Maximum Diaphragm Dimension 1

[300redbeard]

IBC 2006 section 2305.2.3 gives ratios for maximum diaphragm dimensions for wooden structures.

Is there a maximum diaphragm dimension for a roof diaphragm with VERCO steel deck attached to reinforced concrete shear walls?

Thanks

 

[DaveAtkins]

The size of a steel deck diaphragm will be controlled by the magnitude of the chord force, the magnitude of the diaphragm shear per foot, or diaphragm deflection. Another consideration is the need for expansion joints to keep the building from becoming too large.

DaveAtkins

 

[300redbeard]

In regards to expansion joints, when is a building to large? This structure is 350' x 397'.

 

[jike]

Expansion joints should be determined based upon a number of factors:

Can the building grow in only one direction or two? This will depend upon brace locations and construction details.

Is the building heated, air conditioned or exposed to outside temperatures (ex. parking garage).

Types of construction: Prestressed, Post tensioned, conventional cast in place, steel, etc.

 

[msquared48]

That would depend on the lateral load the diaphragm sees as related to the allowable shear values, the chord forces as mentioned, which can be included in the walls or a steel member the diaphragm attaches to, plus the 3.5:1 maximum diaphragm ratio as alloiwed by the IBC.

Mike McCann
MMC Engineering

 

[300redbeard]

What section is the 3.5:1 maximum diaphragm ratio as allowed by the IBC under?

 

[Gumpmaster]

AISC Design guide 7 talks about expansion joints. 350x397 might be at the upper end of having no expansion joints depending on the temperature change and building configuration.

One conversation that's been floating about my office is on these really large buildings, does a light gage steel deck really act like we think it will on such a large diaphragm? Not much testing at that scale. An option is to provide horizontal cross bracing in each direction instead of a diaphragm.

I've done several buildings with diaphragms that size and no expansion joint. We haven't had high winds or earthquakes yet to see how they perform. No complaints from the client though.

 

[300redbeard]

This site is in Calif. site class 'D'. Temperatures range from freezing to 120 degrees fahrenheit.

I'm performing a plan check review. I've used cross bracing as well as standard steel decking with diaphragm capacities. My original concern is that the columns, which are spaced at 50 ft OC each way are only gravity bearing. Typical designs I have used and seen on structures this large are cantilevered column designs, so they are taking up their portion of the seismic instead of perimeter walls taking all the seismic loading/reactions.

 

[msquared48]

That's a new one on me. I would never do it. It would drive up the required lateral force significantly.

Plus, the diaphragm is much stiffer than the columns, so I doubt that the columns would ever be loaded to the degree calculated. You would have to calculate the stiffnesses based on relative deflections and back into the solution. The center columns would take more load than those closer to the walls too based on the deflection pattern of the diaphragm.

Mike McCann
MMC Engineering

 

[msquared48]

Another thought here. It seems to me that with a structure that large, there would have to be some interior concrete fire walls that could be utlized too as shear walls, thus breaking up the diaphragm stresses seen.

Mike McCann
MMC Engineering

 

[300redbeard]

msquared48,

Thanks for the input! The structure is similar to a Super WalMart in design, except reinforcedd concrete walls instead of CMU walls.

Your comment: "That's a new one on me. I would never do it. It would drive up the required lateral force significantly." Are you saying you would or would not use cantilevered column design?

Thanks

 

[southard2]

For expansion joints you've got to get, "Expansion Joints in Buildings" Technical Report No. 65 1975 (See link below). This is what is referenced both in the AISC manual for steel construction as well as Vulcraft's designing with joist and deck book that most of us are familiar with. In appendix B are the temperature tables you will need to determine your maximum length between expansion joints. The controling temperature change is then plugged into the figure on page 5 to obtain the recommended length between expansion joints. This value is then adjusted depending on serveral factors also listed on page 5. This technical report also contains the equation for determining what your expansion joint width should be. In Florida most of the structural engineers I know limit the length to 200ft to 230ft. However with this guide one can usually justify much longer lengths. According to what is stated above you are borderline for sure. If you shear wall stiffness isn't symetric you will probably have to put in an expansion joint at that temperature range.

If you are using unreinforce masonry limit your length to 200ft. Also for masonry the emperical approach limits the diaphragm width to length ratio to 2:1 per table 2109.2.1.2 FBC or IBC.

You said your walls are concrete. For tilt up construction the top of wall drift limitations are .005 x Height generally speaking though I don't think this is codified.

Mike I too would like to get the IBC reference for maximum length to width ratios for steel diaphragms. I have limited mine to a 2:1 ratio.

http://www.nap.edu/openbook.php?record_id=9801&page=5

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[300redbeard]

I'd like to thank everyone who gave input. It's pretty amazing in the engineering field how many factors are involved in proper engineering design. I learned something and am grateful and appreciate your helping me to grow.

Jon