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Lap length for beams
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ramu_se
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Joined: 30 Oct 2017
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PostPosted: Fri May 27, 2022 12:41 pm    Post subject: Lap length for beams Reply with quote

Hi All,

Lap length for beams

At the compression zone - middle L/3 is the lapping zone

At the tension zone - L/4 from the face of the column is the lapping zone

Lapping has to be provided where it is zero bending moment.

Lap length @ Compression zone - 24d

Lap length @ Compression zone - 45d

Is this lap length is correct or do we need to provide 50d on both zones?

Kindly provide your guidance.

Thanks & Regards,
Ramu.
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vikram.jeet
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PostPosted: Fri May 27, 2022 2:48 pm    Post subject: Reply with quote

Lap Length

Better to use Lap length baSed on tensile stress . In earthquake there is reversibility of tension & compression zones .  Actual EQ forces are many times the design values and structure and ductility saves collapse of str.  EQ resistant detailing need to be followed.

Therefore please give only one (tension) lap length in drgs  through drg notes.
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ramu_se
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PostPosted: Sun May 29, 2022 12:21 pm    Post subject: Reply with quote

Thank you sir.

By mistake, i have typed both zones as a compression zone.

As you have told , can we provide the same lap length for tension as well as compression zone?

How much is the lap length for beams as per IS code?

Shall the lap length not exceed the development length?

If yes,

Can we provide 50D as lap length for beam, column & slab?
Thanks,
Sathya.
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vikram.jeet
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PostPosted: Mon May 30, 2022 3:28 am    Post subject: Reply with quote

Lap length / development length / Anchorage length are generally same .

Development Length = Ld

Need to work out Ld from simple basic : Equating tensile strength of reinf bar  = bond force

Pi * D * Ld * sb = ( pi * D^2 /4) * 0.87 Fy

Ld = {(0.87 * Fy ) / ( 4 * sb)} *D

This formula is already there everywhere including IS 456.

This is in Limit state .

sb is permissible bond stress based on conc mix

For Fe 500 reinf and M20 mix
sb = 1.2 Mpa for plain bars and = 1.6 times 1.2 Mpa for deformed bars

Hence bond length = (0.87*500) / (4*1.2* 1.6) =57 *D

You have to specify  Ld in drg notes based on mix used , steel reinf
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vikram.jeet
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PostPosted: Tue May 31, 2022 2:37 am    Post subject: Reply with quote

Also pl note that LAP LENNGTH FOR DIRECT TENSION CASES = 2* Ld
(As per IS 456)
Generally this situation arises in following :

(i) Hoop tension bars of  circular in plan water tanks , but while working out Ld be based on actual stress / reduced stresses in reinf as per IS 3370. Laps shall be 2*Ld

(ii) Ring beams of water tanks subjected to hoop  tension

(iii) Hanging column vertical bars anchored into Master rcc beam at upper floor

(iv) steel shed pedestal vertical  reinforcement bars  under wind uplift case are under direct tension ,if shed is open and Anchorage in footing shall be 2*Ld . However where shed is cladded , rreinf is provided for tension  plus Bending . Thus Anchorage in footing shall be based on weighted principle considering 2*Ld for direct tension in bars due to wind uplift on roof Plus Ld based on Bending action from wind on cladding.

(v) any other situation
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vikram.jeet
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PostPosted: Tue May 31, 2022 2:45 am    Post subject: Reply with quote

Lap Length for reinforcement bars in bundles shall be as per IS 456 stipulations .
2 bars bundle  - - -  1.10*Ld
3 bars bundle  - - -  1.20 * Ld
4 bars in bundle - - - 1.33* Ld
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vikram.jeet
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PostPosted: Tue May 31, 2022 3:19 am    Post subject: Reply with quote

For Bridges and Culverts , Lap length shall be as per IRC 21 stipulations .
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vikram.jeet
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PostPosted: Tue May 31, 2022 7:06 am    Post subject: Reply with quote

Lap Length for bars ,always in compression :

As per IS 456 , the Bond stress for bars in compression  = 1,25 times

Hence For Bars always in direct compression , the Lap length = 0.8 Ld

It may be noted that even in column bars , reinf is subjected to predominantly Bending apart from taking direct compression and therefore reinf is not in pure compression stage . Therefore  lap Length not less than Ld to be used.
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PostPosted: Tue May 31, 2022 7:23 am    Post subject: Reply with quote

Lap Length can exceed Ld  in many situations :

(i) Bars in direct tension , already substantiated

(ii) Bars in Bundles

(iii) Bars subjected to heavy cyclic traffic loading as in Bridges

(iv) EQ resistant Detailing demand Ld plus 10 d

etc
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PostPosted: Tue May 31, 2022 7:27 am    Post subject: Reply with quote

Er Ramu
Lap Length is to be provided for aĺl structural components of rcc viz Foundations , rcc columns , Beams , as well as slabs . Where rcc shear walls are there  it needs to be provided in walls .  Value of Lap Length be based on conc mix  and reinf grade , which is generally same for floor beams and slabs . For rcc columns if building is low ht say upto G+3 ,mix used is same , but in high rise construction column mix is rich compared to floor beam/ slabs and need to have separate higher Lap Length value for column bars.
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