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[vikram.jeet]

Concrete Block Foundation :

Despite many advantages discussed in previous posting , the excavation of block is time consuming as it is to be done manually so that side soil is not disturbed . Also block size need to be at least 900x900 mm or higher for manual working excavating.

In case JCB is used for excavation , side shuttering / 4.5" brick wall is needed before concreting. Also in such situation side backfill around block needs compaction .

But still is an economical and faster when compared to conventional spread footing with top and bottom meshes concreting first and pedestal concreting afterwards.

[vikram.jeet]

Footing Base contact Vs Factor of Safety against Overturning

Footings subjected to Uniaxial Moment AND Centric weight /Loading

Generally footings of sheds , Masts or Concentric Column loadings etc

Eccentricity e = W/M

W is the net con centric weight on Footing including self wt of footing and soil over it.

M is the moment Overturning due to horizontal forces

Contact Length X = 3 ( B/2 - e )
contact C = X/B
B is the Length of footing in direction of moment

Stabilizing Moment = W * B/2

FOS (OVERTURNING) F = (W*B/2) / M = (W/M) * (B/2) = (1/e)* (B/2)

e = B /(2F)

X = 3 (B / 2 - e) = 3 (B / 2 - {B / f (2)})

X/B = 1.5 - 1.5/F

C= 1.5 - 1.5/F

F = 1.5 / (1.5 -C)

Result as under :
If contact length = 50% I.e C = 0.5 , FOS OT = 1.50

If contact length = 66.7% C = 0.667 , FOS OT = 1.80

If Contact length 75% C = 0.75 FOS OT = 2.0

IF Contact length 90% C = 0.90 FOS OT = 2.50

IF CONTACT Length = 99.99% C = 1.0 FOS OT = 3.0


Therefore just by working out Footing base Contact , Fos on overturning is known .

[vikram.jeet]

In continuation of previous post :

BASE contact area is , since reduced , maximum base pressure needs to be within permissible limits


The maximum base pressure = 2W / (X*L)


L is the length of footing in other direction
( B is the length of footing in direction of moment as stated in previous post)

[vikram.jeet]

I DESIGN , HE SUPPORTS


Success of any civil construction is dependent on many factors :

Soil Investigation to be done properly and recommendations need to be firm

Structural design & Detailing needs to be perfect

Construction materials tested and upto desired standards

Experienced Construction Agency with zero tolerance to bad Quality , Corruption and laxity

Workman ship to be best

Quality control to be maintained throughout

Safety standards not be compromised

Only a strong Team can deliver with fulfillment of above .


A wise Doctor always write behind his sitting chair :

I TREAT , HE CURES


A wise Designer do write the same :

I DESIGN , HE SUPPORTS

[vikram.jeet]

Small Shed Foundations

In practice Large / Small shed foundations are in form of spread footings at 1.2m - 2.0 m foundation depths depending on uplift forces from roof and lateral forces on roof and cladding causing moments as result of Wind conditions.

However in some previous posting it was discussed that rcc block foundations are also economical , but these require Block depths in excess of 1.2 to 1.5 m .
But for Small sheds it is further economical and faster to provide Rcc / steel girder tie beams in transverse direction at or just below Plinth Level , tie-ing all opposite columns , which can resist lateral moments from columns fully . The uplift force can be balanced by providing rcc block and a smaller size with shallow depth of rcc block suffice as it balances the uplift force with some fos.

Only thing while suggesting such transverse tie beams introduction is that these beams , though are at PL , shall not interfere with any functional requirement beneath plinth , say in form of furnace etc.

The peripheral rcc beams are to be provided as usual , but above note is regarding provision of transverse tie beams which exclusively provide double benefit , (i) to resist lateral column moments fully (ii) to provide weight to foundation block to resist uplift thereby help in reducing block foundation size.

[vikram.jeet]

Structural engineers patience :

Patience of engineers engaged in structural design profession is always on test. Mostly if everything moves smoothly , there seems no problem but many occasions create unfavorable and unacceptable situations for them causing them to be rigid , blunt , and even losing temperament. Even a very jolly and humorous engineer can reverse gears in technically unfavorable environment , be it at design office or at site execution.

At design office :
(i) In case Senior SE is asking to reduce , design which str engineer engaged feels as under design , he will argue , may become irritated unless instructions are in writing .
Many CR's were spoiled due to this .
(ii) Proof Consultant is asking for some review which SE feels is not in direction of str safety . It seen that prime engineers may lose patience.

EXECUTION:
(i) Execution done wrongly with under reinf , and SE is asked to prove it OK.

( ii) On site visit , many execution flaws in placement and site not agreeing to correct and blaming drgs for poor detailing.

As Review Consultant :
(i)Review designer feel something in direction of undersafety/stability and pin point , but prime designer not ready to appreciate .


( to be continued )

[vikram.jeet]

Structure Engineers patience

Continuing from previous post - many situations are mentioned wherein even a humble , jolly , humorous structure engineer may feel uncomfortable due to reasons none other than purely technical. The main reasons for uncomfort are many :
(i) Responsibility of Structural safety is innate in designers

(ii) Structure work is calculations oriented and only designer is knowing the quantum of physics and mathematics inherent in it . For others not associated with design, it is just a routine work which can be modified here and there.

(iii) structure job is not rosy , any small changes may interfere with long calculations/ analysis just jeopardizing with str safety

(iv)Str design is primarily based on Collapse mechanism . Designer right from college to design office is in thinking to keep materials away from their failure ranges . Collapse / Failure which is a negative aspect , but SE has to think / visualize while working as part of profession .

(v) Designer always think of Concrete , Steel and building materials ( and not of flowers , roses), mostly , while working on designs and this thinking environment prevail in his attitude . That is why it seen that most designers of structures are inclined to music , poetry or any positive activity to disconnect from this materialistic domain and connect to some finer aspects .

(vi) Designers work is 2x2 =4 neither think of providing 4. 1, nor provide less and also sometimes lose patience to resist providing 3.9 . I.e. underprovision . Flexibility domain is very narrow for Str Er as compared to other professionals .

But a professional has to deal all situations . A soldier is battle hard and has to fight without fear , so is the case with Structural engineer whose main aim is to provide safe designs with due diligence to economy and construct ability.

[vikram.jeet]

Structural Engineers Patience - Always on test

Yes , SE 's do not like that design detail given are followed wrongly , but as per modified saying " TO ERR IS HUMAN ,TO CORRECT IS DIVINE " , Mistakes do happen during implementation / execution due to poor supervision , less time frame , laxity . A far- sighted str Engineer always has remedies / plan B to correct certain things executed wrongly . Definitely , remedies consume extra cost , interfere with aesthetics , and also invite mis trust on site execution.

It is seen that mostly , execution team do not report on wrongly executed rcc work items and it is left to God Almighty , but some sincere ones report as - executed position and request for correction. Therefore a str Er must have correction plan .
It is very difficult to correct foundation deficiencies , but these can be corrected to some extent during construction . Once floor finishes are implemented at GF / work completed and occupied ,correction for foundations become more and more difficult , nearly impossible .

Superstructure items can be corrected with some loss to aesthetics .

The idea is that Remedy / correction skills / retrofitting must be known to an able Str Er.

[vikram.jeet]

Shear Check at supports having uplifting reaction . I.e support is subjected to Tension ( and not compression)


Some situations are as under :

(我)光束with hanging column , shear check in beam shall be at face of support at hanging column.

(ii) A steel roof shed - - column pedestal ( with its spread footing ) under wind conditions is subject to Neet uplift force , and therefore rcc footing shall be checked for shear at face of support under wind case. If moment also act in conjuction with uplift , the footing on heel side be checked for shear at face of support.

(3)挡土墙基础板在脚跟上heel side , the tension due to bending moment is at top face but stem wall face on heel side is having Tensile force due to moments from ep or otherwise and weight of soil over it stabilizes. Thus heel portion of base slab shear check be performed at face of supports . *

(iv) Any column footing with low vertical load and high Moment , Stabilizing itself thru self wt + wt of overlying soil , the heel side needs to be checked at face of support.

(i) & (ii) As per spirit of IS 456
( if compressive reaction is induced at support face shear checked at d from face.
And if Tensile reaction is induced at face , shear to be checked at face .)

*Reference can also be made to book RCC design by Prof A K Jain where this aspect was confirmed (in Retaining wall design example)

[vikram.jeet]

TWO WAY SHEAR CHECK **


Situations requring two way shear check to safeguard against local failure
;
(i) Cantilever beam supported by ( ie commimg out ) from thin RCC wall

(ii) A cladded shed Column Pedestal ( rcc) on thin spread footing And subjected to Tension and high wind moments from cladding and roof

(iii) FLAT slab with rcc columns subjected to moments

In all above situations there is moment concentration in beam/ columns to be transfered to beam less slab (thin ) . Had there been rcc beams present in both directions ,the problem of local two way ( punching) shear may not arise , but slab is thin member and it's thickness needs to be sufficient to safeguard against this shear failure.

* * Punching shear concept on footings due high compressive load from column is checked , but this two way shear arising as result of moment concentration also needs to be seen in situations discussed above.

In flat slabs this aspect is included in IS 456 regarding transfer of moments from columns to slab as result of unbalanced gravity moments , EQ , Wind .