Serviceability Requirements under IBC 2006

[ISCK]

I remember coming across a post in Eng-tips forum asking about an ambiguity in footnote "g" of Table 1604.3 of the IBC 2006 code.

Unfortunately, I couldn't find that thread anymore, so I'm just giving that person a heads up on my finding, as I too, have found this to be very intriguing. Perhaps, it may interest you as well.

The deflection limits for "Floor members" under:
1. service live load = l/360
2. unfactored dead load + service live load = l/240

Footnote "g" then says, "For steel structural members, the dead load shall be taken as zero."

You're probably wondering, what?! When do structural engineers start ignoring dead load? Does that mean, if our floor member is structural steel, we just calculate the more stringent deflection limit of l/360?

I would say check both, since it's best to be safe than sorry.

The reason for this footnote, is that I believe it was carried from the previous edition of the IBC. Back then, per "IBC Final Draft Public Hearing:::March 1999", Hank Martin, AISI, provided the following reason:

"The table is a comination of the requirements in the National Building Code and the Uniform Building Code. Both the National Building Code and the Standard Building Codes establish deflection limits only under live load unless additional requirements are placed on a specific material by its associated design standards. The UBC establishes deflection limits under live load and dead load by the application of the load combination L + KD. Table 16-E in the UBC establishes different K factors for wood, concrete and steel. The K factor for steel structures is set at zero. We believe the Table in the IBC has mistakenly added a requirement for steel structures that was never intended. The addition of Footnote "f" will bring the IBC back into conformance with current practice. To our knowledge, steel structures have not nor are currently being desinged to this more stringent criteria."

Just note that when it said "current practice" = somewhere back in March 1999. But at least we know why the IBC 2006 has Footnote "g" now right?

To all structural engineers in practice right now, please share with us which one would you use for steel floor members. If you take Footnote "g" into account, would you use L/360 or L/240 (since D=0, (D+L)/240 = (0+L)/240)?

Thanks!

 

[WillisV]

For typical interior beams (not supporting cladding), I use L/360 live load and L/240 dead+live. Note that AISC Design Guide 3 on serviceability actually allows the use of L/360 under 50% of the live load - which is in direct conflict with IBC for this case.

 

[ISCK]

Hi WillisV,
you said that you would use L/360 for live load and L/240 for (dead + live). Please clarify on L/240. Do you mean (D+L)/240? And do you mean that you'll check both as well? Thanks.

 

[Lion06]

L just means the span of the beam

 

[ISCK]

Thanks for the correction StructuralEIT.
Sorry to WillisV for asking for clarification.

Also to all people who were reading this thread, ignore the part "(since D=0, (D+L)/240 = (0+L)/240)".

Just one question to StructuralEIT.
Do you check l/360 or l/240 for interior steel floor members? where the deflection limit for l/360 is using live load, and l/240 deflection limit is using dead+live loads?

 

[lkjh345]

We do the same as WillisV shows above: We check both Live Load deflection for less than L/360 and Dead Load+Live Load deflection for less than L/240.

 

[Lion06]

Yes, we do check deflections that way.

 

[asixth]

Maybe you don't want to include the dead load so the engineer can determine how much the floor beam will "bounce", when fully loaded.