Diaphragm flexible or rigid?

[SteelPE]

I am in the process of designing a small structure 125’x36’ (with 5 25x36 bays) single story 22’ tall. Code is IB 2009 with ASCE 7-05. I have braces down the long sides with moment frames in the short direction. Due to some architectural requirements, the moment frames are concentrated towards one end of the building (at 0’ 75’ and 100’ from one end)

The problem I have is that I am getting to much drift due to the seismic loads. One of the problems causing the drift is the fact that I am using a flexible diaphragm (steel joists with metal decking). However, I question whether this deck is rigid or not. According to ASCE 7-05 untopped metal decks are permitted to be idealized as flexible. How would I know if this diaphragm is rigid or flexible? Would the diaphragm be rigid if it's not in compliance with section 12.3.1.3?

 

[Lion06]

Seismic drift limits are EXTREMELY generous, like on the order of H/40. You're exceeding that?

 

[SteelPE]

I am violating equation 12.8-15 of ASCE 7-05 using an R=3 system. The bases of the moment frames are pinned and the building has a brick facade (making the structure on the heavy side) with a site class E soil.

Since my frames are not equally distributed, if I assume a flexible diaphragm then the frames are not sharing the load in accordance with their stiffness as they would if the diaphragm was rigid.

 

[SteelPE]

Oh and I am limiting the inelastic deflection to H/50 not H/40. I can never really understand the provision for the interior walls, partitions and ceilings being designed to accommodate the story drifts. What does that entail? The H/40 would solve my problem.

 

[structuralengr89]

On sheet 124 of ASCE7-05 figure 12.3-1 says a diaphragm is flexible if the maximum drift deflection > 2x average drift of the vertical element.

Pull out your Vulcraft deck manual and look in the diaphram section and compute your deck deflection (there's an example. Run your analysis as a rigid diaphragm and if your deck doesn't deflect more than 2 times your drift, your good- At least that's how I do it

It really depends on your length to width ratio of your building.

 

[structuralengr89]

I would never stamp a brick fascide building with H/40 or 50 whether it's addressed in the code or not. I see metal building submittals sometimes where they are using like L/80 or 90, but I wouldn't go less than about L/200 or 240- preferably L/360.

 

[SteelPE]

structuralengr89

The H/50 deflection I am talking about is for the inelastic seismic drift. I had the structure designed for H/500 under 10year wind loads with a site class of D. The geotechnical report came back with a site class of E now I have a seismic drift problem. This should end up pushing my wind drift to less than H/600.

So are you saying that you hold L/200 for inelastic seismic drift?

 

[steellion]

I would assume a flexible diaphragm and beef up the moment frames or design the base plates as fixed to control deflection.

 

[structuralengr89]

What is your SDC? I'm guessing that your site class is now E-that you are in SDC D-F?

If that is the case you can't use R=3.

 

[SteelPE]

steellion:

I have always been told that you would want to avoid fixing the bases of columns.... and fixing the bases has been discouraged many times in this forum.

structuralengr89:

Nope, a site class E puts me in SDC C with a height limit of 65 feet for R=3 (jurisdiction modification to R=3 system).

 

[frv]

SteelPE-

Is there any reason other than cost that you say you'd want to avoid fixing the bases?

It's been my experience that it's darn near impossible to get a typical moment frame to work for drift limits without fixing the bases (obviously the more bays you string together, the more feasible, but the difference is enormous from fixed to pinned)

 

[SteelPE]

frv

I've fixed bases before but most everything I have ever read on this site basically says to not fix the bases of columns.

In this instance I have a partial fixity of 0.4EI/L. This is based upon information from an AISC seminar I attended a few years back.

On top of that, I'm also perpendicular to the exterior wall therefore, the anchor bolts will become a problem.

 

[SteelPE]

frv,

Fixing the bases is going to cause a foundation problem. I am going to have an instance where I am going to have a large base moment with very little axial load to resist the moment which is going to make the foundation to large.

structuralengr89:

I missed your post above about calculating your steel deck deflection. This was basically the question that I was asking. Between my frames I am going to end up with a very small relative deflection of the deck... therefore I would think it would be rigid, however, ASCE 7-05 says that untopped metal decks can be idealized as flexible (which is what I have). Previously, ASCE 7 gave the 2x the building drift as a flexible definition... but that has now been modified. In my instance. My drift is greater than the deflection therefore it should be rigid... This will allow me to distribute the loads equally to all frames (well sort of anyway). If I add another frame then I will be able to get the drift below the allowable.

 

[frv]

SteelPE..


I know that the foundations get bigger. My question was whether it was primarily a cost issue.

Obviously you want to avoid fixing whenever you can, but sometimes you have no way around it. You will try to come up with some odd solutions when in the end the easiest thing to do is to fix the base..

As far as your column being perpendicular to the exterior wall, if there are no property line issues or building footprint issues, you can bury the column below finished floor and extend your footing or pier cap beyond the building footprint line to avoid edge distance issues. Another option is offset base plates, although these can be a hassle.