Expert: Arshad Khan - 6/8/2011

Question
QUESTION: Dear Arshad,
I have to avoid middle columns in a hall, therefore 3 beams (c/c-8.1 m)each 16.2 m long running along the bay is crossed through the middle by a 32.4 m beam (e.g. -l-l-l- ). I am bit confused, Shall I suppose the longer beam (secondary) being supported by short beams (principle), in which case i have to consider point load in the middle of each short beam.And for long beam there will be three supports without point loads.
Thanks.

ANSWER: Hi Ananta,

You have two options here:

1. Keep the slabs as 2-way spanning and use the 32.4m beam as secondary, with supports at 8.1m centres (no point loads). The main beams, 16.2m span should be made deeper/wider than the secondary beam as they will be more heavily reinforced due to the central point loads.

2. Change the slab to one-way spanning and eliminate the central 32.4m beam. This will mean a thicker slab (unless you change it to a very efficient ribbed slab) and easier design and detailing and construction.

---------- FOLLOW-UP ----------

QUESTION: Thanks a lot. I had 1st option in my mind but i would better go with 2nd option. Could you please give me an idea how to incorporate lift shaft with Raft footing? How to cope when raft is deeper than the lift bed (when we have to go deeper for good soil condition) and when raft is higher than lift bed (because of some existing structure). what is a concept in designing raft in both cases?

Answer
Hi Ananta,

1. Where raft is lower than lift pit depth, then extend the liftshaft walls down to raft and either fill up the liftshaft with weak concrete, or else cast a suspended slab for the lift pit bottom strong enough to resist lifts impact loads.

2. Where lift pit needs to be below top surface of raft, simply drop the raft below the footprint of the shaft/pit and slope the bottom of the raft at 45 degrees to join the two different soffit levels of the raft. Depending on the size of the pit and relative depth of pit and raft, you may not need to drop the raft by the full depth of the pit- in other words if the bending/shear stresses so allow, you could just have say half to 2/3rds of the raft depth just under the footprint of the pit.