Examples of vertically applied forces on building elements are the dead loads of the building structure and some live loads. These loads tend to move the structure in a downward direction, i.e. to sink into the ground.
The extent of any such movement depends upon;
– the ability of the foundation to distribute the loads exerted by the building over a sufficient area ensuring the stability for a particular load-bearing capacity.
Different soil types have different values of load-bearing capacities. The functional requirement of foundations of buildings is to ensure that load applied by the building on to the ground should not exceed the bearing capacity of the soil. The bearing capacity of the ground is normally expressed in kN/m2. In most instances, this bearing capacity is very much less than the pressures likely to be exerted by the building structure if placed directly onto the ground.
How to reduce the pressure applied to the ground?
The pressure applied by the building on to the ground is reduced by introducing foundation. It increases the interface area between the building and the ground. Therefore, it results in reducing the pressure applied to the ground.
The need to withstand vertical loadings is not only limited to the lower elements of the building structure. However, such loadings are greater in magnitude at the lower sections due to the effects of accumulated loadings from the structure.
Vertical components under vertical loads
All structural components, such as beams, columns, slabs, and walls must be strong enough to carry loadings imposed upon them. Any building element should not fail under loading and should not show excessive deformation. Columns and walls carry loads of floors and roofs from above. Therefore, they must resist the buckling or crush by the forces exerted.
Whether a column or a wall fails under crushing or by buckling due to the effect of vertical loading depends on the ‘slenderness ratio’ of the element. It means long and slender building elements units will tend to buckle easily.
Short and broad units resist such buckling tendencies. Although the risk of buckling is higher in long and thin elements, the risk can be reduced by incorporating bracing to prevent sideways movement. This is termed as lateral restraint.
In a heavily loaded situation, even short and broad sections also may be subject to failure. Most probably, the failure mode in these situations is due to crushing. However, these are very rare occurrences.
Horizontal components under vertical loads
Examples of horizontal components of a building are floors and beams. These components also must be capable of performing effectively while withstanding vertically applied loadings. This is ensured by the use of materials of sufficient strength, designed appropriately, with sufficient support to maintain stability.
Heavy loading conditions on horizontal elements may give rise to deflection as shown in the figure.
In extreme situations, puncturing of the component, as shown in the figure, may results from shearing at a specific point.
When deflected in a simply supported situation, upper sections of beams and floors are under compression while lower sections are under tension. This may limit the full use of the strength of some materials, such as concrete, which resist well only compressive forces. Under these conditions, composite units such as reinforced concrete may be used to give a more economical design.
Vertical forces in an upward direction
Vertical forces on a building are not always in a downward direction. Sometimes they may act in upward directions. These upward forces can be resisted, by using the weight of the building. Upward loadings generate from the ground, due to changes and resulting forces of shrinkable clay or soils prone to expansion due to frost. These upward forces exerted by the ground is termed as heave.
Horizontal forces acting on buildings generate from many sources. In the case of the basement wall, it may be due to sub-soil pressure. Or on to a roof or wall, it may be due to wind pressure or due to physical loading on the building.
These horizontal loads may affect the stability of a building in one of two ways:
– by overturning or rotation of the building or its components
– by horizontal movement or sliding of the structure.
Anyhow, the effects of these acts are highly undesirable. Therefore, the occurrence of these situations must be avoided by carefully designing the building. The type of foundations and the level of lateral restraint or buttressing incorporated into the design are essential to the prevention of such modes of failure. Additionally, in framed structures, the use of bracing to prevent progressive deformation or collapse is essential. This is described as the resistance of the ‘domino effect’.
In some areas of a building, the external forces act at an inclination. Normally this happens when pitched roofs support on walls. A common example is wind loading. These oblique forces produce vertical and horizontal components of the applied loadings at the point of support. These can be resisted by the introduction of buttressing and/or lateral restraints.