CIVL 2230 Introduction to Structural Concepts

1
CIVL 2230 Introduction to Structural Concepts
Design
The University of Sydney
Department of Civil Engineering
2
LECTURE 2 PERMANENT AND IMPOSED ACTIONS

• A PERMANENT ACTION (G) also known as a dead load
  is an action acts continuously through the design
  working life and for which variations in
  magnitude with time are small. Permanent actions
  shall be taken to include the self-weight of the
  following
• (a) The structure.
• (b) All other materials in the structure (walls,
  floors, roofs, suspended ceilings and other
  permanent construction).

3

• An IMPOSED ACTION (Q), also known as live load,
  is a variable action resulting from the intended
  use or occupancy of the structure. It may be
  removed or replaced on a structure.
• The magnitude of the imposed action to be applied
  to a structure is specified by the Standards, and
  it is mandatory for the designer to adopt it
  undiminished, except for any concessions that are
  listed.

4

• Probably the most common imposed action is that
  of uniformly distributed actions on floors and
  roofs. These are always given in units of kN/m2
  (kilopascal, kPa) and are listed in Table 3.1 of
  AS/NZ 1170.12002 typical values are 1.5 kPa for
  general areas of dwellings, 3.0 kPa for offices
  in general use and public areas with tables, up
  to 5.0 kPa for corridors, stairs, balconies,
  dance halls etc., and 7.5 kPa for stages in
  public assembly areas.
• For non-trafficable roofs, the action is
  generally 0.25 kPa, but full details are given in
  Table 3.2 of the Standard.
• A concentrated action shall also be considered in
  the position giving the most adverse effect. A
  typical value is 4.5 kN in factories.

5
CALCULATION OF PERMANENT ACTION

• calculated from the volume and density of the
  components of a structure.
• the design process is cyclic, so that a
  preliminary design based on assumed initial
  dimensions will be re-assessed and most likely
  re-calculated using the 'final' derived
  dimensions.

6

• In design practice for buildings, it is not
  possible to calculate permanent actions
  precisely, and some approximations are made.
• For example, if the floors consist of reinforced
  concrete slabs and beams, the self-weight of the
  concrete is calculated on nominal dimensions,
  using density of 2500 kg/m3 for normal weight
  concrete, which also includes the weight of the
  steel reinforcement.
• The slab may be covered with a topping and/or
  tiles or other covering, which add significantly
  to the load.
• Partition walls also add to the permanent
  actions, and an allowance is normally made for
  them by replacing the 'line' load of the walls by
  an equivalent uniformly distributed action
  typically 0.5 kPa over the entire floor area.

7
Example of a RC building
8
(No Transcript)
9
(No Transcript)
10
Permanent action for design of the slab

• Slabs normally sustain only their own
  self-weight, and any directly supported load such
  as topping, tiles, ceiling under, services such
  as pipes, cables etc.
• Of course the slab is supported on beams and/or
  walls, but the slab sees only the actions
  directly on its surface

11

• In this case, the slab is supported by beams or
  by the shear core. It is unusual for the slab
  thickness to change over the extent of a floor.
  A critical slab panel is rectangular in plan view
  and has approximate dimensions 8m x 8m. An
  initial estimate of the required slab thickness
  is 175 mm. Therefore, the initial estimate of
  permanent action on the slab, expressed as
  kilonewtons per square metre of slab
  (kiloPascals, kPa), is

12

• Bare slab0.175?2,500?9.8/1,000 4.3 kPa
• Topping tiles (estimate only) 1.0 kPa
• Ceiling under services (estimate) 0.5 kPa
• Partitions (estimate) 0.5 kPa
• Total permanent action on slab 6.3 kPa
• Normally, the actual self-weight of the
  quantities marked 'estimate' are not known
  accurately at the time of the initial design, and
  therefore an initial estimate must be made to
  allow the design to proceed.

13
Permanent action for design of the beams
14

• In a square panel (e.g. 8m ? 8m), the figure
  becomes 4-sided of total area 32 m2, and the
  total permanent action on that beam transmitted
  by the slab becomes 6.3 ? 32 201.6 kN. This
  load is however not distributed uniformly over
  the span of the beam, but increases linearly from
  zero at the ends (Figure 5) to a maximum of 6.3?8
  50.4 kN/m at midspan.

15
RECTANGULAR INTERIOR PANEL
Wmax 6.3x8.1
l
l
Wmax 6.3x8.1
SQUARE PANEL
16
Useful formula for replacing the 6 or 4-sided
load distribution by a uniformly distributed load

• L is the span of the beam, l is the distance from
  the end supports at which the distributed loading
  reaches its maximum, wmax is the maximum value of
  the load distribution ( 50.4 kN/m in this
  example), and w' is the equivalent load uniformly
  distributed over the entire span.

17

• The formula ensures that the maximum bending
  moment on the beam at midspan, i.e. wL2/8, is
  equal to the maximum bending moment at midspan in
  the actual non-uniform load case. It is a good
  exercise for the student to verify this equality.
  It must be noted that the equivalent load
  distribution w' does not give the correct value
  of maximum shear, which must be evaluated from
  first principles.
• Also, fixed-end moments of wL2/12 can be subject
  to a maximum error of 5 (triangular load),
  reducing to zero error as the load distribution
  approaches uniformity.

18

• To this permanent action on the beam must be
  added the self-weight of the stem of the beam
  (the upper part of the beam has already been
  accounted for by the slab load calculation). The
  total equivalent permanent action on the beam
  therefore becomes

19

• The maximum moment at midspan is therefore wL2/8
  41.6?82/8 333 kNm.
• The student should verify that the correct
  maximum shear force at the support is 132.8 kN,
  including the effect of the beam stem.

20
Permanent actions on columns

• Estimation of permanent actions on columns is
  especially critical, as errors accumulate when
  the load carried by columns in the lower levels
  of buildings are estimated by summation of each
  level above. There are several ways of
  calculating permanent actions on columns,
  depending on the accuracy required.
• A full computer frame analysis is normally
  warranted for the final design, but more
  approximate methods are acceptable for a
  preliminary design.

21
(No Transcript)
22

• If the building has a regular array of columns,
  then permanent actions on interior columns may be
  calculated by considering a plan tributary area
  of slab around each column (Figure 6).
• If for example a building has 50 stories, and
  the columns are spaced 8 m apart, then an
  interior column at ground level would carry a
  permanent action due to the slabs (only) of
  50?8?8?6.3 20,160 kN.
• Add self-weight of the beam stems and of the
  columns themselves.

23

• Exterior columns carry lower loads than interior
  columns. AS3600 allows a simplified method that
  states the negative moment at the first interior
  support may be taken as wL2/10, implying that the
  load on the exterior columns is 20 less than the
  load calculated from a tributary area.
• This also implies an increase in the load on the
  first interior columns. Global equilibrium of the
  entire structure must of course always be
  satisfied.

24
CALCULATION OF IMPOSED ACTIONS

• Imposed actions (live loads) are that part of the
  load that may be imposed because of the use or
  purpose of the building or structure. It
  includes the loads specified by the loading
  standards (e.g. AS/NZS 1170.1-2002) for various
  uses and occupancies of the building.
• These specified imposed actions cover the
  occupants, furniture, movable equipment,
  fixtures, books, etc,.
• Imposed actions include impact and inertia loads,
  but exclude wind, snow and earthquake loads,
  which are covered in other standards.

25
FLOOR LOADS IN BUILDINGS

• Floors in houses 1.5 kPa
• Stairs and landings in houses 2.0 kPa
• Office buildings (office use) 3.0 kPa
• Shop floors 4.0 kPa
• Office buildings (file rooms) 5.0 kPa
• Equipment floors 5.0 kPa
• Public assembly (no fixed seating)) 5.0 kPa
• Grandstands 5.0 kPa
• Stages in public assembly areas 7.5 kPa
• Non-trafficable roof (flat or pitched,
• providing shelter from the elements only) 0.25kPa

26

• A second type of action is also to be considered
  by the designer it takes the form of a
  concentrated action on the surface, listed
  alongside the distributed load, in Table 3.1 of
  AS1170.1.
• Normally, the concentrated action is taken as a
  point action, or distributed over the small area
  of 0.01 m2.
• For example, the concentrated action to be used
  on ordinary office floors is 4.5 kN, located in
  the most damaging location for the design of a
  specific structural element (slab, beam, column
  etc.).
• It is rare for the concentrated action to have a
  more severe effect than the distributed action.

27
LIVE LOAD REDUCTION

• LIVE LOAD REDUCTION FACTOR
• ? 0.3 3/?A, not greater than 0.5
• A is the sum of all areas supported by a
  structural member, in m2.
• Does not apply to roof and balcony actions, to
  places of assembly, and to areas on which the
  imposed action exceeds 5 kPa.

28
HEAVY MAINTENANCE HANGAR AT BRISBANE
AIRPORTMAINTENANCE OF 3 LARGE AIRCRAFT FOR
QANTAS AIRWAYSSTRUCTURE BEFORE LIFTCLEAR SPACE
BETWEEN FRONT COLUMN 160 m
29
FRONT, SECONDARY AND DISTRIBUTION TRUSSES
30
ROOF IN FINAL POSITION