Structural Support and Connections

Structural support is a part of a structure providing the necessary stiffness and strength in order to resist the internal forces (vertical forces of gravity and lateral forces due to wind and earthquakes) and guide them safely to the ground. 

External loads (actions of other bodies) that act on structures cause internal forces (forces and couples by the rest of the structure) in which support structures. Supports can be either at the end or at any intermediate point along a structural member or a constituent part of a structure and they are referred to as connections, joints or restraints. A structure depends less on the weight and stiffness of a material and more on its geometry for global stability. Whatever the condition is, a specific rigidity is necessary for connection designs. The support connection type has effects on the load bearing capacity of each element, which makes up a structural system. Each support condition influences the behavior of the elements and therefore, the system. 

It's easy to draw bending moment and shear force diagram of ideal fix support (rigid) or pinned support (flexible). Rigid connection restrains rotation and transfer moment in structural members whereas flexible connection allows structural members to rotate therefore cannot transfer any moment. Ability of any beam to transfer moment depends upon relative stiffness between connecting members.  

It is difficult to design and detail any connection to give justice to type of support considered in structural analysis. It is often very easy to forget that the assumed idealization can be strikingly different than reality!

Types of supports are classified into two categories: 

• External Support
• Internal Support

1. External Support

The supports which are usually provided externally without disturbing the structural members are external supports. Different types of external supports are as follows.                 

a) Simply Supported

Simple support is just a support on which structural member rests. That could be idealized by some to be frictionless surface supports. They cannot resist lateral movement and moment like roller supports. They only resist vertical movement of support with the help of gravity. The horizontal or lateral movement allowed is up to a limited extent and after that the structure loses its support. It’s just like a brick resting longitudinally on two bricks. This type of support is not commonly used in structural purposes.  The built reality often depends upon gravity and friction to develop a minimal amount of frictional resistance to moderate lateral loading.   

                                                                  Representation of simply supported member and reaction exerted on that member  

                                                                                                   Practical example of simple support

b)  Pinned Support or Hinged Support

A pinned support can resist both vertical and horizontal forces but not a moment. They will allow the structural member to rotate, but not to translate in any direction. Many connections are assumed to be pinned connections even though they might resist a small amount of moment in reality. Best example for hinged support is door leaf which only rotates about its vertical axis without any horizontal or vertical movement.                  

                             

                              Representation of pinned supported member and reaction exerted on that member   

                 

                                      Practical example of pinned Support or hinged Support

c) Fixed Support/Moment connection

Fixed supports can resist vertical and horizontal forces as well as a moment. Since they restrain both rotation and translation, they are also known as rigid supports. This means that a structure only needs one fixed support in order to be stable. All three equations of equilibrium can be satisfied.                      

                         

                                         Representation of fixed supported member and reaction exerted on that member

                        

                                        Reinforcement detailing in reinforced concrete member at support location

                        

                                                    Practical example of fixed support  

d) Rolled Support

Roller supports are free to rotate and translate along the surface upon which the roller rests. The surface can be horizontal, vertical, or sloped at any angle. The resulting reaction force is always a single force that is perpendicular to, and away from, the surface.

Roller supports are commonly located at one end of long bridges. This allows the bridge structure to expand and contract with temperature changes. The expansion 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 designed to allow a limited amount of lateral movement.

                                                                                    Representation of roller supported member and reaction exerted on that member

                      

                                          Practical example of roller Support in steel bridges

e) Wind post

A wind post is a structural item used in the design and construction of masonry walls to increase lateral wall stability and protect them against damage from horizontal forces imposed by wind pressure, crowd or handrail loads.

                          

                                               Practical example of wind post in masonry structure

2. Internal Support

Internal supports are provided internally in the structural member which means an internal support divides the full member into parts. So, the external reactions can be found for each part which will be quite easier for the analysis.

a) Internal Hinge

Similar to hinge support internal hinge also resists translation in both directions and allows only rotation. In structures, for axial members internal hinges are provided and for beam members middle hinges are provided. These can be widely seen in arch type bridges at the center of arch.

                          

                                        Representation of internal hinge in the arc member 

                             

                                          Practical example of internal hinge in steel arc bridge