1800 x 600 wide RC Beam - Deflection 1

[80smetalfm]

Iam currently designing a 10 storey reinforced concrete building to be located in a highly active seismic zone with around 0.35g. Today iam looking closely at a RC transverse beam loaded with a structural 200 series blockwall above it at the carpark on the 2nd floor. The client wants to maximise parking hence the depth to my beam is limited to 600mm deep and columns limited to 300mm wide!
Im experiencing problems with long term deflections. I came up with a beam 1200wide x600 mm deep for my ultimate limit bending strength. However, deflection fails so i have to bump up the size to 1800 x 600mm to accomodate for deflection. My span is 10 metres. Whats the best thing to do in this case?

 

[swearingen]

Look at embedded carbon fiber reinforcement, or carbon fiber overlays. You can effectively have the reinforcement OUTSIDE of the concrete on the very bottom, thereby giving you the best resistance to moment and deflection for a given depth. It is more expensive, though...

 

[UcfSE]

300mm seems extremely small for use in a ten-story structure. Your client needs to understand how inefficient the structure will be from a load-carrying point-of-view, not to mention constructibility as well. That means its cost will be way too much compared to what it should be. That's all IF you can make it work.

 

[dik]

Can the CMU wall be designed as a deep beam? or can reinforced concrete be used and hang the floor from the wall?

Dik

 

[AUCE98]

You may also want to look at the long term deflection based on when the CMU load is applied. Based on ACI-318, after a month from the time concrete has been placed, the LT creep factor reduces to 1.5 from 2. You may also be able to provide additonal compressions steel to reduce LT deflection.

AUCE98

 

[dik]

Often a CMU wall will arch between spans and little load will be applied to the span. I've reviewed buildings where the supporting beam has deflected an inch or so and the wall has clear spanned it leaving a gap between the slab and underside of the masonry wall.

dik

 

[JAE]

80smetalfm,

For your span of 10m (32.8 ft) the 600mm (23.6 in.) depth seems adequate. For simple span concrete beams, the minimum depth required so as to not have to check deflections is L/16 = 625mm (24.6 inches).

Usually, if you have deflection problems in concrete beams you can either

1. Deepen the beam
2. Widen the beam
3. Add reinforcing in the tension zone (i.e. increase effective moment of inertia, Ie)
4. Find ways to redirect the load on the beam to other structural members or load paths (i.e. dik's suggestion to use the wall).

Be sure to take advantage of any monolithic slab flanges.

Also, for long term deflections, you can add steel in the compression zone to reduce long term creep.

 

[rapt]

80smetalfm

By my calculations, increasing the width to 1800 would only reduce the deflections by about 15%.

You could put in 4 times the amount of bottom reinforcement you need for strength and an equivalent amount of top reinforcement and you will get the deflections down enough (assuming I have modelled something similar to what you have briefly defined)

If you want to rely on the wall spanning by itself, make sure you reinforce the bottom of it to make sure it can span by itself and that noone is going to want to cut holes at the bottom of the wall in the future.

It may be easier to just fill the cores and reinfroce the wall and even supply starters out of the slab into the wall.

 

[80smetalfm]

Many thanks to all of you esp rapt & JAE. I will now look at increasing my reinforcement but still within the maximum reo allowed within the beam. Just a note, seismic areas im involved with require all CMW to be filled and starter bars in most cases used between walls to beam connections. Good point also from UCfSe. Will ensure column width is adequate!!!

 

[JAE]

Oh yes, I was going to agree with UcfSE's point about the columns as well....very very small, even for a one story building.

 

[80smetalfm]

Speaking of columns the size i initially thought of was 2000mm x 300mm (6.6ft x 1 ft) in plan. ie more like a "blade wall"! I havent done the numbers yet!! Anyway, would like to hear your comments again on this column size. My wide beam mentioned above will be sitting ontop of this thin "column".

Concrete compressive strength to be used is 40Mpa. All reinforcement bars yield strength fsy= 500Mpa. Stirrup(ligs, fitments in other words) will be R12 bars.