Member deflection vs floor deflection

[WWTEng]

Typically floors (steel or composite beams with concrete on metal deck) are designed for deflection limits of TL/240 & LL/360 per IBC 1640.3. But I am wondering if these are deflection limits for individual members for the floor system? Please see attached for a graphical representation of my question. If I was design for TL/240, I could have a 2.5” deflection at some point in floor.

I thought that these deflection limits applied to the individual members but 2.5” worst case deflection for the grid such as the attached seems excessive.

 

[slickdeals]

There was a discussion on this not too long ago. Usually member deflections are relative to the ends of the member. However, a beam's end will move along with the girder and will have it's own deflection in addition.

As a result, IMHO, your true deflection should factor in the deflection of the girder and added to the beam to figure out a bay's deflection.

I will post the paper once I find it.

 

[slickdeals]

See this recent thread thread507-321581

 

[slickdeals]

See paper below

http://www.concreteconstruction.net...tion of Composite Deck Slabs_tcm45-343273.pdf

 

[MiketheEngineer]

For floors - I usually want at least 1/480 to cut down on deflections. Do I take into account the total deflection - joist plus beam - not usually but on longer spans - say over 16'-18' I do.

 

[woodman88]

The deflection limits IMHO in the 2009 IBC Section 1604.3.1 to 1604.3.6 deals with the individual members deflection. Table 1604.3 also deals with individual members, except as noted in footnote e for ponding and footnote h for each glass lite vs total length.
Of course this section begins with “1604.3 Serviceability. Structural systems and members thereof shall be designed to have adequate stiffness to limit deflections and lateral drift.” So the total structural system needs to have “ adequate stiffness to limit deflections” but these limits (except for special cases) are not well defined in the code, that I can find.
So we are left with applying our engineering judgment/experience, for the most part, in designing the structural system for adequate stiffness.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[WWTEng]

Considering that this for a school, where this may be changed into a library (stacks) I think I ought to look for the maximum deflection in the bay. Diagonal from column to column (39') would limit the maximum to 1.95" @ TL/240 or more stringently to 1.3" @ TL/360. Not sure if 1.3" for total would be too conservative for this bay.

Slick: Thanks the paper you linked. Looking at that I think the somewhat easier solution would be to camber the girders only. Not sure though, I gotta think about this.

 

[structSU10]

Librarys typically have a fairly high load compared to a standard classroom or cooridor. If it is likely to be changed, it may be worth the stricter deflection criteria to save some headache down the road. A few thousand dollars now could mean a lot less retrofit later if there is a proposed change.

Also a cursory check on vibration never hurt anything if the bay is large enough for concern.

 

[abusementpark]

Are most engineers really looking at total floor deflection? I doubt it.