long span concrete beams 2

[csstruct]

Hello

I was looking for some tips on designing large, long span concrete beams. ie 1000mm deep x 17m long.

Is the design process as simple as scaling up the design for 300mm deep x 6m long beams for example or is there additional items i need to consider?

 

[csd72]

The behaviour is the same as it depends mostly on the span to depth, though with a deeper beam you can get some cracking in the side faces. The code will specify minimum side face reinforcement for this.

The behavior only changes when you have a span to depth of about 4 or less (code definitions vary), it is then called a 'deep beam' and behaves quit differently (more like a strut and tie models). I dont know why they call it a deep beam as it is the span to depth that is important.

If it is an upstand beam then you may need to check the slenderness limit (yes there is usually on in the concrete codes).

 

[dik]

I've done lots of conc beams spanning 59' to 60' for parkades and they have a pile of reinforcing... Typical Geometry beams spaced at 15' centres with 5-1/2" conc slab over. T-beams are 36" deep x 16"-18" wide stem and design loads are 50 psf LL for parking loading. They are typically part of a multi-storey frame with horizontal members at 10' spacing. The columns are roughly 16" x 36"(perp to span) with reinf approaching max.

In the beams, you have to check for flexure and shear (both can be high) as well as deflection, cracking, vibration wasn't an issue. You also have to check for rebar placing because of congestion of bars at the columns. Beam to column fixity assumed to be monolythic and spread footing assumed to be rigid at base (good granular material). Effective EI for columns calculated taking into effect of crackingas well as stress level of concrete. Maximum reinforcing occurred at upper most column due to lack of vertical load.

Other than that, nothing out of the ordinary.

Dik

 

[UcfSE]

In the US, the side face reinforcing is called "skin reinforcement" and is required for any beam 36 inches deep or greater. Our code also limits the span compared to the width of the beam. If the unbraced length exceeds this amount you may have to design for lateral-torsional buckling. I would think that other codes around the world would have similar provisions. Provided that your beam is not classified as a "deep" beam it essentially is just regular beam design. Deep beams have other design theories to be used since they exhibit much more nonlinear behavior compared to the theory used for typical beams.

 

[ash060]

Also the code allows you to use skin bars as part of your tension reinforcing using strain compatability. Also make sure to check long-term deflection with such a long span beam (compression reinforcing can help with this).