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Some general view
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vikram.jeet
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PostPosted: Wed Mar 31, 2021 2:07 pm    Post subject: Some general view Reply with quote

For a Bending member , cg of effective section is the  Neutral Axis.

For members with Bending + axial force , cg of effective section  do not coincide with NA.
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vikram.jeet
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PostPosted: Thu Apr 01, 2021 3:45 am    Post subject: Reply with quote

A Bending beam member , in slant ,between two supports
(e.g.bending mbr supprting lean to roof)

For gravity loading - supports do not subject to any horizontal reaction from beam.( Even though member is subject to bending and axial force).

For wind loading - which is normal to slant member transferred from sheeting ,the supports will be subject to horizontal force.
(Even though member is subject to bending only).
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vikram.jeet
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PostPosted: Thu Apr 01, 2021 6:31 am    Post subject: Reply with quote

+ve Moment reinfreinforcemat bottom face in slab panels:
Design of slab panels where DL is much lower than Live Loading
( e.g. industrial loading bays ,storage areas, library floors etc)

Industrial- DL is to the extent of 500 to 6oo kg/ m2.  But Live Loading may be 1000 to 2000 kg / m2

Assuming panels of approx equal spans
Supports moments (top) - As routine using (DL+LL) with coeffs given in IS456 for existing edge condition  as per drg.

Bottom  face moments- These can be based on panel loaded  and adjoining  panels unloaded . Approx it can be worked out  considering DL with existing  edge condition  of continuity/discontinuity. For LL which is much higher, coeffs can be read with edge condition as discontinuity on all edges. The moments thus worked out for bottom face will be slightly higher and a judacious % (80 to 90%) can be added to DL moments at Bottom face.
Views expressed above are personal &shall be ignored otherwise.


Last edited by vikram.jeet on Sun Sep 12, 2021 3:50 am; edited 1 time in total
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vikram.jeet
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PostPosted: Thu Apr 01, 2021 4:56 pm    Post subject: Reply with quote

Shed structures & PEB the steel structures with sheeting roof

Loding conditions : loading case DL + LL + wind load This case is not a practical  since during high winds /storms ,there is no possibility of presence of LL on roof.. PEB designers generally include this case which end up in uneconomical design.
Earlier Book on steel structures  by prof Ramachndra clearly states that during heavy winds No live load is expected on roof.
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vikram.jeet
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PostPosted: Sat Apr 03, 2021 5:10 am    Post subject: Reply with quote

Changes in Permissible shear stress with time as result of research

IS 456(1964), Permissible shear stress  under working loads was 5 kg/cm2 for M150 conc mix  AND 7 kg/cm2 for M200 mix.  Though Tc bd relief concept was not there . ALSO shear was to be checked st face of support in beams .(However in footings it was checked st d from face.).
IS 456(1978) Major changes in shear concept were incorporated and Perm stress was based on tension reinforcements%. Tc bd relief offered by conc in shear force was permitted.Shear needs to be checked at d from face .

Major effect of this provisions was on open foundations.
Earlier Perm stress = 7kg/cm2 (1964 version)
Now Perm stress = 2.2 kg/cm2 for 0.20% tension reinforcement  working stress design. A drastic change.
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PostPosted: Sat Apr 03, 2021 12:37 pm    Post subject: Reply with quote

Bridge Substructure design :

In manual design era , Bridge Substructure were designed considering
Maximum LL case
Minimum LL case
No LL case

Design calculations are presented for all three cases in conjunction with two cases of water levels
Water at HFL
Water at LWL

Minimum LL case is a case where Full LL is present on Bridge but near to other support ,thus giving rise Minimum value of
P but full braking & Temp forces ( based on bearing type)

Designs if not presented with all above cases ,wether.governing or not were returned back by MOST / State PWD's.

Now a days software, probably must be taking care of these combinations.
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vikram.jeet
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PostPosted: Sat Apr 03, 2021 2:18 pm    Post subject: Reply with quote

Frame analysis - Building frames
Effect of presence of rcc slab

If a frame is analysed in two separate cases , one without any slab , ( seldom a case )in buildings) AND other with slab as T  or L beam
There is going to be difference in deflection of beam in these two cases. And so is in joint rotations at ends. I think , keeping this aspect in view , the earlier manual designers use I value as follows
Beam without any slab I = Igross
Beam with slab on both sides( T beam) I = 2.0 Igross
Beam with slab on one side (L beam) I = 1.5  Igross
However CPWD manual on design of Milti storeyed buildings vol 1
provided  multiplying factors based on extent of slab in section of beam.
In case modelling of structure is done with slab elements , results will be more realistic , but generally modelling is done for frames w/o incorporating the Slabs.
This may lead to high moments ( especially in end columns) than
realistic ones with Slabs.
Latest IS 456 is asking to use cracked MOI.  But still Slabs effects needs to be incorporated.
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PostPosted: Sun Apr 04, 2021 5:27 am    Post subject: Reply with quote

Minimum +ve moments in rcc beams
In view of many factors like (I) lack of provision of Ld reinforcements length into end columns  - mostly seen at sites where supervision is not strict (ii) Alternate span loaded /Adjacent span unloaded cases are not done during
structural  analysis /design

For two or more span beams
End span -.  wl2/12
Interior span -. wl2/14

For portal frames (single span)-. wl2/10
Many Earlier consultants have adopted this practice. However the idea was good and act as sort of quality control at design office stage.

Vikramjeet


Last edited by vikram.jeet on Sun Sep 12, 2021 4:07 am; edited 1 time in total
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PostPosted: Mon Apr 05, 2021 7:42 am    Post subject: Reply with quote

Vertical extension of building
Column reinforcements lapping

A building where rcc columns & their foundations are designed for future vertical extension( including rcc beams for seismic) , column bars of extension storey are lapped with existing col bars.

(I) If Existing column bars are of varying lengths,  ( say 50% bars of length Ld and rest 50% of 2*Ld - - -Only lapping will suffice

(ii) If Existing column bars  are of length Ld - - Lapping only will not suffice and Welding of at least  50% bars be needed to satisfy the codal provision of  'Not more than 50% bars be lapped at one section'
(iii) If existing column bars are less than Ld, All such bars will require Welding with new bars

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PostPosted: Tue Apr 06, 2021 4:24 am    Post subject: Reply with quote

Detailing of reinforcement - Must be execution friendly
The designers work in  design office in a comfortable  A/c environment  with tea & coffee available all times. But the labour / supervisors work in a very tiring & extreme  conditions ,be it weather or accessibility .Hence it is the duty of designer to provide drawings of structures with details easily executable.
Special note shall be given in bold and need to be boxed.
Footings - sizes can be different but it is preferable to use single diameter bar ( spacings as per design) ,unless some footing is of abnormal size.
RCC columns - Bar dia's can be two at the most in a single column.
RCC beams - Providing continuous frame beams in tabular form is Very disturbing for field team & design office shall avoid it. Apart from field staff exerting , chances of mistakes are many OR designer will get frequent phone calls from site regarding clarifications. Thus drawings for beams must give Longitudinal view with  sections at midspan and support.

Slabs - Reinforcements must be shown on plan with top bars shown by firm lines and bottom bars shown by dotted lines .Bars can be written  as a,b,c etc and a table giving Bottom bars AND Top bars giving bar mkd. detail. Top  bars must be mentioned with their length required. Now a days cranking is not done.
Tie bars for tieing top reinf where other direction main bars are not available shall also be mentioned in note form.

Main idea is to provide details easily understandable and chances of mistakes be avoided.

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