Permanent actions and imposed actions

Asst. Prof. Hang Thu Vu

hang@civil.uwa.edu.au

Required reading

AS1170.0 AS1170.1 AS4100 Supp – Section 3 If you log in university computers, you can

access e-copies of these AS/NZS standards at

http://www.saiglobal.com/online/autologin.asp

Lecture outline

Permanent actions are usually known by the name ‘dead loads’

Imposed actions are usually known by the name ‘live loads’

When the structure is in use, it is subjected to different load types. Dead load (DL) G and live load (LL) Q are the most significant load types.

The factored load combination of dead load and live load in most of the time acts as the critical load case for the limit states that the member/structure are designed for.

Lecture outline In this lecture we first consider each load type separately

Dead loads: What to include, Calculate load magnitudes to use in design, Proposed load factors

Live loads What to include, Proposed load magnitudes to use in design, Proposed load factors

We then consider possible load combinations for dead load and live load for use in designing structures for Strength limit state Serviceability limit state Stability limit state

Lecture outline Exercises for practising to design members for

serviceability limit state and stability limit state are provided in the tutorial.

Depending on the applied load combinations and the support conditions, the structural members may behave differently. For example: the member can be under tension/compression/buckling when subjected to axial load; under bending when subjected to lateral load ..

You will learn how to design members for strength limit state later in this course. Different member behaviour requires different design procedure. For example: design for tension member, design for flexural member ..

Dead loads

What to include Dead loads are permanent loads that do not change

in the structure’s life. They are self weight of The structure Material incorporated into the structure: walls, floors, roofs,

ceilings and permanent constructions Permanent equipments: fixtures, fittings, electrical wiring,

plumbing tubes, ducted air system .. Partitions, fixed and movable Stored materials

When there is significant design change, dead loads should be reassessed and followed by a fresh structural analysis.

Calculate of self weight

Volume x Density Volume is computed from the element

dimensions Density: Refer to AS/NZS 1170.1: 2002,

Appendix A “Unit weights of materials” Example: Self weight of brick masonry wall

10 m long, 2.5 m height, standard thickness 0.1m = (10*2.5*0.1)*1.9=4.75 kN

Movable partitions and removable items Movable partitions: Self weight of the

movable partitions is the calculated actual weight but not less than a UDL of 0.5 kPa over the area being considered.

Removable items: Consideration shall be given to the effect of removing permanent items that are not essential parts of the structure, such as water tanks, stored materials, service equipments, partitions and similar.

Load factors for dead loads

As the variability of dead loads in structure’s life is minimum, there is no factors to account for variability of dead loads

Dead loads are only factored to generate factored load combinations when design members for various limit states.

Example: load combinations for strength limit states 1.35 G 1.2 G + 1.5 Q

Live loads

What to include? Live loads generate from the use of the structure They are time-dependent and space-dependent.

Their magnitudes and distribution vary significantly with occupancy and function.

The loads shall be not less than the greater of the following:(a) The prescribed values resulting from the use of the structure.(b) The nominal values given in AS/NZS 1170.1.

AS/NZS 1170.1 provides a uniform, statistically based approach to determine live loads.

What to include? For each intended use, two load types are noted:

uniformly distributed load (UDL) and concentrated load.

The reason for considering concentrated loads is that there are some localised loads (e.g. heavy items of furniture, equipment or vehicles) that may not be adequately represented by a UDL.

The distributed and concentrated imposed loads shall be considered separately and design carried out for the most adverse effect.

Example: Lecture theatre, live load for floor design: 3 kPa UDL and 2.7 kN concentrated load (Source: AS/NZS 1170.1:2002, Table 3.1, Type C2, Specific use: lecture theatre)

Concentrated load

A concentrated load shall be applied as follows:(a) At its known position or where its position is not known, in the position giving the most adverse effect.(b) Distributed over the actual area of application or if the actual area is not known or otherwise stipulated in Tables 3.1 or 3.2, over an area of not greater than 0.01 m2 for floors and roofs.

Partial load

Where the live load Q varies from one span (or room) to another, to account for the most adverse load cases, analysis is carried out for Factored live load on all spans Factored live load on two adjacent spans Factored live load on alternate spans

The second case results in high BMs over the support between the two loaded spans.

The third case results in high BMs at mid-span in the loaded beams

Partial load

Proposed load magnitudes to use in design for buildingFloors Reference load values for Q is given by values in

AS/NZS 1170.1: 2002, Table 3.1 and Appendix B multiplied with the area reduction factor ψa (Clause 3.4.2). 0.5≤ψa≤1

Example: offices for general use, class room, lecture theatre: UDL = 3 kPa, concentrated load = 2.7 kN, where ψa is

, A (m2) is area supported by the considered member

Aa

33.0

Proposed load magnitudes to use in design for building

Roof and supporting elements Roofs are considered non-accessible except for normal

maintenance and minor repairs If roofs are frequently accessible and used for floor type

activities are treated as floors Reference live loads for roofs are given by values in

AS/NZS 1170.1: 2002, Table 3.2 (3.5.2) Extra concentrated load 1.4 kN may be applied

for roof trusses, ceilings, skylights and similar structures to account for supporting one person load

Example: For structural elements in roofs of houses, the UDL is 0.25 kPa and the concentrated load is 1.1 kN.

Proposed load magnitudes to use in design for buildingBarriers, parapets, balustrades and railings Reference live loads for barriers (top edge

and infill) are given by values in AS/NZS 1170.1: 2002, Table 3.3

Example: Handrail in building carpark: Top edge live loads: 1.5 kN/m (horizontal), 0.75

kN/m (vertical), concentrated load 0.6 kN (positioned for worst effect)

Infill live loads: 1.5 kPa (horizontal), 1.5 kPa (random direction over 2000 mm2 or two adjacent balustrades)

Proposed load magnitudes to use in design AS/NZS 1170.1 Clause 3.7 nominates values for

dynamic factors for vertical loads and load factors for horizontal loads due to activities of installed cranes, hoists, lifts and machinery

Guidance is also provided for estimating live loads for specific types of structure: Carpark: Clause 3.8 Grandstands: Clause 3.9

Section 4 gives methods for calculating the action of liquids, ground water, rainwater ponding and retained earth. The methods are straightforward from basic solid mechanics and soil mechanics. No load factors are proposed.

Load factors for live loads

There are factors to account for variability of live loads in terms of time-dependences Ψl : long term factor

Ψs : short term factor

and space-dependences Ψl : area reduction factor

Example: load combinations for serviceability limit states G + Ψs Q where Ψs =0.7

Load combinations for dead loads and live loads

Note: This lecture mentions only the load combinations for dead loads and live loads. The appearance of other load types such as wind load in the load combinations are not considered.

Strength limit state

If only dead load is considered: 1.35 G

If both dead load and live load are considered, 1.2 G + 1.5 Q

It is generally the most significant load combination among load combinations for strength limit state design

Serviceability limit state

G + ψl Q

G + ψs Q

Where ψl and ψs are factors to account for long term service used and short term service used subsequently

Example: for structural member within an office building G + 0.7 Q (ψs =0.7)

Serviceability limit state

AS4100 Supp1:1999 Clause 3.5.3 and Appendix B nominates deflection limits structure under static loads

Vertical deflections

Horizontal deflections

Stability limit state

Stability described in AS4100 - 3.3 refers to the general stability of the structure such as overturning, uplifting or sliding of the structure.

In general, all structures should satisfy the following requirement :

T* < TR

where T* refers to the destabilising actions, such as overturning moments and uplifting forces, and TR refers to stabilising actions,

Stability limit state

The load combination for dead load and live load is

The live load Q should not be included in calculation if it produces a restoring action.

Stabilizing and unstabilizing load