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provision of reinforcement in columns

 
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jagan0819
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Joined: 30 Mar 2016
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PostPosted: Mon Sep 23, 2019 3:55 am    Post subject: provision of reinforcement in columns Reply with quote

hi sefians

we are designing another floor for a office building (which has only ground floor)
and the issue is for column which is located in ground floor(existing floor) has 8 nos-20mm dia bars and we have done analysis in staad and for same column in 1st storey requires Ast of 3200mm2...

i need a clarification that can i place extra steel at junction of ground and first storey to satisfy steel requirement as per bending moment in top storey or can i lap 20mm dia bar which is at bottom storey with 25 mm dia bar in top storey


regards



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es_jayakumar
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PostPosted: Tue Sep 24, 2019 1:06 am    Post subject: Reply with quote

Does the requirement of 3200 sq-mm of steel occur at the base of the FF column ? If so, neither placing the extra bars at the base, nor lapping them with existing bars will suffice. One method will be to anchor additional bars into the column at the base (slab) level. As the embedding of bars cannot be done for Ld, the bars so anchored will not return you the full tensile capacity. Hence, you have to calculate the number of additional bars to be anchored.

E S Jayakumar
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jagan0819
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PostPosted: Tue Sep 24, 2019 5:12 am    Post subject: thank u for valuable suggestion sir Reply with quote

es_jayakumar wrote:
Does the requirement of 3200 sq-mm of steel occur at the base of the FF column ? If so, neither placing the extra bars at the base, nor lapping them with existing bars will suffice. One method will be to anchor additional bars into the column at the base (slab) level. As the embedding of bars cannot be done for Ld, the bars so anchored will not return you the full tensile capacity. Hence, you have to calculate the number of additional bars to be anchored.

E S Jayakumar




sir the requirement of 3200 mm2 is at base of first floor column


thank u for valuable suggestion sir
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es_jayakumar
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PostPosted: Wed Sep 25, 2019 1:28 am    Post subject: Reply with quote

You are welcome, Er.Jagan.
You can use the "Post installed rebar" technique of HILTI or FOSROC (Lockfix) for embedding additional bars at the FF level in the column base. You can calculate the additional reinforcement details in your case as follows :
Shortage of steel area = 3200-8*314 (ie, 8-#20) = 688mm2.
Please refer Table-3 of the attached literature of HILTI (got from google). Suppose, you use #16 bars anchored through 200mm depth in M20 concrete and Fe415 steel. The factored (ultimate) strength for the anchorage, as per the Table = 33.8 kN.
The ultimate tensile strength of one #16 bar = 0.87 fy Ast = 0.87*415*201 N = 72.6 kN.
Efficiency of anchorage in this particular case = 33.8/72.6 = 0.47 (47%)
The additional steel area you have to supplement = 688/0.47 = 1464mm2.
Number of #16 bars to be anchored = 1464/201 = 7.3, say 8 Nos. You can use higher dia bars and increased depth of anchorage to reduce the number of extra bars.
The Applicator will help you out in working out the theoretical calculations in this regard for using their latest products. They will even carry-out pull-out test on a test rebar to find out the practical ultimate  pull-out capacity. We have done such test using HILTI rebar anchorage in rock, that I have already mentioned in a few of my previous postings.


E S Jayakumar



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