parts of a structure

7/28/2019 Parts of a Structure

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Design of multi storeyed residential building.

Parts of a structure:

1) Dead loads: The dead load includes loads that are relatively constant over time, including theweight of the structure itself, and immovable fixtures such as walls, plasterboard orcarpet. Dead

loads are also known as Permanent loadsWeight of structure includes slabs, beams, columns, footing, and brickwork.

2) Imposed loads: The imposed load includes loads that may be moved from one place toanother. They include partition walls, miscellaneous, etc.

3) Live loads: The live loads are temporary, of short duration, ormoving. These dynamic loadsmay involve considerations such as impact, momentum, vibration, etc.

Roof and Floor live loads are produced

1. during maintenance by workers, equipment and materials, and

2. during the life of the structure by movable objects such as planters and by people.

Bridge live loads are produced by vehicles traveling over the deck of the bridge

(We can reduce live loads in a multistoreyed building because the loading in each floor in notsame or each floor is not loaded fully or equally every time.)

[Water load: It is considered as dead load and not as live load in worst case scenario. ]

[Lift load: The lift load is considered as dead load because load of the lift is transferred to the

top gutter and from that to the beams.]

4) Environmental loads: These are the loads that act as a result of natural phenomenon such

as weather, topography, etc. They include:a) Wind loads:These are the forces on a structure arisingfrom the impact of wind on it.

Wind loads are considered for the structures (buildings) exceeding four floors.

b) Seismic loads: Seismic loading is one of the basic concepts of earthquake engineeringwhich means application of an earthquake-generated agitation to a structure. It happens at
http://en.wikipedia.org/wiki/Plasterboardhttp://en.wikipedia.org/wiki/Carpethttp://en.wikipedia.org/wiki/Moving_loadhttp://en.wikipedia.org/wiki/Dynamics_(mechanics)http://en.wikipedia.org/wiki/Impact_(mechanics)http://en.wikipedia.org/wiki/Momentumhttp://en.wikipedia.org/wiki/Vibrationhttp://en.wikipedia.org/wiki/Vibrationhttp://en.wikipedia.org/wiki/Momentumhttp://en.wikipedia.org/wiki/Impact_(mechanics)http://en.wikipedia.org/wiki/Dynamics_(mechanics)http://en.wikipedia.org/wiki/Moving_loadhttp://en.wikipedia.org/wiki/Carpethttp://en.wikipedia.org/wiki/Plasterboard


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contact surfaces of a structure either with the ground or with adjacent structures or with

gravity waves from tsunami.

c) Snow loads: The load due to the weight of snow on a roof; included in the design

calculations.

Structural Designing: It is an art and science of designing with economy and elegance, asafe, a serviceable, and a durable structure.

Stages in Structural Design:1) Structural planning.

2) Estimation of loads.

3) Analysis of structure.

4) Member design.

5) Detaining, drawing and preparation of schedules.

[Note: Length of steel rod = 40 (12.192m). Detaining deals with anchorage length (overlapping of rods).]

Density/Unit

wt.

Kilograms per

cubic meter

Kg/cum

Kilo Newton

per cubic meter

KN/cum

Tons per cubic

meter

T/cum


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Steel 7850 78.50 7.850

Concrete

(Plain)

2400 24 2.4

Concrete

(Reinforced)

2500 25 2.5

Water 9810 or 1000 9.8 or 10 1

Brick 1900 19 1.9

1) Structural planning:Structure is planned based on the following things:

a) Type of building (Residential/Industrial/Commercial).b) Type of structure (Steel/RCC).

Steel (Traditional steel/ Pre- engineered building).

c) Function.

d) Soil strata [Index/Engineering properties (Safe bearing capacity (SBC) which is the load bearing

resistance per unit area.]

Soil type [Clay/Sand/Silt or C/S/Mho ()]

e) Folded section or R.C.C shell [Domes, RCC shells, Intze tanks.]

f) Beam slab grid system [This system is used for longer spans.]

g) Pre-stressed hanging roof.

Principle elements of R.C building frames:i. Slab.

ii. Beams to support slabs and walls.


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iii. Columns to support beams.

iv. Footings to support column loads.

I. Slab: A flat, reinforced-concrete structural member, relatively sizable in length and width, but

shallow in depth; used for floors, roofs, and bridge decks. Slabs are classified into

a) One way slab: A concrete slab in which the reinforcing steel runs perpendicular to the supporting

beams, that is, one way.

Aspect ratio: B/L

Or 2 (where ly is the longer span and lx is the shorter span )

[Shorter direction controls the slab as the load has to be distributed to beams quickly, thus main steel is

laid on the shorter side while as minimum reinforcement or the rods on longitudinal side are used to resist

thermal expansion]

b) Two way slab: A concrete slab supported by beams along all four edges and reinforced with steel

bars arranged perpendicularly.

Aspect ratio: B/L< 2

< 2 (where ly is the longer span and lx is the shorter span )

[Loads are distributed equally in both directions]

Determining the items once the planning is decided:1) Column position.

2) Beam location.

3) Spanning of slabs.4) Layout and planning of stairs.

5) Types of footing.

Column positioning: Positioning of the columns depends on the various factors such as External boundary conditions.

Increase number of column positions.

Recommended spans

Beam Type Cantilever Simply supported Fixed/Continuous

Rectangular

3m 6m 8m

Flanged (T,L beams)

5m 10m 12m

Avoid columns inside big halls.

[Grid slab, pre-stressing]

Column location very near (especially at the corners of the building).then we have to choose

single column (transfer load to columns).

Orientation of columns:


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Column should be orientated in such a way that beams and walls are flushed.

Column projections outside or inside should be avoided.

Columns are governed by slenderness ratio ().

=L effective/r min

Radius of gyration(rmin

) = I/A

If < 3, pedestals are used

If =3-12, short columns are adopted.

If >12, long columns are adopted.

Minimum thickness of column should be 9, otherwise bending occurs.

The column should be orientated that the depth of column is perpendicular to the major axis of

bending so as to get larger moment resisting capacity(i.e., the depth of the column should be in

the plane of bending).

D Major axis

b Minor axis

M.Ixx =bD

3

/12M.Iyy = Db

3/12

(M.I xx >> M.Iyy)

(Moment generated on yy axis should be transferred to major axis (xx).

For square, we use square columns usually because load distribution is equal.

Positioning of beams:

Provide beams always under the walls(solid walls) or below the heavy concentrated load(pointload).

Heavier loads should be always applied to the shorter path (because load distribution or

dispersion is quick to the ground).

Span of the beam depends on the slab panel (i.e., beams should be located according to slab

panels ).

Support conditions Cantilever Simply supported Fixed or continuous

Slab type One way Two way One way Two way One way Two way


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Maximum span inmeters

1.5 2.0 3.5 4.5 4.5 6.0

Spanning of slabs:

One way slab: 2

Two way slab: < 2

Spanning of slab controls beam spanning.

Staircase: A flight or series of flights of steps and a supporting structure connecting separate levels.Also called stairway.( A set of stairs and its surrounding walls or structure.) Components of staircase: It consists of following components;

i. Tread:The part of the stairway that is stepped on. It is constructed to the

same specifications (thickness) as any other flooring. The tread "depth" is measured from the

outer edge of the step to the vertical "riser" between steps. The "width" is measured from one

side to the other.

ii. Riser:The vertical portion between each tread on the stair. This may be missing for an "open"

stair effect.

iii. Nosing: An edge part of the tread that protrudes over the riser beneath. If it is present, this

means that, measured horizontally, the total "run" length of the stairs is not simply the sum of

the tread lengths, as the treads actually overlap each other slightly.

iv. Stringer, Stringer board or sometimes just String: The structural member that supports the

treads and risers. Types of footing: Footings are of several types and they are as;

i. Isolated footing: used to support single columns. This is one of the most economical types of

footings and is used when columns are spaced at relatively long distances .
http://en.wikipedia.org/wiki/Specificationhttp://en.wikipedia.org/wiki/Stair_riserhttp://en.wikipedia.org/wiki/Stair_riserhttp://en.wikipedia.org/wiki/Stair_riserhttp://en.wikipedia.org/wiki/Specification


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ii. Raft footing (or mat): consists of one footing usually placed under the entire building area. They

are used, when soil bearing capacity is low, column loads are heavy single footings cannot be

used, piles are not used and differential settlement must be reduced. Raft foundations are used to

spread the load from a structure over a large area, normally the entire area of the structure. They

are used when column loads or other structural loads are close together and individual pad

foundations would interact. A raft foundation normally consists of a concrete slab which extends

over the entire loaded area. It may be stiffened by ribs or beams incorporated into the foundation.

Raft foundations have the advantage of reducing differential settlements as the concrete slab

resists differential movements between loading positions. They are often needed on soft or loose

soils with low bearing capacity as they can spread the loads over a larger area .

iii. Pile foundation: These are those deep foundations that are relatively long, slender members

that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock

strata having a high bearing capacity. They are used when for economic, constructional or soil

condition considerations it is desirable to transmit loads to strata beyond the practical reach of

shallow foundations. In addition to supporting structures, piles are also used to anchor structures

against uplift forces and to assist structures in resisting lateral and overturning forces. These are

thick slabs used to tie a group of piles together to support and transmit column loads to the piles .


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Under-reamed piles: These are those deep foundations that are used in soils having low bearing

capacity such as Black cotton soils. They have mechanically formed enlarged bases that have

been as much as 6 m in diameter. The form is that of an inverted cone and can only be formed in

stable soils. The larger base diameter allows greater bearing capacity than a straight-shaft pile

iv. Combined footing: usually support two columns, or three columns not in a row. Combined

footings are used when two columns are so close that single footings cannot be used or when

one column is located at or near a property.

v. Continuous footing: support a row of three or more columns. They have limited width and

continue under all columns.

Loads and materials: Materials Rigid and Non Rigid

Material is rigid when there is no deformation due to load.

It is not rigid when there is deformation.

Non Rigid materials Elastic & Plastic.

If deformation entirely disappears on removal of load it is elastic.

Deformation does not disappear after removal of load, have a permanent set is called plastic. .Characteristic loads and design load: The anticipated load levels due to self-weight, contents

and users, snow and wind are called characteristic loads.


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For the Ultimate Limit State, the design loads are taken as the characteristic loads multiplied by

the appropriate safety factor; for the Serviceability Limit State un-factored characteristic loads are

used.

Factors of safety: Safety factors provide security against collapse and allow for unavoidable

differences between assumed and actual structural behavior.

Or Design load = Characteristic load x Factor of safety.

.

Important aspects of structures:

Concrete mixes(N/mm2):

M5, M10Pavement & bed concrete (Lean mix).

M15, M20 Nominal mix (1:1:3) [M15 -20mm coarse aggregate]

M25 (1:1:2) [10mm-20mm coarse aggregate]

M30 & AboveDesign mix[5mm-101mm coarse aggregate]

Load combinations:a) Dead load(DL) & Live load(LL): For DL+LL, we have to increase load combination as

1.5(DL+LL). Where 1.5 is the factor of safety.

b) Dead load(DL), Live load(LL) & Wind load(WL):We have load combination as1.2(DL+LL+WL)

c) Dead load(DL), Live load(LL) & Earthquake load(EL):We have load combination as1.2(DL+LL+EL)

Critical load combinations:a) Maximum support moment:

A

Live load varies anytime, so we dont have to provide LL all over the span. So we have to provide LL in

such a way maximum support moment occurs (e.g., in mid spans for getting at A). We can reduce the DL

then by 10% (i.e., Dead load=0.9DL).

b) Maximum span moment: For maximum span moment for span AB, Live load has to be imposed on

that span only.

A B

1.5(DL+LL)LL

DL=0.9DL

1.5(DL+LL)DL=0.9DL


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c) Maximum support shear: It is obtained when we have maximum sagging moment.

A B

If AB = 1.5(LL+DL), i.e., maximum sagging moment, then we have support shear at A & B.

Codes:

IS 456-2000 (code of practice for plain and reinforced concrete)

SP-16 (Standard practice),(Design Aids)

IS 875 (code of practice for design loads)

Part-1 Dead loads

Part-2 Imposed loads (Live loads)

Part-3 Wind loads.

Part-4 Snow loads.

Part-5 Special loads and combination

IS 1895 (earthquake resistant design of structure)

SP-22 Design Aids for earthquake resistant design of structure(Explains methodsor procedures)

IS 1080 (Code of practice for design and construction of shallow foundations) IS 1888-1972 Methods of load tests on soils(Plate load test, etc.)

IS 1904 Pile foundations

IS 4326 (Code of practice for earthquake resistant design)

IS 13920 (Code of practice for ductility)Code of practice for ductility of R.C structure and sub-earthquake.

SP-23 Mix Design.

SP-25 Cracks in building and their repairs.

SP-34 Detaining in R.C.C structures.

SP-38 Design of steel trusses.

1.5(DL+LL)

Submitted by:

1. Syed Gous Andrabi.

2. Khaja Imad ud din.

3. Feroz Khan.

4. Abdul Salman.

parts of a structure

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