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Hanger Design for Lateral Seismic Loads

Hanger Design for Lateral Seismic Loads

Hanger Design for Lateral Seismic Loads

I was wondering if I could get some input on this model I've been having trouble with.

Some background on the project (refer to attached sketch):

The structure I'm analyzing is a system of stainless steel beams and hangers for "cassettes" (basically boxes made up of HSS sections that holds water filtration membranes), that are suspended in rectangular concrete water treatment tanks. The beams consist of HSS8x4x0.25 sections, and the hangers consist of bent plate forming a 4"x3"x0.25" channel.

The beams are anchored to the concrete tank walls at either end, and have brackets (c/w pins) along their length upon which the hangers are supported. The hangers have a 1" thick top plate with an oversized slotted hole that is supported eccentrically on the beam bracket. A double-split shaft collar is added to the pin to prevent the cassette from popping off. The hangers are connected to the cassette below with a 0.75" thick plate and a single 0.75" diameter bolt, with double nut and washers. There are three different hanger lengths depending on the tank configuration: 29", 41", and 60".

This is an existing design that the client has been using for years, but it was designed according to UBC 97, and they now need it to comply with IBC 2006. It has to be designed for both gravity and seismic loads. The worst case is when the tank is empty, as the hanging weight of the cassette is around 9000 lbs, compared to only 1000 lbs when the tank is full (due to buoyancy). They've asked for the design to be checked for a range of Sds values (0.25g to 2.50g), as this system is used in water treatment plants all over the US. The idea is to keep the same design, and just increase the member sizes depending on the Sds value.

Regarding my model, I used dimensionless rigid members down and out from the centerline of the beam to try to mimic the bracket plate with the pin. I also used a rigid member for the top plate of the hanger. At the connection of the top plate to the bracket, I've put in a full moment release, to account for the oversized hole and the fact that the ¾" diameter pin really can't take any significant moment. In order for my model to be stable, I have to provide a fixed connection from the hanger bottom plate to the cassette below. The problem with this is my moments are quite large (especially for the 60" long hanger), and the connection consists of only a single bolt. I did an FEA for the bottom plate, and it's overstressed even at low Sds values. The bolt is not working for a lot of cases either, even if I go high-strength.

Ideally, I want to just consider the bottom plate connection as pinned, and add in cross-bracing to take the lateral seismic forces. Unfortunately, the client does not want to make any changes to the design, because any additional steel results in significant costs (it's all stainless steel, and there can be hundreds of these systems in a given treatment plant). They don't want to change the design because they've been doing it this way for years and the likelihood of a seismic event occurring when the tanks are empty is quite low.

I'm struggling with how this system was designed for seismic in the first place, there doesn't seem to be much lateral stiffness at all. Am I missing something? Any input is appreciated.



Sidenote: I used a static seismic analysis. I determined my seismic loads from ASCE 7-05 Chapter 13, Eqn 13.3-1. I used ap = 1.0 and Rp = 3.0, z/h = 0.84, Ip = 1.0, so my load ranged from 0.09W (Sds = 0.25g) to 0.89W (Sds = 2.50g)

RE: Hanger Design for Lateral Seismic Loads


A couple of thoughts:

1.  See the attached sketch...just wondering about how you modeled the hanger to tube connection with the moment pin.  I think my suggestion appears better but what do you think?

2.  Your seismic loads appear to be OK except would Rp = 1.5?  (other mechanical)?

3.  With a single bolt you can sometimes use the bolt/plate combination - sort of like a reinforced concrete beam where the bolt serves as the tension element and the compression side is taken by compressive stress between the bolted plates.  This does then develop a moment connection of sorts.

4.  Hearing "We've been doing it this way for years" sucks.  You might ask them if any of their assemblies have survived a major earthquake and ask them to direct you to the site - you could call to ask how well they survived.  The only way I could see, if truly there are excessive moments at the hanger bottoms, would be that the high sway tendency in the assembly actually negates the seismic forces

5.  Check out ASCE 7-05, section 13.1.4 for exemptions to M/E components....with and using "flexible connections between components" you might argue that the hanging "chandelier" here will simply sway in an earthquake and as long as you have all pins...good to go.


RE: Hanger Design for Lateral Seismic Loads

Thanks for your reply JAE,

In response to you comments:

1. Your modeling suggestion would make my life easier, but I don't think I can model it that way, because it doesn't account for the eccentricity of the top plate. See my mark up of your sketch.

2. I originally had Rp = 1.5, but after a discussion with some colleagues I increased it to 3.0, since it's a pretty ductile system with flexible connections. I did this to try to make my loads more manageable. What are your thoughts on this? Rp = 1.5 seemed to be more applicable to more rigid systems. I still designed my anchors into the tank wall using Rp = 1.5, as per 13.4.2.

3. That's what I'm trying to do, I designed the bolts based on the tension due to the moment divided by the distance from the bolt to the edge of the hanger. The problem is this distance is quite small (1.5" in the weak axis of the hanger) so the tension on the bolt gets extremely high for the higher seismic values. Also, from the FEA of the plate, I'm getting very high localized stress around the bolt hole.

4. Yeah I should ask my client about the seismic performance of their systems. One thing I'm struggling with is he told me that for the previous design, the engineer told him that the 29" long hanger is the worst situation, and can be increased to any length beyond that without issue. However, in my model, the 41" and 60" long hangers result in significantly higher moments in the hanger and at the bottom connection (ie. M = P*L). His argument is the longer the hanger gets, the more flexible it gets and therefore transfers less force. Intuitively, I guess this makes sense, but I have no way of modeling this in RISA. He's basically saying that the system should behave similar to a suspended cable system, which would be somewhat valid if the model was like your sketch. However, because of the eccentric top plate, the hanger has to be stiff enough to transfer the associated moment...otherwise it would just kind of fold in on itself.

5. This is a good point, and it's something I'm aware of. For most instances (Seismic Design Categories A to C), this system is exempt from seismic requirements. However, they need the seismic design for Categories D to F, or when the importance factor is greater than 1, or when the building engineer specifies that the system be designed for seismic even if it would be otherwise exempt (which apparently has happened before).

Again, thanks for your input.  

RE: Hanger Design for Lateral Seismic Loads

Your sketch makes sense...the RISA plot wasn't clear on which end the pin was on.

Have you tried having RISA calculate the period of the hanging system?  A large T can possibly justify reducing the lateral seismic demand.  I don't have time right now to look through the flow of seismic demand calculations for something like this - but usually "palm trees" require much lower seismic demand that "oak trees" and that might explain what the former engineer was talking about.

RE: Hanger Design for Lateral Seismic Loads

For the longest hanger, the period is 0.627s in the Global X direction, and 0.313s in Global Z. Do you have a reference or anything for how to determine seismic demand based on the period?  

RE: Hanger Design for Lateral Seismic Loads

T is in the equations for seismic demand right?

Alternatively you could perform a dynamic analysis (using RISA).  They have a good help menu on the procedure.   

RE: Hanger Design for Lateral Seismic Loads

Are you talking about the Design Response Spectrum, ie. determining Sa for a specific period?

I attempted a dynamic analysis, but the problem is it's not site specific, so I only have a range of Sds values, and no information on the Sd1 or TL values. The TL doesn't really make a difference, but I get different results for different Sd1 values.  

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