Systems to give stability under horizontal load

The structural system as a whole must also be capable of maintaining strength, stiffness and stability. These conditions will be satisfied by selection of suitable members and methods of connection, but the building will also require to resist horizontal loads, normally by the general form of the structure, method of connections, or additional bracing elements.

The simplest stable system to resist horizontal load - wind - is an inclined triangular "tent" form; a tripod in three-dimensions. Often this form leads to functionally waste space in the bottom corners. From the historical stone arch or timber "cruck" frame has developed the steel arch and later the inclined rafter portal frame with either 3-pin; 2-pin or "fixed" joint systems.

 
Systems for stability - sloping roof structures

With vertical columns and walls, there are three principal options, vertical cantilevers, rigid jointed portals, and bracing:

Stability can be provided in one of three basic ways:

• Vertical cantilever about the foundation. The column is "rigidly" fixed to the base. Such columns are "pin" jointed to the beam at the top, sharing the applied load, and are designed as "propped cantilevers". Note that the foundation size will be greater than for the systems outlined below to limit soil pressures due to the transfer of bending moments to the foundation
• Rigidly jointed "portal" frames of various types in which the column is rigidly connected to the roof `beam'
• Braced by "pin" jointed slender tensile or compressive elements. With two crossed tensile elements, one will always go into compression and be ineffective when wind loads are acting Note that combined horizontal and vertical loading will increase the tendency to sway horizontally. A frame will also sway sideways under eccentric vertical loading.

Columns acting as vertical cantilevers about foundation

Rigid frame action

Diagonal bracing

Additional structural elements may also be used to resist reactions to horizontal loads, such as bending, swaying and torsion. These include shear walls and cores.

In tall buildings elements such as lifts and fire escape stairs are also required and these can be used to provide lateral stability. Additional elements such as shear walls can also be incorporated into the design.