long span concrete beams

[JM_W]

thread507-176530
need to design ordinary reinforced concrete beams spanning 30 meteres, spaced at 1.2metres, to support terraced concrete floor (for amphithetre seating)
The headroom is not an issue and we are allowed beam depths of up-to 2.4 metres.
Have done an analysis and design and the re-bars area is not excessive.
Is there something I need to look out for?

 

[jayrod12]

Long term deflection due to shrinkage and creep. On a span of that magnitude, I'd expect it to be fairly substantial.

 

[CANPRO]

I'm not a concrete expert by any stretch, but for that span I suspect you'd be best suited with some sort of post-tension beam rather than plain reinforcing. How far off the ground is it? The formwork for this sounds expensive. Would it be an option to cast it on the ground and lift in place?

 

[WARose]

ACI mandates bracing spacing at 50*b (where b is the width of the compression flange). In excess of that is a LTB situation you need to check out.

 

[rapt]

Even at 2400 deep, in a quick calculation assuming simply supported 30m span, without doing anything special for deflection control I would expect deflections of about 120mm.

So you definitely need to do something about that.

 

[MIStructE_IRE]

I’d avoid ordinary reinforced concrete at this length. Consider PT or perhaps prestressed bridge beams (if they come in that length..?) if it has to be concrete.

Personally I’d be looking at composite steel trusses. Watch vibrations!

 

[GC_Hopi]

Seems like a situation for precast prestressed concrete. Or steel. They both dominate the stadium market for good reason. Ordinary reinforced concrete would be a tough and perhaps uneconomical depending on the circumstances of this design.

 

[Ron]

Agree with GC...most are done this way. Pre-cast, pre-stressed concrete is commonly used for these applications.

 

[bones206]

Or have sloped CIP girders as the primary members and span the seating from girder to girder with shallow secondary members (i.e. the seating). I think this would be more economical and practical. Something like this:

 

[BridgeSmith]

Prestressed beams with a CIP composite topping could be a good option. By the time they are placed, most of the cambering due to creep has taken place, and generally the guys who design these for the precasters are fairly good at balancing them out so they don't camber or sag much after the topping/top flange is in place.

 

[bones206]

I agree with HotRod. A prestressed soffit beam is sometimes a good option for long spans in unique situations. The precast bottom speeds up construction and allows for customization in the CIP top portion, while reducing deflection with the prestressing. I think these are a pretty useful structural tool but they do require design collaboration with the precaster.

Here is one manufacturer I know of: Link