How to design long span slab with big opening?

[zakmuh]

Hi everyone..
I have to design a long span roof slab (18m x 12m) with big opening (10 x 3). There is no parapet wall or any other loads on the roof.
Roof height is 5m. Architect doesnt want any downstand or upstand beams in the roof. So should I just take each side as a cantilever slab and design it or take it as 2-way slab and provide cross rebar around the opening?

Thanks in advance

 

[briancpotter]

Which are you referring to as "each side"?

Regardless, if there are supports on all 4 sides, the slab should be designed as a 2-way slab. None of the slab portions in the layout you seem to be describing will act as a cantilever.

Brian C Potter, PE

http://simplesupports.wordpress.com

 

[rowingengineer]

Is this a normal reinforced slab or are we talking post tensioned?

At the spans discussed you are going to have a thick slab if this is a single plate arrangement. I would give consideration to making the slab a little thicker and putting in a slab beam on the edge of the opening.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[BAretired]

I would give a little thought to replacing the architect.

BA

 

[hokie66]

What is the opening for?

 

[Trillers]

I agree with BARetired - that Architect is nuts - he is forcing the structural costs to increase because you will now be forced to design the opening edges as wide flat beams, which are OK, but you also need to add diagonal reinforcement at the corners which will require a thickened slab anyway to attain minimum thickness. Change the ARCHITECT!!!

 

[zakmuh]

Thanking everyone for the replies

Well we can't change the architect. It'e the client is well loaded ( owns 3 Bentleys), whos insisting him net to let the engineer give downstand beams lolz!I like it because it forces me to learn new/difficult designs.
I'm talking about 200mm reinforced slab and the opening in the slab is for skylight ( aluminium glazed roof system).

How about providing rebar like...bottom shortspan 16-140mm,longspan 16-200mm, top-at supports 16-400mm. Makes sense??

 

[hokie66]

You will need a lot more than a 200 slab to span 12 metres. Suggest you find a structural engineer in your area who has dealt with long span concrete structures.

 

[frv]

Your numbers (and your architect) are way off. Bentley's or not, mo amount of money will change statics/physics/material properties. You are proposing an L/60 span to depth ratio. That should be triggering loud warning sounds in your head. Don't ignore them. Or you're going to end up with a very compact Bentley owner.

 

[frv]

"...no amount..."

 

[slickdeals]

Just thinking out loud here...

Best case, assuming continuity exists on all sides of this 18m x 12m roof, each side of the roof extends 4 m or 4.5 m from the support to the skylights. Can a sloping slab (PT) tapering from say 350mm to 200mm at the support be designed to cantilever off the supports? Parallel to the 18m direction, run post-tensioned cables to "pick up" the sagging edge?

 

[BAretired]

How about a truncated pyramid with the 3m x 10m rectangle elevated about a meter above the perimeter? Or, better still, set the skylight rectangle at a jaunty angle giving different slopes to each side of the pyramid.

BA

 

[ztengguy]

remind me not to go looking at Bentleys in whatever area this will be built, unless they are outside.

 

[paddingtongreen]

It will be deep to avoid visible deflection. You could use a deep slab but fill a significant portion of the middle depth with empty Sonotubes, I've done this in some retrofits where the top and bottom of my slab were predetermined and the resulting depth was much more than I needed and there was much more weight than I wanted.

Doing this, the beams and slab are hidden within the slab.




Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[zakmuh]

I appreciate all of your comments on this topic. Thanks a trillion!
Being a learning engineer, I've learnt alot of useful and important info and design approches.
Nothing impossible, until we try it eh? We tried to design a long span slab with opening, without having beams but failed! We didn't change the architect but changed the design with upstand beams and covered them with parapet walls