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Some general view
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vikram.jeet
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PostPosted: Sun Jul 25, 2021 7:48 am    Post subject: Reply with quote

DURABILITY CONSIDERATION BY STR ENGINEERS

5.0 PROVISION OF  MINIMUM POSITIVE MOMENT REINF FOR MID SPAN IN RCC BEAMS

RCC beams which are simply supported are the beams without practically  any restraint at ends and are designed for Full moment which act at mid span in udL case/ central point load case . In such cases such Consideration is not required.
However where RCC beams are continous over supports OR  RCC columns   , such Consideration as a QA measure from design office is generally considered by majority consultants and even some Govt design offices.
Minimum span moments are as under:
For Interior span    (  w*L^2 / 16 ) to (w*L^2 / 14)
For End spans.        ( w*L^2 / 12)
PORTAL FRAME BEAMS   w*L^2 /10

Higher value of Midspan BM be adopted , Design analysis values Vs Above stated values.

The main reasons for such are:
(I) Alternate spans unloaded , the loaded span will get higher Midspan BM. Such cases are generally not carried out ,though in manual design era , there was emphasis in Govt  Design Deptts to carry out the same , but Pvt were not doing. Now a days with computer such cases are not tackled.
(ii) In Framed construction , the Column monolithic to beam at ends
is taken in design and in dtails , reinf bars are embeded Ld into column..But at many sites due to poor supervision / no supervision/ laxity , the L bends are not provided and hence a case of partial fixity at site. Such provisions also take care of such poor dealing at site , though it shall not act as a reasoning .
The basic reasoning by designer is (I).
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vikram.jeet
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PostPosted: Sun Jul 25, 2021 7:54 am    Post subject: Reply with quote

Previous posting :To be read as

(I) SPAN (in question)  LOADED WITH ADJACENT SPANS UNLOADED  will yield maximum + ve BM at Midspan of span in question.
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vikram.jeet
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PostPosted: Sun Jul 25, 2021 8:56 am    Post subject: Reply with quote

DURABILITY CONSIDERATIONS BY STR DESIGNERS

6.0 WATER/LIQUID RETAINING STRUCTURES -
For making the task of Structural engineers easy , BIS has already taken these measures in Codes on LIQUID RETAINING STRUCTURES , namely IS 3370 in 4 parts.

The only difference in latest version from newer one  are:
Minimum Concrete Mix is M25  (earlier it was M20)
Strength calculations: Stress in steel permissible -  130 MPA ( earlier it was 150 MPA)
Limit state design Method also stated while retaining Working Stress Method ( earlier it was only WSM )
Annexure for crack width calculation given
Some more , as per Codes
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vikram.jeet
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PostPosted: Mon Jul 26, 2021 2:55 am    Post subject: Reply with quote

DURABILITY CONSIDERATION BY STR DESIGNERS

7.0 STEEL STRUCTURES - CORROSION
Steel structures are ready built , factory manufactured (with best Quality control  ) Structural members available in Angles, Channels , I-Girders , Tee girders ,Circular hollow sections, SQ & rectangular hollow pipe sections .
But steel can live long if it is protected from CORROSION. The menace of corrosion eats away the steel , if not protected from moisture , wetting , corrosive environment. As such , Structural designers are more concerned about Longevity of steel structures.

Consideration to durability is expressed in Steel Structural drawings :
(I) Painting of steel members by PRIMER COATS OF RED OXIDE . Generally this is done in fabrication workshop or at site prior to erection of steel members. One or two coats as per specification.
(ii) After Erection , painting with two to three coats with enamel paints.
(iii) Re- Painting of entire steel structures by one or two coats , AFTER EVERY THREE - FIVE YEARS  as per environmental / exposure of steel members .

(to be continued)
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vikram.jeet
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PostPosted: Mon Jul 26, 2021 3:27 am    Post subject: Reply with quote

DURABILTY CONSIDERATION BY STRUCTURAL DESIGNERS

8.0 STEEL STRUCTURES - WELDING
As already discussed that Quality control on steel is needed during its production at factory , and obviously it is easy to QA under factory conditions . But the success of quality in Quality Assurance is not achieved unless fabrication at workshop as well as at site is upto the required standards.
Fabrication of steel members at workshop is also , to great extent , controllable provided welders are competent . But fabrication at site require further skill of welders.  Overhead welding is also challenging for welders and specialised A category welders are put on such jobs.
Even BIS Code IS 800 ,  itself address the Quality/ Durability concern by allowing only 80% of permissible stress for SITE WELDING  , whereas  for SHOP welding it is 100% . Thus the designers Consideration  to site welding is taken care by the Indian Standard.
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vikram.jeet
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PostPosted: Mon Jul 26, 2021 3:47 am    Post subject: Reply with quote

DURABILITY CONSIDERATION BY STRUCTURAL DESIGNERS

9.0 STEEL STRUCTURES - MAINTENANCE (REPAIRS)
Painting  after every three to five years with two coats of enamel paint to protect the steel from corrosion , has already been discussed in previous post.
But the repairs to corroded steel members after thorough inspection of steel sheds, steel buildings , steel staging of water tanks, steel bridges , PEB structures  and so on is very important part of Maintenance drill and activity. It generally seen that pipe columns of semi cladded / uncladded steel sheds got corroded with passage of time due to contact with rain water time to time . There is need to weld pipes collars of higher dia in half round needs to be welded from base plate to 300 to 450 mm above corrosion level . Like wise repairs with steel collars / plates / angles need to be carried out after thorough inspection.

If such inspection is carried out by Structural engineer , drawing details can be given , but any civil / mechanical engineer  can also suggest such repairs.

A STICH IN TIME , SAVES NINE
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PostPosted: Mon Jul 26, 2021 4:17 am    Post subject: Reply with quote

DURABILITY CONSIDERATIONS BY STR ER

10.0 STEEL STRUCTURES - CORROSION ALLOWANCE
This is a DESIGN STAGE PROVISION  for those steel Structural members  under following situations:
(I) Exposed to aggressive environment  , coastal areas
(ii) Inspection & Maintenance is very difficult due to  Accessibility problems
(iii) Inspection & maintenance result in  prolonged power disconnection and heavy revenue loss to owner , mostly in POWER PROJECTS , especially HYDROPOWER pressure shaft steel liners.

Corrosion allowance is prescribed in some steel Structural Codes of various countries in view of above mentioned situations , together with durable coatings.
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PostPosted: Mon Jul 26, 2021 12:08 pm    Post subject: Reply with quote

DURABILITY CONSIDERATIONS BY STR ER

12.0 STEEL STRUCTURES - SLOPING CORRUGATED SHEET ROOFING

The sloping roofs of sheds , PEB structures , Industries , Plants , hutments , Bus /Railway /Metro Stations , their permanent sheeting roofs also need attention , though these are mostly  not designed if supporting members I.e PURLINS , to which this members are hooked/ screwed are provided at spacings less than 1.35m to 1.5 m . But the durability of these sheetings is definitely a considerations to the structural engineers.
Corrugations in thin sheeting , generally in range of 0.5mm to 1.0mm make them FOLDED PLATES , and provide desired structural strength .
Structural designer is concerned to see that these sheetings are not stressed beyond limit due to deflection of supporting members viz PURLINS / GIRTS. This is to ensure durability of the sheeting.

( to be continued)
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PostPosted: Mon Jul 26, 2021 12:49 pm    Post subject: Reply with quote

DURABILITY CONSIDERATIONS OF STR ER

STEEL STRUCTURES  - CORRUGATED SHEETING

12.0(in continuation from previous posting)
The durability of sheeting depends on following factors :
(I) Slope of sheeting to drain off the rain water efficiently
(ii) Deflection of supporting members , Purlins and girts
(iii) sheeting thickness / corrugation pattern
(iv) Pre Coating on sheeting

Slope depends on areas of rainfall intensity , but as a limit to the slope :
For AC sheeting roofs  - -  1:7 , preferable 1:5 or still steeper
For GI, MS metro type  - - - 1: 10 , or steeper
color coated

DEFLECTION LIMIT  OF PURLINS / GIRTS (As per IS 800 - 2007)
WIND LOADS
For Elastic sheeting ( metro ms sheets, GI /AL)  = L /150
For Brittle sheeting (Asbestos Cement  sheets)  = L / 180
LIVE LOADS (Simple spans)
For Elastic sheeting/cladding =L / 240
For Brittle Cladding/ sheeting = L / 300
LIVE LOADS (Cantilever spans)
For Elastic sheeting / cladding = L / 120
For Brittle sheeting  / cladding  = L / 150

Thickness needs to be increased if purlins are spaced more .
Precoating must be of approved quality as per standards , but it is factory product , must be ISI mark from BIS.
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PostPosted: Mon Jul 26, 2021 2:36 pm    Post subject: Reply with quote

SOME GENERAL VIEW  - POSTINGS

The postings within this title , are generic in nature . Aim is just to enlighten issues of structural  / Civil Engineering for further thinking . Some times posting are on non tech issues , just to express  some divergent views.

Inspired from Serial postings by sri Suraj Singh saheb on his 'Field Engineering talks on FB.

Also truly inspired by SIR U H VARYANI SAHEB  the OCTOGENARIAN structural expert on sefi.

No other member seems willing to share his experience s as Civil/ Structural engineer in form of SERISL POSTINGS  , especially those who are superannuated from active day to day working , must be having time for Structural Community .
Request for the same. Sir UH Varyani saheb has already made such request.
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