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Seismic Design of Elevated Silo Framing
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- Subject: Seismic Design of Elevated Silo Framing
- From: "Jim Lutz" <Jim.Lutz(--nospam--at)bhcconsultants.com>
- Date: Thu, 14 Oct 2010 10:43:18 -0700
Rich, first of all you don't want to be using methodology meant for liquid storage tanks. Dividing tank silo loads into impulsive and convective components only applies to liquids. Silos full of sand only have impulsive forces. Convective periods relate to the sloshing component of accelerated liquids. It sounds like you are new to tank design standards. It's a little confusing at first. Look at ASCE 7-05 section 15.3.2 for starters, since the silos will undoubtedly weigh more than 25% of the total structure. Treat the silos filled with sand as if they were rigid, which won't be too far off, and model as described in paragraph 1. ASCE-7 allows you to use the R value for the framing system, but if you wanted to be more prudent, I would go to a reference like AWWA D100 and check the recommended R value for impulsive loads for ground supported or elevated tanks and use the lowest of all the R values. A very flexible support framing system is not what you want, since the tanks will wave all over the place under seismic loads. I would use braced frames on each side and not consider moment frames if you can help it. If you are using modeling software, after you set up your frame model, you can model the effects of each silo's mass by creating a point at each silo center of gravity, and then "locking" them to their appropriate connection points. The vertical location of these points will vary with the assumed fill depth for each silo. Apply silo weight and seismic loads at the center of gravity points, and the frame model will distribute them. You can get away with this because the tanks are essentially rigid compared to the frame, especially when full of sand. You could probably get some peculiar effects depending on which tanks are full and which are empty. Orthogonal load effects are significant in a structure like this, so you will need to consider a lot of potential load combinations. One of the complexities in this sort of problem is that the tanks may not vibrate in phase, since their natural periods will change depending on how full they are. I would not normally get into that level of analysis, but it's something to consider. To model those sorts of effects, you would need to make your model a lot more complicated, and I doubt your client is interested in paying you to do that. The braced frame approach is a good one. You will also want good bracing of the platform, which is a challenge with silos because the usual location for horizontal bracing elements will probably conflict with the funnel location. If you have to design the silos themselves, there are other issues, but it doesn't sound like that is part of your problem. Jim Lutz, PE, SE Senior Structural Engineer BHC Consultants, LLC 1601 5th Avenue, Suite 500 Seattle, WA 98101 Ph: 206-505-3400 Fax: 206-505-3406 www.bhcconsultants.com This email and all attachments are confidential. For further information about emails sent to or from BHC Consultants or if you have received this email in error, please refer to http://www.bhcconsultants.com/Email+Disclaimer+/default.aspx >From digest Oct 8, 2010 ------------------------------------------------------------------------ -- 2 Message:0002 2 ------------------------------------------------------------------------ -- From: <SEAInt05(--nospam--at)lewisengineering.com> To: <seaint(--nospam--at)seaint.org> Subject: SEISMICE DESIGN OF ELEVATED SILO FRAMING This is a multi-part message in MIME format. ------=_NextPart_000_0051_01CB6737.B8428320 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit I working on the seismic design of a steel frame support structure for silos. It will have 6 silos, 12 feet diameter and 38 feet tall. Sand is stored in the silos. I am using ASCE 7-05 and NEHRP. I am also using API 650 and AWWA D100 as a reference. I am using concentrically braced frames for lateral stability. I'm struggling to find determine the seismic forces. Sometimes I'm a slow learner and need good examples to understand technical documents. I can't find good examples for elevated silos or tanks. I know Harold helped write the EXAMPLES chapter 12 for the NEHRP provisions. Unfortunately, the examples for silos or tanks are for ground supported ones, not elevated ones. Perhaps he can expand on it with this post. As I see it, I have two seismic forces. The first is impulsive and the second is convective. The NEHRP example refers to AWWA D100 for some design information. It appears to me that the live load weight in the silos is broken down into 2 categories, one is impulsive weight and the other is convective weight. The 2 add up to 100% of the total live load. Apparently the chart for this breakdown is in AWWA D100 Commentary, Fig A.5 and the API 650 Commentary, Fig E6-8. It would be nice if this was included in ASCE 7 or NEHRP. Instead I had to spend big bucks for the two standards.. But I digress. The charts calls this a liquid ratio. I am assuming that this will also work for sand. Is that a correct assumption? ASCE 7 says you can't estimate the fundamental period with the equations from section 12.8.2.1. They must be calculated. Here is a challenge for me since I just haven't had to do this before. To get the fundamental period of a 3 bay braced frame I started out with determining the stiffness of the assembly. I created a plane frame of the braced system, through 10 kips horizontal on the system, analyzed and printed out the horizontal displacement. I calculated the stiffness K = FORCE/DISPLACEMENT. I then calculated the natural frequency from OMEGA = SQRT(g*K/W). For W I used the dead weight of the frame + tank and the live load of the product. I then calculated the fundamental period T = 2*PI/OMEGA. Is this the right approach? I then stepped through the rest of the calculations to determine the impulsive seismic shear. Next came convective calculations. First, what is the convective period? ASCE 7 15.7.6.1 has an equation in the ground supported tank section. This same equation is mentioned in the ground supported tanks section of AWWA and API, but no mention of it in the elevated tank section, except a statement that it is to be greater than 3 times period T . After much searching it dawned on me that maybe the convective period is just related to a tank and is independent of whether it is ground supported or elevated. So I used the ground support equation to calculate the convective period. Was that a correct assumption? Convective is based on sloshing. The equation for the period does not have anything in it related to the material being sloshed. API 650 is for petroleum and AWWA D100 is for water. The period equations are the same. Is it okay to use this same equation for a granular material such as sand? I would think the wave of the slosh would be smaller with sand than with water. Again, the elevate tank section of ASCE 7 does not have the equations to calculate the convective base shear, like the ground supported section. I used the same equations as the ground supported section ASCE 7 Eq 15.7-10, 11, 12, to get the convection base shear. Was this the right approach? To sum it up, I then added the impulsive and convective base shears to get a total base shear for by braced frame system. Fortunately I did all this in MathCAD so I can make quick modifications. So where might I have to revise this approach. While I'm on my soap box, here is another wish list item I have. I wish someone would write a paper or tutorial on how to use a typical frame analysis program to do a dynamic analysis like an seismic analysis. Again, I like set-by-step procedures that explain what to do and why it is being done, what information is needed, and what does the output actually tell me. I have several books on structural dynamics and I thumb through and see all the matrix equations and spectral response curves, but not practical step-by-step procedure needed for real-world application with a common frame analysis program. I typically use RAM Elements for my analysis, but I have experience using several other packages. If anyone knows of any published papers on the Internet or books to purchase please let me know about them. Look forward to seeing some of the responses. Thanks for your help. Rich ******* ****** ******* ******** ******* ******* ******* *** * Read list FAQ at: http://www.seaint.org/list_FAQ.asp * * This email was sent to you via Structural Engineers * Association of Southern California (SEAOSC) server. To * subscribe (no fee) or UnSubscribe, please go to: * * http://www.seaint.org/sealist1.asp * * Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you * send to the list is public domain and may be re-posted * without your permission. Make sure you visit our web * site at: http://www.seaint.org ******* ****** ****** ****** ******* ****** ****** ********
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