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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 / (2F)}]

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.