Even though these pulleysdo not exist, they are useful to enable learning about certain issues. Thus,friction and mass are often ignored in the consideration of the behaviorof a connection or support. It is important to realize that all of the graphicalrepresentations of supports are idealizations of an actual physical connection.Effort should be made to search out and compare the reality with the grpahicaland/or numerical model. It is often very easy to forget that the assumed idealization can be strikingly differentthan reality!
The diagram to the right indicates the forces and/or moments which are"available" or active at each type of support. It is expectedthat these representative forces and moments, if properly calculated, willbring about equilibrium in each structural element.
ROLLER SUPPORTSRoller supports are free to rotate and translate along the surface uponwhich the roller rests. The surface can be horizontal, vertical, or slopedat any angle. The resulting reaction force is always a single force thatis perpendicular to, and away from, the surface. Roller supports are commonlylocated at one end of long bridges. This allows the bridge structure toexpand and contract with temperature changes. The expansion forces couldfracture 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 designed to allow a limited amount of lateralmovement.
A roller support cannot provide resistance to a lateral forces. Imaginea structure (perhaps a person) on roller skates. It would remain in placeas long as the structure must only support itself and perhaps a perfectlyvertical load. As soon as a lateral load of any kind pushes on the structureit will roll away in reponse to the force. The lateral load could be a shove,a gust of wind or an earthquake. Since most structures are subjected tolateral loads it follows that a building must have other types of supportin addition to roller supports.
PINNED SUPPORTS
A pinned support can resist both vertical and horizontal forces but nota moment. They will allow the structural member to rotate, but not to translatein any direction. Many connections are assumed to be pinned connectionseven though they might resist a small amount of moment in reality. It isalso true that a pinned connection 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 one direction andprovides resistance to lateral movement. The design of a pinned connectionis a good example of the idealization of the reality. A single pinned connectionis usually not sufficient to make a structure stable. Another support mustbe provided at some point to prevent rotation of the structure. The representationof a pinned support includes both horizontal and vertical forces.PINNED CONNECTIONS
In contrast to roller supports, a designer can often utilize pinned connectionsin a structural system. These are the typical connection found in almostall trusses. They can be articulated or hidden from view; they can be veryexpressive 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 to resolvea number of tensile forces. Upon closer examination one can notice thatthe connection is made of a number of parts. Each cable is connected tothe node by an end "bracket" which is connected to a large pin.This is quite literally a "pinned connection." Due to the natureof the geometry of the bracket and pin, a certain amount of rotational movementwould be permitted around the axis of each pin.
One of the connections from the pyramid of I.M. Pei's Loiuvre addition followsbelow. Notice how it too utilized pinned connections.
Pinned connections are confronted daily. Every time a hinged door ispushed open a pinned connection has allowed rotation around a distinct axis;and prevented translation in two. The door hinge prevents vertical and horizontaltranslation. As a matter of fact, if a sufficient moment is not generatedto create rotation the door will not move at all.
Have you ever calculated how much moment is required to open a specificdoor? Why is one door easier to open than the another?
FIXED SUPPORTS
Fixed supports can resist vertical and horizontal forces as well as a moment.Since they restrain both rotation and translation, they are also known asrigid supports. This means that a structure only needs one fixed supportin order to be stable. All three equations of equilibrium can be satisfied.A flagpole set into a concrete base is a good example of this kind of support.The representation of fixed supports always includes two forces (horizontaland vertical) and a moment.
FIXED CONNECTIONS
Fixed connections are very common. Steel structures of many sizes are composedof elements which are welded together. A cast-in-place concrete structureis automatically monolithic and it becomes a series of rigid connectionswith the proper placement of the reinforcing steel. Fixed connections demandgreater attention during construction 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 is welded and the other is comprised totwo screws. Both are considered to be fixed connections due to the factthat both of them can resist vertical and lateral loads as well as developa resistance to moment. Thus, it it found that not all fixed connectionsmust be welded or monolithic in nature. Let the hinges at locations A andB be examined in closer detail.
SIMPLE SUPPORTS
Simple supports are idealized by some to be frictionless surface supports.This is correct in as much as the resulting reaction is always a singleforce that is perpendicular to, and away from, the surface. However, arealso similar to roller supports in this. They are dissimilar in that a simplesupport cannot resist lateral loads of any magnitude. The built realityoften depends upon gravity and friction to develop a minimal amount of frictionalresistance to moderate lateral loading. For example, if a plank is laidacross gap to provide a bridge, it is assumed that the plank will remainin its place. It will do so until a foot kicks it or moves it. At that momentthe plank will move because the simple connection cannot develop any resistanceto the lateral loal. A simple support can be found as a type of supportfor long bridges or roof span. Simple supports are often found in zonesof frequent seismic activity.
The following movies illustrate the implications of the type of supportcondition on the deflection behavior and on the location of maximum bendingstresses of a beam supported at its ends.
Simple Beams that are hinged on the left and roller supported on the right.
Simple Beams that are hinged on the left and fixed on theright.
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 DonaldPetingCopyright © 1996, 1997, 1998by Chris H. Luebkeman
