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Some general view
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vikram.jeet
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PostPosted: Mon Apr 12, 2021 3:43 am    Post subject: Reply with quote

Learning from Drafting brothers

Structural & civil engineer learns a lot from his drafting colleagues. How to present the details is an art  and a budding  engineer ,in initial stages of career is ignorant about this art. Engineer mostly concentrates on analysis & design , but the end product is proper presentation of details. A senior experienced draftsman is also a teacher  to budding engineer in r/o of the  detailing/ presentation .

Steel draftsman  have to do many detailing calculations themselves
, Of course to be checked by design engineer .

Engineer , if develops habbit of occsional drafting himself will have added advantage . Correction in drgs can be done. But if freehand sketches are prepared and given to D/M will enhance his degree of visualisation which ,ultimately helps in design/analysis .

Main aim is that engineer must take drafting/drgs , the end product of design,  with  same enthusiasm as analysis/design
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vikram.jeet
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PostPosted: Mon Apr 12, 2021 4:40 am    Post subject: Reply with quote

Water retaining structures
Casting length without contraction joints

As per IS3370 part 1&2, The maximum length between contraction joints using minimum reinf% given in this Code is 7.5 m and maximum length between expansion joints be 30 m I.e. every 4th joint be expansion joint.
Contraction joints are to be provided with water bars, for clear water it can be steel bar but for sewage works it needs to be PVC.

In case casting length is to be increased  beyond 7.5 m, the % of minimum reinf to take tensile stresses developed due to contraction of concrete needs to be increased. As per curve given in FINTEL Handbook ,the curve can be referred for casting length Vs %of minimum reinf required without contraction cracks.

In view of RMC there is higher casting lengths requiremts  in water retaining structures  and hence this view.
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vikram.jeet
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PostPosted: Mon Apr 12, 2021 8:58 am    Post subject: Reply with quote

Structural engineer's cup

As we know , the work of engineer of structures is highly demanding , yes other disciplines are equally too.But structures engineer obligation to provide safe but economical structures put him in tremendous self descion making  to work between these two aspects .  Still there is Criticism , comments , observations  by non experts and are the part of daily life.(corrections by experts   is always welcome) ,Responsibility to deliver design drgs in limited time in ever demanding atmosphere together with signing for safety, economy ,stability ,correctness , with sometime restrictions (on design features)by fellow Architects /clients  literally make him a hard nut to bear stressful situations.personal satisfaction to do the the demanding job is the major return to S.E.

Some poetic touch nearer to reality :  

A structural engineer  told his fellow professional :

Do not pour the wine in my cup , it may boil.
Do not touch my cup ,your hand may burn.
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vikram.jeet
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PostPosted: Tue Apr 13, 2021 6:09 am    Post subject: Reply with quote

Temperature movements

Bridges
Many years back, the bearings provided were Rocker bearing at one end (allowing rotation of deck but no displacement) and Roller bearing at other end allowing both rotation and displacement due to thermal variations. These bearings were made of forged steel.

The Neoprene pad bearings were used for smaller spans and these allow rotation as well as some little displacement of top surface of bearing wrt to bottom surface fully unmoved. Both ends share equal forces.

Pot bearings & then PTFE bearings are Fixed type as well as free type wrt to temp movements as being used now.

Buildings  & Sheds  
Supporting truss  or steel girder
One end is provided with shoe plate with circular holes to match the bolt size and other end is provided with oval shaped slotted holes to allow for sliding of shoe plate of this end over column top plate.
Only frictional force will be exerted on columns and truss member due to temp movement.

Small bridges & culverts
For slab bridges / culverts of spans upto 10m , bituminous top painted concrete pads. Friction coefficient as per IRC 6.
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vikram.jeet
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PostPosted: Tue Apr 13, 2021 6:46 am    Post subject: Reply with quote

Shear check at face of support

As per IS 456 the shear check needs to be done at face of support in cases where tensile reaction is generated in support.

A steel shed structure under wind case is subjected to high  wind uplift reaction and net reaction on supporting column is upwards creating tension in supporting mbr. For footing design , the shear needs to be checked at face of support under wind case .

A rcc beam supported by hanging member(column)  , the beam needs to be checked for its shear at face of support.

A footing  of retaining wall or column subjected to high bending moment will undergo tension at bottom face on toe side whereas tension will be on top face on heel side. The check for shear on heel side need to be at face of support as per some rcc books of late and seems in line with the concept . But this is  still a mater
of  discussion.
In this case  rotation of cantilever heel is upwards .  Compared to a cantilever beam where  rotation is downwards.
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vikram.jeet
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PostPosted: Wed Apr 14, 2021 1:34 pm    Post subject: Reply with quote

,Theories of RCC design

Working stress Method  (WSM)
Analysis load  is Working  loads   i.e. 1.0 DL + 1.0 LL
Permissible compressive stess-conc in bending taken  (28 days strength )/ 3
Permissible stress in steel reinf - taken   =about 0.55 times yield stress

Ultimate load Method (ULM)
Analysis loads - Ultimate Loads = DL*1.5 + LL*2.2
compressive stress in conc in bending taken equal to  (28 days strength*2/3)
stress in steel reinf =  Fy ( i.e yield strength )

Limit state Method  (the present time ) - -LSM
Analysis Loads - Limit state loads =DL *1.5 + LL*1.5.
Comp stress in bending in conc using partial safety factor=  (28 days strength*2/3)/1.5
Stress in steel using partial safety factor = fy/1.15


On  comparing the three , the overall effect of safety can be summarised as under
WSM - FOS on conc stress in bending compression =3.0
           - FOS in steel tensile stress   = around 1.80
USM - FOS on conc stress in bending = 1.5*(1.5+2.2)/2 = about 2.775
           - FOS on  steel tensile stress = (1.5+2.2)/2 =1.85
LSM - FOS on conc stress in bending comp =(1.5*1.5.1.5)= 3.375
         - FOS on steel tensile stress = (1.15*1.5)=1.725

The comparison is only mathematical and LSM is the product of latest research and takes into account the conditions of collapse as well as serviceability.
Overall effect on design:
For a given BM , the concept requirement for a singly reifd.section is WSM>LSM>USM  due to concept of stress block
Steel consumption for a given section is marginaly different , LSM>WSM>USM

Just a quantitative comparison multiplying safety factors with load factors . ( For USM only average value of load factor taken assuming DL & LL of same intensity , though seldom ).
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PostPosted: Wed Apr 14, 2021 3:51 pm    Post subject: Reply with quote

Some view on History
( As I know,  but  may be different  )

Civil Engineering

In Mogul Era many beautiful & asthetic mega structures were constructed by Mogul Emperors. Taj Mahal is known to one & all as a wonder of world. At that time there was no distinction between architect & engineer. A Person who conceive the concept of structure had to give all the details of structure, it's concept it's aesthetics ,it's structural features . The engineer and architect role was to be carried by same person, though experienced in the profession. Mohamad Isaa from Iran was the engineer cum architect of  the Taj.

In British era , the concept of engineer first came. The engineer serving Military was Military/ Army engineer whereas engineer serving civil population was CIVIL engineer.  A civil engineer has to look to all mechanical and later electrical works . Civil engineer was highly occupied & then came the segregation of roles of Architect , civil engineer , Mech Er, & Elect Er.

The roles were further subdivided and today each  one has specific role related to his field. The education from College level take care of these branches of engineering.

But in the begining was Civil engineer & Military engineer.
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vikram.jeet
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PostPosted: Thu Apr 15, 2021 2:57 pm    Post subject: Reply with quote

Tender stage design

This is important to  see that tender stage design be carried out in brevity to arrive at concrete members  sizes & thicknesses.

Reinforcement is taken on basis of concrete volume of various members. Values  just a guide.
Reinforcement quatty :
RCC Buildings:
Rcc footings  - 50 to 60 kg/m3 (mostly bottom mesh)
Raft foundation - 120 to 140 kg/m3
RCC columns -  200 to 250 kg/m3
Slabs reinf  - 80 to 100 kg/m3
Beams reinf - 225 to 275 kg/m3 ( quatty on beams full depth volume)
Basement retaining walls - 100 kg/m3

Liquid retaining structures - 90 kg/m3. (mostly uncracked sections)


Steel shed rcc footings: 70 to 80 kg/m3 ( top as well as bottom mesh)

The reinf calculated must be further factored for :
(I) Laps
(ii) chairs for slabs
(iii) wastage
Around 5 to 10 percent extra to take care of above. Column laps shall be calculated extra.
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vikram.jeet
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PostPosted: Fri Apr 16, 2021 4:13 am    Post subject: Reply with quote

Quantity estimation and cost estimate

In Govt departments the design is carried by structural engineers but job of quatty estimation is left to junior engineers not engaged in str design. The designers ,since not dealing in quatty/cost estimation , are not aware of project cost ,or even structural item cost.
But in Pvt consultancy, the estimation work is also done by the str engineer handling the job and it gives him more insight of project . Some design offices ask str engineer to provide at least structural items like conc volume & steel reinf, (which is now ready to get from staad) . Earlier it was to be worked out manually .

Job of quatty survey & costing   also enhance the visualisation of designer & shall be taken with same enthusiasm as design work . It not only provides some break from routine str design work but also make designer more confident and aware of  extent of detailing vis a vis cost i.e more detailing with meager saving can be avoided and vice versa.

Cost matters since it can be treated ,to larger extent , as physical impact of design .
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PostPosted: Sat Apr 17, 2021 6:29 am    Post subject: Reply with quote

Footings at boundary line (Edge footings)

For large projects on bigger plots this situation is seldom but for small works , the edge columns have to be dealt by structural engineer. Discussions on this subject were available on sefi & can be referred .Going by the mechanics of structure , the cg of edge column do not coincide with cg of footing area and eccentricity between two is the additional moment on footing. Some options are discussed hereinbelow as food for thought.
(A) Providing trapezoidal footing (shape in plan) thereby reducing the eccentricity as discussed in many books. Not a practical solution
(B) Assuming pressure distribution as triangular with max pressure underneath footing and zero at inner edga. May help to reduce ecc moment but not a solo solution.
(C) Provision of Strap beam at foundation level itself and to be tied to inner column . This is mostly preferred solution of structural designers. But at site where there is no supervision/ semi supervision , there is  unwillingness to dig a trench between edge column and inner column to accommodate strap beam & these are liable to be missed & left to mother Earth to bear.
(D) Provision of entire ecc moment at ground storey in edge column. Generally not adopted due requirement  of very high reinf in edge column at G/stroey.

(E) Provision of Counter fort shaped column  from footing to plinth beam level with width of counter fort same as column dimension in that direction but width of counter fort in ecc direction at bottom shall be same as width of footing . At top counter fort has same dim as rcc edge column . Column reinf shall be as per design and vertical in footing with extra tie bars at sloping face of counter fort(inner side). The unbalanced eccentric moment = P*e , P col load and e is ecc of col cg with footing cg.
If d is the difference in foundation level & plinth beam level, then due to formation of couple  direct tension in plinth beam = P e/d . Plinth beam to be designed for this additional direct tension . This force will be negated by force from other side edge column. Footing shall also be checked against sliding for this force , may not govern as footing is under column load.
Counter fort shall be checked for ecc moment P e.

(F) Some Relief in eccentric moment can be taken for passive resistance of soil using suitable FOS.

(G) A paper on edge footing analysis was  authored by  our respected UH Varyani Sahib   and it may provide provide more insight in to mechanics of structure wrt to edge footing.
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