Technical Guidance Note 3 Level 1 - Derivation of imposed loads.pdf

› Note 3 Level 1

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TheStructuralEngineer February 2012

Technical Technical Guidance Note

Derivation of imposed loads Introduction

This Technical Guidance Note concerns the derivation of imposed loads. This is a core guidance note and as such, subsequent notes will make reference to this one. It is therefore important to understand the contents of this note before attempting to digest any of the others. Please be aware that this note does not cover lateral loads onto barriers, balustrades and axle loads from vehicles. These will be covered in a forthcoming note. All of the guidance notes in this series have an icon based navigation system, designed to aid the reader.

Design principles Imposed load is defined as the load that is applied to the structure that is not permanent and can be variable. In Eurocode phraseology, it is described as a quasi-permanent variable action.

Definition of Imposed Load (quasi-permanent variable action) The items listed below can be considered to be imposed loads:

• moveable partitions • furniture and occupancy • livestock • plant room installations, such as air handling

units and boilers • cradles for maintenance access (but not the supporting structural elements they are fixed to such as rails and secondary support beams) • sports and gym equipment such as goal posts, basketball hoops and punch bags • audio and video equipment installations including speaker clusters, televisions and lighting rigs Elements that can often be mistaken for imposed loads when they are in fact dead load elements include the following:

• finishes, such as screeds and suspended ceilings. These are sometimes referred to as ‘super imposed dead loads’

• building services installations, such as ducts, cable containment and lighting • mass of soil • fixed partitions, e.g. those that cannot be demounted and placed elsewhere and non load bearing walls Please see Technical Guidance Note 2, Level 1 for more details on dead loads/permanent actions. Typical Imposed Loads Imposed loads are sub-divided into categories A-H in Eurocode 1-1 and their values can be found in Tables NA.2 to NA.7 (inclusive) in the UK National Annex. They are based on the structure’s use. Table 1 is a list of common imposed loads that are applied to building structures: Imposed load type

Area load (kN/m2)

Concentrated load (kN)**

Residential

1.5

2

Office (above ground)

2.5

2

Car park ††

2.5

10

Retail

4

3.6

General storage areas †

2

1.8

Restaurants

2

3

Plant rooms

7.5

4.5*

Theatre/stages

5

3.6

Staircases

4

3

Roof access

0.6

0.9

Partitions

1

n/a

Table 1 List of typical imposed loads

Icon Legend

• Design principles

• Applied practice

• Worked example

• Further reading

• Web resources

* This load is taken from the now withdrawn BS6399 Loading for Buildings Part 1: 1996 Code of practice for dead and imposed loads, Table 1. The Eurocodes currently advise to determine actual plant loads from building services engineers rather than apply an assumed blanket load. In the event this information is not available, the area load suggested in Table 1 can be used. ** Concentrated loads are point loads that can be supported anywhere on the structure to the exclusion of the corresponding area imposed load. They should only be considered when checking local effects that would be induced by such loads. † General storage refers to category E11 i.e. for static equipment that does not include book and paper storage or other specific types of items listed in Table NA.5 †† For car parks that are limited to vehicles with a gross weight that is less than 30 kN. The Table contains only a sample of the most common types of imposed loads. The reader is directed to Eurocode 1: Actions on Structures – Part 1-1: General actions – densities, self weight, imposed loads for buildings and the relevant National Annexe for a more comprehensive list.

Partial Factors (γQ) for Imposed Loads (Quasi-permanent variable actions) Partial factors are used within Limit State design methodology. They are applied when designing elements based on their capacity

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to resist stress, be it shear, bending, torsion or a combination of the three. They are also used when checking for stability within the structure. The Eurocode’s approach to these factors considers the nature of the load before any factors are applied to it.

discussed previously. The appropriate partial factors to Ultimate Limit State (ULS) design of super-structure elements have been applied to the loads placed upon it.

The following partial factors apply to imposed loads that are commonly found within building structures:

Consider Fig. 1 below, where ‘Gk’ refers to the dead load and ‘Q ’ the imposed load:

k

Where n is the number of storeys of structure above the level that is being considered, excluding the roof. This factor is applied to imposed loads only, prior to any partial factors being applied to them. Note: Reduction factors can only be applied to imposed load categories A-D, as defined in Eurocode 1-1 and the UK National Annex.

1.5 (γq) is the base partial factor for imposed loads for superstructure element design when it is ‘unfavoured’ 1.3 (γq) is the base partial factor for imposed loads for substructure element design when it is ‘unfavoured’

Stability Analysis Partial Factors

αn = 0.5 for n > 10

Imposed Load Reduction for Element Design

Figure 2 Ultimate Limit State partial factors

This analysis concerns the design of the element and its supports. In this instance the worst case bending moments and shear forces are based on the maximum partial factors (γQ) that Eurocode 0 will allow, which are: ‘Unfavourable’ Imposed Load – 1.5 Q k Again the ‘Gk ’ in Figure 2 refers to dead load that is covered in Technical Guidance Note 2, Level 1.

Eurocode 1-1 allows for the reduction of applied imposed load to be applied when designing elements within a structure. The UK National Annex states that the area being supported by the element can be reduced using factor A, that is defined in Equation NA.1:

αa = 1.0 - A/1000 > 0.75 Where ‘A ’ is the area of structure that is being supported. It is not the tributary area of imposed load the element is subjected to. Figure 4 below explains this further:

Partial Factors for Substructure Element Design Figure 1 Equilibrium partial factors

Finally consider Figure 3 below and note the same load condition with reduced partial safety factors:

It can be seen that the load in the main span is acting favourably to resist the load being applied to the cantilever section. This cantilever load is causing instability in the structure, as support ‘A’ is unable to resist an upward vertical force.

Figure 4 Imposed load area reduction factor; supported area vs. tributary area

Figure 3 Sub-structure partial factors

Figure 1 indicates what factors are to be applied to loads that are dependent upon the nature of the analysis being carried out. In this instance the stability of support point A is being checked for uplift. This falls under the category of checking for equilibrium in a structure, and hence the following partial factors (γQ) apply: ‘Favourable’ Imposed Load – 0 Qk ‘Unfavourable’ Imposed Load – 1.5 Qk The ‘Gk’ in Figure 1 refers to dead load that is covered in Technical Guidance Note 2, Level 1.

These factors apply when considering foundation design, specifically for what loads are being applied to the footings. Hence the partial factor (γQ) is: ‘Unfavourable’ Imposed Load – 1.3 Qk

Imposed Load Reduction in Multi-storey Structures

Partial Factors (Q) for Super Structure Element Design

Imposed loads in multi-storey structures can be reduced, based on the likelihood of all of the floors in the structure being fully occupied. The factor that can be applied to the imposed loads as they are considered from a lower level in the structure, are as follows:

In the design of structural elements, the worst case loading pattern is used to determine the maximum shear and bending forces the structure is to be subjected to. Figure 2 is of the same structure for the stability analysis

αn = 1.1 – n/10 for 1< n < 5 αn = 0.6 for 5 < n < 10

Applied practice The applicable codes of practice for the derivation of imposed loads are as follows: BS EN 1990: Eurocode Basis of Structural Design BS EN 1990: UK National Annex to Eurocode: Basis of Structural Design BS EN 1991-1: Eurocode 1: Actions on Structures – Part 1-1: General actions – densities, self weight, imposed loads for buildings BS EN 1991-1: UK National Annex to Eurocode 1: Actions on Structures – Part 1-1– densities, self weight, imposed loads for buildings

› Note 3 Level 1

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TheStructuralEngineer February 2012

Worked example Determine the characteristic and ultimate imposed loads that are applied to the columns at Level 1 of the structure in Figure 5 below:

Technical Technical Guidance Note

There are a total of 6 storeys to this building above Level 1; the 400mm square column layout is a grid measuring 8m x 12m. Levels 2,3,5,6 have raised floors and suspended ceilings with services installed within them. Level 7 is the roof and has a suspended ceiling and services fixed to its soffit and a 50mm screed and 15mm thick tiles on its external surface. Level 4 is a plant room with no finishes to the floor or lowered ceiling at Level 5. There are building services hung from the soffit of Level 5.

The first loads to determine are the imposed loads from each level:

Then the reduction factor is applied to the column at Level 1:

Finally the characteristic and ultimate imposed loads are calculated:

Figure 5 Worked example structure

Glossary and further reading

certain types of imposed loads that allows for the likelihood of full occupancy of a structure.

Action – The consequences of an applied load.

Ultimate load – A load that has had partial factors applied to it.

Characteristic load – A load that has had no partial factors applied to it.

Unfavourable load – A load that does increase the bending/shear stresses within an element or generates instability within a structure.

Favourable load – A load that does not increase the bending/shear stresses within an element or create instability. Reduced imposed load – Factor applied to

Further Reading Manual for the design of building structures to Eurocode 1 and Basis of Structural Design – Institution of Structural Engineers – April 2010

Web resources For more information on this subject, please visit: http://www.istructe.org/resourcescentre/library