Deflection limits...

[Diquan]

Hi!,

Any ones knows a book, a web page or reference that recomendate deflections limits for different types of strutures? (ie. buildings, bridges, industrial storages, etc.)

Diquan

 

[40818]

I'm not sure i can answer your question without further explanation of the problems. Different structures/systems will have different allowable deflections depending upon the materials used, structural design etc.
Try to be a bit more clear if possible.

 

[csd72]

AISC design guide 11.

 

[Lion06]

AISC Design Guide #3 is the one on serviceability and gives a great deal of recommendations of deflections limits for different types of construction (masonry, curtainwall, etc...). There is also a good discussion about calc'ing the 10-year winds (used for serviceability) from the 50-year wind map.

 

[Diquan]

40818 (Aerospace) -> I have to design a structure that will work as a freezer, if has in front 40m by 40 deep and 15m tall (131ft x 131ft x 50ft) and cannot have columns inside.

The structure needs to support A/C and special panels.

Now, if i design the structure to support all the equiment the structure will deflect more than 50cm (20in) at the top of roof, so the design is govern by deflection not by work.

And the question is, what is a good limit for deflection? for this types of structures?

L/120? L/150? L/200? L/240? L/300? or L/360?

So, is there book, or something that will say something for structures this high?

Diquan

Diquan

 

[Lion06]

Again, the height has nothing to do with it. H/500 is teh same whether teh structure is 5" tall or 500' tall. 20" on a 50' structure is way too much. That is H/30. What type of finish/cladding will be on this structure?

 

[csd72]

What are the walls/roof made out of. Whats on top.

Is there heavy equipment on the roof?

The absolute most deflection I would allow for this type of structure is H/150 and L/200 but this would be more stringent depending on the details.

csd

 

[TCPhoenix]

International Building Code (IBC 2003) gives deflection limits based on height for a variety of different structures.

 

[40818]

I would recommend adapting the design to eliminate the excessive deflections. I would probably be a little more stringent than csd72, probably in the order of L/300.

However, for small deflection theory, you can actually have rather large linear deflections, as it is the beams radius of curvature that is the divider between small and large deflection theory.

So if you can prove that the structure is acceptable by analysis then you should be ok. However if you have to comply with certain building regs then you might be constrained somewhat.

 

[Lion06]

40818-
It is not just what is acceptable by small/large deflection theory. This is a serviceability consideration. You don't want the finish of the structure (whatever that may be) to crack or otherwise fail. Different materials require different drift ratios.

Diquan-
What is the finish of the structure?

 

[civilperson]

Most tall freezers have shelving inside for pallets which are forklift located. The IRS will allow a faster depreciation of shelving than buildings so many freezers are supported by the shelving columns. A truss or joist-girder can span 40 meters with acceptable deflection limits if the depth of the truss can be fit inside or above the insulated envelope.

 

[Diquan]

StructuralEIT (Structural) -> The walls are made of foam (polyurethane)

csd72 (Structural)-> Made of foam, the air conditioner equiment is attached to the roof. You end up with that deflection based on experience?



Diquan

 

[Lion06]

The exterior finish of the structure is foam? What kind of lateral system are you using? How are you getting the wind loads into that lateral system?

 

[UcfSE]

Your building code should have limits for deflection of members under wind and gravity loads.

Seismic loads will have their own deformation checks, again in your building code.

Drift under wind loads is not something I have seen typically codified but you may find other sources with recommendations. The AISC has a design guide which addresses this with some recommendations.

If you have equipment on the roof, you may want to check if the vibration will be an issue.

If you have any partitions or anything that could be damaged by excessive deflection, you may also need to limit the deflection to an absolute number, in addition to limiting it to a given fraction of the span.