LOADS ON BUILDINGS

The load on buildings is due to dead, imposed, wind, dynamic, seismic and accidental loads. In the UK, multistory buildings for office or residential purposes are designed for dead, imposed and wind loads. The design is checked and adjusted to allow for the effects of accidental loads. The types of load are discussed briefly. 

Dead load 

Dead load is due to the weight of roofs, floors, beams, walls, columns, floor finishes, partitions, ceilings, services etc. The load is estimated from assumed section sizes and allowances are made for further dead loads that are additional to the structural concrete. 

Imposed load 

Imposed load depends on the occupancy or use of the building and includes distributed loads, concentrated loads, impact, inertia and snow. Loads for all types of buildings are given in BS6399: Part 1.   

Wind loads 

Wind load on buildings is estimated in accordance with CP3: Chapter V: Part 2.

The following factors are taken into consideration: 

1.  The basic wind speed V depends on the location in the country. 

2.  The design wind speed Vs is VS1S2S3 where S1 is a topography factor normally taken as 1, S2 depends on ground roughness, building size and height above the ground and S3 is a statistical factor, normally taken as 1. 

The ground roughness is in four categories in which category 3 is a location in the suburbs of a city. The building size is in three classes. 

 Class B refers to a building where neither the greatest horizontal dimension nor the greatest vertical dimension exceeds 50 m.

Class C buildings are larger, the height may refer to the total height of the building or the height of the part under consideration. In a multistory building the wind load increases with height and the factor S2 should be increased at every floor or every three or four floors.

3.  The dynamic pressure q=0.613 Vs 2N/m² is the pressure on a surface normal to the wind and is modified by the dimensions of the building and by openings in the building? 

4.  Pressure coefficients are given for individual surfaces. External pressure coefficients C_pe that depend on dimensions and roof angles are estimated for external surfaces. Depending on whether openings occur on the windward or leeward sides, internal pressure or suction exists inside the building. Tables and guidance is given in the code for evaluating external and internal pressure coefficients C_pe and C_pi. 

5.  The wind force F on a surface is 

F=(C_pe−C_pi)qA

 where A is the area of the surface and C_pe and C_pi are added algebraically. The force acts normal to the surface.

6.  Force coefficients C_f are given to find the wind load on the building as a whole.

The wind load is given by 

F=C_fqAe 

C_f is the force coefficient and Ae is the effective frontal area of the building. The use of force coefficients is an alternative to determining wind loads on individual surfaces. This method is used for multistory buildings. See Fig. 3.13(b) where the wind load is applied as point loads at the floor levels. Wind loads should be calculated for lateral and longitudinal directions to obtain loads on frames or shear walls to provide stability in each direction. In asymmetrical buildings it may be necessary to investigate wind from all directions. 

Load combinations 

Separate loads must be applied to the structure in appropriate directions and various types of loading combined with partial safety factors selected to cause the most severe design condition for the member under consideration. In general, the following load

combinations should be investigated.

(a) Dead load Gk+ imposed load Qk 

1.  All spans are loaded with the maximum design load of 1.4Gk+1.6Qk; 

2.  Alternate spans are loaded with the maximum design load of 1.4Gk+ 1.6Qk and all other spans are loaded with the minimum design load of 1.0Gk. 

(b) Dead load Gk+ wind load Wk 

If dead load and wind load act in the same direction or their effects are additive the load combination is 1.4(Gk+Wk). However, if the effects are in opposite directions, e.g. wind uplift, the critical load combination is 1.0Gk−1.4Wk. 

(c) Dead load Gk+ imposed load Qk+ wind load Wk. 

The structure is to be loaded with 1.2(Gk+Qk+Wk).