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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Wed Jul 28, 2021 2:30 am Post subject: |
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DURABILITY CONSIDERATIONS BY STR ER
13.0 BEAMS IN END GRIDS
These beams in end grids support slab on one side only , unless some cantilever slab projection is provided for balcony . But mostly side grids carry slab on inner side , I.e. one side only. These beams are L beams. Due to partial fixity of slabs with beam , the development of compatibility torsion is obvious. As per spirit of IS 456, this torsion can be taken care by minimum reinforcements ,stirrups and top bottom bars, if slab panels are designed discontinuous at this beam.
But still , some designers cover this in detailing , by ignoring Tc bd relief in end grid beams. This provide little extra stirrups required to reduce torsional cracking. Also if end beam webs are >450mm, 8mm dia. side face bars are provided.
Thus Durability concern is addressed through detailing . |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Wed Jul 28, 2021 2:45 am Post subject: |
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DURABILITY CONSIDERATIONS BY STR ER
14.0 MAIN BEAMS SUPPORTING SECONDARY BEAM ON ONE SIDE
Main beams are supported on Columns /Brick walls but secondary beams are supported on Main beams . If secondary beams are continuous over main beams , the BM at continuous support balances. But if secondary beam ends in main beams , the monolithic concrete , but with partial embedments of reinf of top bars , create torsion in Main beam.
As stated in previous post , If secondary beam is designed considering hinged connection with main beam, the torsion developed due to partial fixity is compatibility torsion and can be taken care by minimum shear reinforcement .
But still , some designers wish to minimise torsional cracking by ignoring Tc bd relief in such beams . Also is beam web depth is > 450mm , 8mm / 10mm dia side face bars are also provided in such main beams carrying sec beam on one side . |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Wed Jul 28, 2021 3:34 am Post subject: |
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DURABILITY CONSIDERATIONS BY STR ER
15.0 SUNKEN SLABS -
SUNKEN slabs are needed mostly at toilets , but sometimes to have aesthetic ceiling of Balconies , Porticos , Front open concourse , sunken slab is recommended.
In early times light weight filling is proposed in structural drawings , but non availability of such light weight material like cinder fill , foam concrete etc , the filling by brick ballast lime concrete , sand filling is done. To address this problem , designers take filling load of sand fill /lime concrete during design stage for slabs and beams design around sunken slab.
Also to facilitate drainage of any water ingressed accidentally during future , one or two 25 to 40 mm drain pipes are indicated in drawings just at top of sunken slab.
The Plumbing pipe Sleeves are indicated in Beams around sunken
slabs in structural drawings with instruction notes.
The Durability considerations from Structural designers are , though , seldom , appreciated . |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Wed Jul 28, 2021 11:46 am Post subject: |
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DURABILITY CONSIDERATION BY STR ENGINEER
16.0 MINIMUM SURFACE REINFORCEMENT IN MASS CONCRETE
Although as per standards , minimum surface reinforcement in mass concrete is 3.6 cm2/ m both ways at surface as per the spirit of IS 456 wherein it is stated that sections of thickness more than 1.0m shall be provided with this reinf , on faces . If other requirements need higher reinf at any face , higher to be provided.
IRC 78 requires Mass concrete abutments to have 2.50 cm2/m as minimum surface reinf. As per earlier 2000 version. Latest version needs to be checked for any revision.
Even SP 34 on Conc block foundations for Machine foundation require 25 kg/m3 to 40 Kg/m3 surface reinf , but minimum 12mm dia at 200 /250mm c/c.
The spacing of 200mm is generally preferable in mass concrete .
But sometimes mass concrete especially in aggressive exposure , extreme weather , continually drying and wetting conditions - e.g.abutments at river side , are provided with higher reinf on surface than stipulated in IS 456 / IRC 78.
The considerations for Durability are always behind the design / detailing of Structures. |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Thu Jul 29, 2021 3:30 am Post subject: |
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DURABILITY CONSIDERATION BY STR ER
17.0 CONCRETE TECHNOLOGY -
Though IS Codes especially IS 456 gives Durability recommendations in its clauses , Structural engineer / his team when asked to visit a site for a day's inspection on already placed reinforcement ready for concreting the next day , not only check the deficiencies in reinf placement , but also discuss the Quality of concrete to be used in construction.
The scaffolding adequacy, bracing of scaffolds , shuttering s, sealing so that concrete pour shall not leak, are also checked from underneath .
The cement , steel grade , any rusting of reinf , are checked.
If concrete is to be produced at site , the Quality of coarse sand and presence of silt to be seen , though visibly, and instruction for use of coarse sand after washing to remove silt , are also given.
The grading of stones using 20mm down and 10mm down stone aggregate is generally discussed.
If concrete is from RMC , the details are checked , extent of fly ash needs to be restricted ad per permissible standards.
Quality of bricks is also checked by hitting two bricks with each other and ringing sound indicates the quality. If bricks break , the quality is poor .
CURING OF RCC WORK AS ALSO BRICK WORK IS EMPHASISED BY THE STRUCTURAL TEAM.
FEW SUCH DURABILITY CONSIDERATION AS EXPRESSED AT SITE BY DESIGNERS OF STRUCTURES leave high impact . Site engineers
who are better experts of concrete technology and concreting operations , are more assured after designers visit. |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Thu Jul 29, 2021 6:06 am Post subject: |
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DURABILITY CONSIDERATION BY STR ENGINEER
18.O STRICT FOLLOWING OF CODES
Designers are mostly labelled as very rigid professionals compared to others like Architects and MEP engineers .They are sometimes seen vocal while checking and proof checking of structures designs.
Nobody taken pain to see the plight of structural engineer who would like to work and provide designs in accordance with NATIONAL CODES, INTERNATIONAL CODES, SPECIALIST LITERATURE IF CODES ARE SILENT ON SOME ASPECT .
On one side strict adherence to Standards ensure safety/stability to structures , whereas on other side issues of aesthetics, economy strech them , but The firmness of Designer of structure , though not liked by owner/ other professionals , always prove RIGHT , in creation of safe structures based on Standards of Nation.
STRICT ADHERENCE TO CODAL PROVISIONS , may be respected . |
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vikram.jeet General Sponsor
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Posted: Thu Jul 29, 2021 6:59 am Post subject: |
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DURABILITY CONSIDERATION BY STR ENGINEER
19.0 DURABILITY FOR BRIDGES
Bridges are designed for a service life of not less than 100 years. Structural Bridge engineers are highly conscious about the structural longevity. River Bridges with their substructures mostly in waters , experience extreme weather throughout the year as also flash floods during rainy seasons.
Their superstructure is constantly under fluctuating stresses due to to heavy traffic loadings , day in and day out. Over above is the Structure abuse by virtue of Overloadings on vehicles.
Fortunately INDIAN ROAD CONGRESS with its STANDARDS / CODES and technical publications hasprovided enough guidance to engineers towards design and construction of durable Bridge structures and aspect if durability has been covered exhaustively.
Few durability provisions at design stage are:
Crack width to be within limits for structural members
(In early times , Mild steel Fe 250and Torr steel Fe415 were available . For all Substructure items in contact with soil/water are proposed in MS , though costly, so as to limit crack width ensuring durability )
Whereas , Fe 415 has a permissible stress of 2300 kg/cm2 as per IS 456 , the permissible stress as per IRC 21 for bridge structures is 2000 kg/cm2 in WSM. |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Fri Jul 30, 2021 2:48 am Post subject: |
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STRUCTURAL SAFETY CONSIDERATIONS BY STR ER
The prime job of a structural designer is to ensure STRUCTURE SAFETY AND STABILITY , at all costs. His entire studies , continuing - education , experience ,expertise are aimed to produce Safe Structures , while keeping in view the durability , economy, functional and aesthetic considerations already discussed.
Some Safety aspects are discussed hereinbelow :
1.0 STABILITY OF STRUCTURES -
It is an overall /global aspect of structural safety. Even if , the structural components are adequately designed and detailed , if Structure Stability is deficient , The designed structure is NOT SAFE at all.
(to be continued)0 |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Fri Jul 30, 2021 4:19 am Post subject: |
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STRUCTURAL SAFETY CONSIDERATIONS BY STR ER S.
1.O STABILTY OF STRUCTURES -
(in continuation)
Stability of structure as whole (GLOBAL STABILITY ) is required from following considerations :
OVERTURNING
SLIDING
UPLIFTMENT FROM FLOATATION
UPLIFTMENT FROM WIND ACTION IN SLOPED METAL ROOF / LIGHTLY LOADED STRUCTURES
LOCAL STABILITY OF CANTILEVER PROJECTIONS |
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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3835
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Posted: Fri Jul 30, 2021 4:59 am Post subject: |
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STRUCTURAL SAFETY CONSIDERATIONS BY STR ER S
2.0 STABILITY (GLOBAL ) - OVERTURNING
Overturning is the most important among all other stability considerations. Structure can not stand and tend to collapse due to overturning , if adequate stability do not exist in structure.
Forces causing overturning of structures are:
(I) Horizontal forces in form of wind, Earthquake , Hydrostatic pressures, Earth pressures and any other horz force.
(ii) Eccentricity of structure due to gravity loads.
The overturning always takes place about a point called TOE.
There is need to see that OVERTURNING MOMENTS caused by above stated forces to be countered. Thus the STABILIZING MOMENTS in structure are worked out and a FACTOR OF SAFETY as prescribed in Codal provisions must be provisioned in the structural designing.
FACTOR OF SAFETY AGAINST OVERTURNING
=( STABILISING MOMENTS / OVERTURNJNG MOMENTS)
Normally a FOS of 2.0 under normal conditions AND FOS of 1.50 under Wind / seismic conditions is prescribed as FOS against OVERTURNING |
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