Expert: Arshad Khan - 5/30/2011

Question
Dear Sir,

We had designed a Mosque, Requiring column free hall, resulting in main girder beams spanning 22.5 M...

the Columns are 11 M high...

Now for detailing these beams  I propose the following:-

Beam is 1500 MM Deep X 800 MM Wide and 22.5 M Length

Reinforcement 14 bars of 32  @ bott
                     14 T 25  @ Top
Considered as framing into Column.

Beams -
Bottom Bars

OPT 1.

-> 2 Layers of 7 bars each.
-> provide 12 M length at center
-> Lap length 50 X 32 = 1600 mm
-> at both ends to provide 6.85 M length pieces

OPT 2.
-> 2 Layers of 7 bars each.
-> provide 12 M length at center Alternate bars
-> Provide 10 M length at center alternate bars
-> Lap length 50 X 32 = 1600 mm
-> at both ends to provide 6.85 M length pieces for 12M length
-> at both ends to provide 7.85 M length pieces for 10M length

Please advice which of the above two option is better or I request you to suggest something which is better then above two options..

Top Bars

-> 2 layers of 7 bars each
-> Provide 10 M lengths at each end Alternatively of which 1.6 M goes as Ld
-> Balance at center is 5.7 M , add 1.6 for splice length both sides => Provide 8.9 M Length Piece Alternatively.
-> Provide 9M length at each end alternatively again 1.6M as Ld.
-> Balance at center is 7.7M, add 1.6 at both sides for splicing and hence =>  Provide 10.9 M Piece at center alternatively.



For the Columns
I plan to stagger the splicing  within the mid height of the column.

Please advice.


Further i belive welded splice is better than wire tying splice in this case..

Waiting for your Guidance...

Answer
Hi Syed,

For your bottom bars, option 2 is better.

For top bars, again it is better to stagger the laps.

There is no need to weld the laps, so long as you meet code requirements, especially related to bundled bars. Remember that tying wires do not contribute to lap/bond performance- it is only the concrete that transfers stress from one bar to the other at laps.

Make sure you confirm that the column can take the support moments and that the beams rebar is well lapped into the column. I would also suggest that you incorporate a pre-camber when casting the beams to cater for self-weight deflection and also long term deflection due to creep.

You may also consider designing the beam with notches at the sides so that it looks like an I-beam as used in bridge beams- once again you will save weight and cost.