An Architect's Approach to Structures

7/28/2019 An Architect's Approach to Structures

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:AN ARCHITECTS APPROACH TO

STRUCTURES-by Prof. A.J.Sanyal:

An Architect is basically space planner.

He has to co-ordinate works of various

consultant e.g. structural, HVAC, Mechanical

& Electrical, Acoustics & Illumination etc.

Hence he should have at least working

knowledge of above fields.

Structural options & possibilities & their

behavior under the influence of loads being

most important.


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Structural design of building may be defined as to

channelized all types of loads acting on building during its

lifetime by shortest possible route to the foundation below so

that bearing capacity of soil is not exceeded & structure is safeto use during its life time.

The loads which will be acting on a building during its lifetime

can be classified as:

a) Dead load such as self weight, permanent fixtures etc.

b) Live load such as occupancy loads, movable loads etc.c) Dynamic loads such as wind loads, Earthquake loads etc.

d) Hydraulic loads such as load due to water table in rainy

seasons etc.

e) Impact loads etc.


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Loads can be also classified as:

UDL (Uniformly distributed loads e.g. self weight of

structural members.

Point loads (Loads which acts on very small area e.g.

secondary beam transferring loads on main beam.

Triangular loads e.g. water pressure in water tank,

soil pressure on retaining wall etc.Trapezoidal loads e.g. loads transferred on long beam

of two way slab.

Combination of above loads.


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Due to action of loads three types of stresses are set up in structural

members :

1. Tensile e.g. shell structures, cables etc.

2. Compression e.g. column, struts etc.

3. Bending & tension or bending & compression e.g. beams, truss etc.

4. Stress is force per unit area, while strain is change in length divided by

original length.

5. Standard bending formula is: M/I=F/y or M=fZ , where M= External B.M,

I=Moment of inertia, f= allowable stress in structural members , y=

position of extreme fiber from N.A; Z= Modulus of section.6. In building we are encountered with members as mentioned in type 3

stress category.


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Bending moment & shear force diagrams:

Bending moment about any point can be defined as force x

perpendicular distance from the point in question.

Shear force can be defined as unbalanced force to the right or

left of the section.

B.M. & S.F. diagram is graphical representation of B.M. & S.F.

In general B.M. is maximum where S.F. is zero & S.F. is

maximum where B.M. is zero (Point of inflection).

In cantilever structure B.M. & S.F. is maximum at support.

In a continuous beam/slab advantage can be taken of

cantilever projection, which reduces span moments of adjacent

span.

Propped cantilever is better option as compared to cantilever

projections.


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In simple beam moment is zero at ends & maximum at centre:

wl2/8, while in fixed beam at ends FEM=wl2/12 & centre it is

wl2/24.

In reality beam/slab is neither completely free or simply

supported or completely fixed, but it can be assumed eithernominally fixed or partially fixed.

Hence for practical design for initial sizing of members can be

based on wl2/10 for UDL.

Deflection of beam/slab depends on load , span & end condition

of beam. Fixed beam is stiffer as compared to simple beam.

If the load is doubled BM in beam/slab is also doubled , but

deflection becomes 16 times as deflection= k.wl4 where k= 5/384

for simple beam & 1/384 for fixed beam.


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Important components of any building are :

1.Roof/Floor-truss with roofing materials,

slabs, plank. Decking etc.2.Beams plinth, floor beams etc.

3.Columns/wall-corner column, side column,

central column etc.

4.Foundation/footing-strip footing, isolated

footing, combined footing etc.


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Slabs/beams can be classified based on spanning e.g. one way slab, two

way slab etc.

It can be also classified as per shape e.g. rectangular, square, triangular,

circular etc.

It ca be also classified as per support condition e.g. simple support, fixed

support, continuous support, cantilever etc.

Columns can be classified as/ its effective length e.g. short or long column.

It can be also classified as per position e.g. corner column, side column or

central column etc.

Foundation can be classified as shallow or deep foundation/ pile

foundation. Again it can be strip foundation, isolated footing, eccentric

footing, combined footing, raft foundation etc.

Roof can be sloping roof in A.C./G.I. sheets or tiles, r.c.c. slabs, folded slabs,

shell structures, pneumatic/fabric structures etc.


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Most of the structure falls under the category of bending & tension or bending

& compression , general guide lines can be given as under:

1. Prefer short span as compared to long span from deflection point.

2. Doubling of load will double the BM but doubling of span will increase the

deflection 16 times for UDL.

3. Prefer propped cantilever in place of isolated cantilever.4. Isolated cantilever structure should be limited in use & adequate counter

weight shall be provided for the same generally 1.5 times the load on

cantilever.

5. In a continuous structure overhang reduces the BM in the adjoining span.

6. Beams/slabs are not completely free to rotate or completely fixed but it is

either nominally/partially fixed.7. Prefer ULD as compared to point load as BM & SF is less in common types

beams/slabs.

8. If point loads cannot be avoided then try to ensure that is more than 10,

which is practically very near to UDL.


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9. In corner column there will be biaxial bending, in side column it be uniaxial

bending & in central column it may be axial loading for symetrical beam &

loading.

10. Short column if preferable to long & slender column.

11. Braced column if preferable to un-braced column.

12. Concentric isolated footing is preferable to isolated eccentric/strip footing.RULE OF THUMBS:

a) For isolated footing Length x breadth = 1.1nwA/B.C.

b) Plinth beam (lintel at 2.1mt) d= 50mm/mt run & Ast= 10mm , no. being

equal to span in mts.

c) Tee or beam= 60mm/mt Ast= d/3 for tee beam & d/4 for ell beam

d) Column with 1% reinf. = 15.57Sq. Cms/tonne load.e) Slabs= 40 mm/mt. run


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STRUCTURES-by Prof. A.J.Sanyal:Sr. No. Member Span/Overall

depth ratio.

1. Plinth beam 15 to 18

2. Tie beam 18 to 20

3. Floor beam 12 to 15

4. Grid beam 20 to 30


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STRUCTURES-by Prof. A.J.Sanyal:Sr.No. Slab type Span to overall

depth ratio.

1. One-way simply

supported

30

2. One-way

continuous

35

3. Two-way simply

supported

38 for L/B=1.5

35 for L/B>1.5

4. Two-way

continuous

40 for L/B=1.5

38for L/B>1.5


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Miscellaneous structures :

Truss depth/span= 10

Virendell truss = 1/8 to 1/10 span, compression & tension

at top & bottom cords is given by: C=T= qL2/8h, where

q=UDL & L=Span in mts & h=depth of V. truss.

Arch. Optimum rise= 1/6 to , d=Span/40 to span/70

Folded plate: depth of fold= Span/15 to Span/10.

Flat plate: d=Effe.Span/22 to E.Span/18.

Two way ribbed slab= Eff. Short span/30 to 1/25.Flat plate ribbed slab= Eff. Span/20 to 1/17

Domes: Rise/span=1/8 to for nomal dome & 1/5 to for

shallow domes.


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Rule of thumbs for M20concrete mix (AJS):

1. Beam in general: d= 15.41[w/k]1/2 , Ast=1.94[w/k]1/2 & of bar= 0.884[w/k]1/4

2. Plinth beam: d= 50mm/mt. run & Ast=10mm , number =span in mts.

3. Tee or Ell beam: MR=Qbd2, where Q= 40xy(2-y), x=Bf/br, y=ds/d, Bf=Effe. Flange

width, br=width of rib, ds=thickness of slab, d= effe. Depth of beam.

4. dofTee/Ell beam=12.77[w/{40kxyb(2-y)}]1/2, Ast=6.527[w/{40kxyb(2-y)}]1/2

d/2

5. Column: Load carrying capacity Pu=[72+24.3p], where Pu= Ultimate load, p=% of

reinf. 15.57 sq. cms./tonne load with 1% reinf.

6. Slab: d= 12.2474l[w/k]1/2=3.873l[w]1/2, Ast

7. Approximate UDL on columns: Corner-2T/M2, Side-1.5/M2, Central-1.1T/M2


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Sorry I could not go in details due to constrain of time. If you want detail seminar

it should be minimum 2-3 days seminar, where basic theory, derivation of

common formula, numerical & assignments with probable solutions etc. can be

covered comfortably.

Any way if like my presentation, tell your friends, else tell your enemies so you can

Pleasure of revenge. But tell somebody.

THANK YOU

An Architect's Approach to Structures

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