Question - I-Beam Stiffening/Strengthening, Bolted connections rather than welded? 3

[human909]

Hi all. I though I'd throw this question out to the masses for a few ideas.

I need to seriously stiffen some members due to some vibrational issue. Cost of steel isn't really the issue it is more labour and looking for the easier and quickest solution to stiffen some existing beams. I'm mostly looking at three options and are after opinions but other suggestion are welcome.

I also want to know how effective bolting beams together can be and whether adequately rigid shear connection can be achieve to ensure that the beam acts compositely with the added member. I would presume slip critical bolting would be required and so make the whole process more difficult and possibly impractical.

Options I'm considering:
1: Welded T piece
2: Welded Wide flange (advantage being downwards facing welds)
3: Bolted member identical to existing (advantage being avoids site welds, but might be negated but number of bolts and slip critical requirements)

Overall I'm thinking option 2 is best but I'd like further input, particularly regarding option 3. Thanks.

 

[CrabbyT]

I don't think bolted is a good idea. Vibration will inevitably loosen the nuts and relieve pretension.

Not sure how bad your vibration issue is, but conceptually, I think #1 with a CJP weld is probably the soundest option. Gut says #2 could be susceptible to cracked welds.

If it's an option, I'd consider isolation dampers for whatever is vibrating before doing anything structurally though.

 

[human909]

Thanks for your response CrabbyT.

I don't think bolted is a good idea. Vibration will inevitably loosen the nuts and relieve pretension.

I agree it isn't the best idea. While I'd expect and hope that suitable tensioned bolts should maintain their tension is it worth the risk? Besides it isn't clear the amount of work required would be more economical than welding.

Gut says #2 could be susceptible to cracked welds.

Would you mind elaborating on why you feel this in you gut? Loads overall are relatively low.

If it's an option, I'd consider isolation dampers for whatever is vibrating before doing anything structurally though.

That have and are still being considered. When I've designed whole systems for this equipment previously I've achieved excellent outcomes when I've combined isolation dampeners with appropriately stiff supports. I've seen less ideal but passable outcomes when only isolation or only floor stiffness is used. (except slabs on ground!)

The current situation with negligible isolation and floor with a natural frequency close to the equipment frequency is no at all suitable. It is my belief (gut feeling) that while isolation mounts will help alot, there will still be a resonance problem as the floor will be closely matching the resonance frequency of the equipment.

I would welcome further thoughts if you have them.

 

[saplanti]

I agree with CrabbyT,

If you are after stiffness Option 1 is the way to follow. Think about other option on how they will transfer load and vibration. Especially the third option is not going to provide proper stiffness, but it may partially or fully dampen vertical vibrations with friction if the bolts can stay without getting loose (that is a question).

 

[dik]

Option 1 is common and with normal fillet welds. Depending on the loading and use the fillet welds can be intermittent. Because of vibration intermittent weld should not be used. Option 2 is less common but can be done due and has the advantage of downturn welds (or whatever people call them), with the same caveats as Option 1. I've not seen Option 3, but about a year back I was involved in a project where the owner had an S section crane beam that the bottom flange was badly worn and he wanted to remove the bottom flange and bolt a WT section to it using slip critical fasteners. (There's a thread somewhere on the site) I didn't like the T to T connection due to eccentricity and used plates on each side. The work they did in the plant precluded welding. In 50 years, it the only time I've used bolts for this type of work.

thread507-492757

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[human909]

Option 1 is common and with normal fillet welds. Depending on the loading and use the fillet welds can be intermittent

Yes. I think it is best to stick with Option 1. I had already figure that was the preferred structural solution, but requests were made for a solution that could be more easily implemented. I think I'll stick to the tried and true approach rather than being all too clever.

 

[dik]

OP See the added note about intermittent welding...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[ALK2415]

For third type solution
bolts loosening (due to vibration) could be prevented by Nord-Lock [Wedge-Locking Washers]
Link

 

[desertfox]

Hi human909
My only concern about option 1 is whether you can size the welds such that fatigue cracks aren't an issue, also how do you know that the section you are providing is correct to stop the vibration that the beams are seeing?

If you were to bolt a section onto the existing beams and use Norlock washers as adn26 mentioned you could try different sections till you get a solution whereas once a stiffener is welded its not as easy to replace.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[human909]

Again, I thank all those who have commented. I am taking all comments onboard.

My only concern about option 1 is whether you can size the welds such that fatigue cracks aren't an issue,

What indications are there that fatigue cracks are at all an issue compared to any other weld on any other beam in an industrial setting? Yes there is vibration here, but the point of stiffening it is to mitigate/remove the vibrational issues.

also how do you know that the section you are providing is correct to stop the vibration that the beams are seeing?

Through calculation. Increasing the stiffness such that the member's modal natural frequency is well away from frequency of the equipment vibration. Right now the flooring is sufficient close that it is vibrating severely with the same frequency as the equipment.

I've decided to place less emphasis on the calculation of the natural frequency of the existing beams and instead use direct measurement to see how the current structure behaves. Why calculate approximately when you can directly measure?

If you were to bolt a section onto the existing beams and use Norlock washers as adn26 mentioned you could try different sections till you get a solution whereas once a stiffener is welded its not as easy to replace.

I prefer to provide a solution once. Hence gathering sufficient data for an effective solution. As previously said I've very effectively designed supports for this type of equipment in the past. This one is a retrofit on a structure that I did not design.

 

[Brad805]

Unless you are not permitted to weld, I am not sure that your bolts would be cheaper. If the existing section is bent, bolt alignment would be great fun. So would the reality of using a mag drill to drill and an endless number of bolt holes in the field that need to align with the off site fabricated parts +- 1/16".

 

[KootK]

@human909: is this a vibration problem associated with truly vibratory loads (machines etc)? If not, more permissive things may be possible.

Lots of folks seem to like your option #2 these days, for exactly the reason that you quoted. And, properly handled, I think that could work here.

Gut says #2 could be susceptible to cracked welds.

Would you mind elaborating on why you feel this in you gut?

I'll let CrabbyT speak for himself but I know where my concerns for cracked welds come from. Last year, we had a wildly entertaining theoretical discussion about the need for the low piece in option #2 to transfer it's shear up to the high piece if the reinforcement was partial, as I suspect it may be. Cool stuff that I suspect you'll enjoy if you have the time to digest it: Link.

Long story short = my concern goes away with some simple, strategic stiffener detailing.

 

[human909]

Thanks KootK to give me more confidence to go with option #2 and intermittent welds. I'll take on board your comments regarding strategic stiffener detailing. I will take the time to digest that thread later, thanks. Also good to know others are doing option #2, I hadn't actually seen examples of it, you know me just making up stuff as I go along...

@human909: is this a vibration problem associated with truly vibratory loads (machines etc)? If not, more permissive things may be possible.

Yes it is very much a true vibration loads. It is a sieve, two in fact on the floor, with several tonnes vibrating of sprung mass with a frequency of 16hz.
So yeah, kinda a big deal when it comes to vibrations that agitate supporting beams, entire floors and once that all gets going, entire buildings. Apparently it is teeth chattering to stand next to it at the moment and can be readily felt 25m+ away.

Lets not go into WHY this new equipment was installed without the owner checking that the floor was up to task. We are about fixing problems here.

Isolation dampeners might also be needed but the client is not keen on this so I'll hold off on insisting on them for the moment. Like I said earlier, the majority of installs I've seen don't have them and the owners simply put up with an unpleasant but not intolerable vibration. The recent design and build install that I personally designed we used isolation dampeners and increased stiffness and standing next to the equipment you basically can't feel a thing.

 

[canwesteng]

I would assume the sieves already have isolators on the feet, in which case adding dampers will only make things worse, unless you find a way to damp the beams (maybe sandwich some fabreeka between the two beams?). If you have no isolators, it would be trivial to select some that are appropriate for 16 Hz. I've only ever used option 1 as a repair myself - if you to go cheap, you can just sit a beam directly below the existing beam and assume no composite behaviour at all - just the additive stiffness of a second beam. Personally, I'd bite the bullet and weld the T.

 

[human909]

Thanks again canwest. A star for your input.

I would assume the sieves already have isolators on the feet, in which case adding dampers will only make things worse, unless you find a way to damp the beams (maybe sandwich some fabreeka between the two beams?).

They have "isolators" as in SPRINGS that largely isolate their movement. But not dampers that absorb energy as that would defeat the purpose of making the equipment vibrate. To properly isolate and dampen the transferred vibration you need a secondary isolation and damped system, AKA SPRINGS and DAMPER. Suitable dampers will not make things worse and are quite effective on these machines.

If you have no isolators, it would be trivial to select some that are appropriate for 16 Hz.

With the energy an mass involved I haven't found it trivial and certainly not cheap to find suitable dampers. If you could point me to an example then I'd be interested. We will be stiffening the structure, but dampers are also open to consideration in addition.

I've only ever used option 1 as a repair myself - if you to go cheap, you can just sit a beam directly below the existing beam and assume no composite behaviour at all - just the additive stiffness of a second beam. Personally, I'd bite the bullet and weld the T.

I want composite behaviour to get suitable stiffness. I'll be going from a natural beam frequency of 12-14hz (a calculation, TBC on site) to at least double that.

Just to confirm. Your preference for the T is because of fatigue cracks in option #2? Or is it some other reason.

 

[canwesteng]

My preference for the T is just because that's what I've done in the past. I'd have no issues with sketch #2 if the weld is easily accessible.

For this, it's important to differentiate between dampers and isolators. Isolators will have a much lower natural frequency than the forcing frequency and reduce the force transmissibility to the structure below. Likely for a sieve operating at 16 Hz, force is probably reduced 70%-90%. Dampers will remove energy from the system, which is important at resonance but as the ratio of forcing frequency to natural frequency exceeds 1.414, increased damping increases force transmissibility (not very much...). So assuming the isolators are properly sized, probably not much is to be gained adding dampers there. What you need in the system is some way of damping the beam - not exactly impossible but I've never gone that route. The most effective way to solve the problem is to stiffen the beam as you've shown - unless there are no isolators on the sieve, in which case it's probably an easy fix.

 

[human909]

Thanks again canwesteng. And I think you have answered all my questions.

For further information the vibration is driven by a 2 six pole motors operating at 2.2kw. The vibrating mass is ~3000kg.

You are correct the existing springs DO reduce the force transmission as naturally you want most of the energy going into the vibration of the equipment and not the structure. According to the manufacturer they reduce it by 95-98%. So we are clearly most of the way there. But that 2-5% remains an issue when you have a structure that is close to the resonance frequency. Both the manufacturers recommendations and my own experience is evidence that that remaining 2-5% can be reduced by a factor of 5-10x with secondary spring & dampeners. So that is an option, but I'm prioritising the stiffening of the local structure.

Once I can obtain the TRUE natural frequency of the floor beams I will be modelling the whole system with harmonic inputs and see what I get. I can approximately calculate the natural frequency but measurements will give me the observed value.

I expect the solution I will choose be option #2 and I have already done initial calculations for suitable member sizes. But I am doing site investigations at 2 different sites to ensure I can properly model and understand the behaviour before just blindly throwing steel at the problem.

 

[saplanti]

In case of vibration I do not like fillet weld goes in bending in cross direction. Option 2 will have this. You need to think about the vibration contribution of the lower beam (mass)to the system since they are connected at flange ends, and behaviour of fillet weld. You may probable be obliged to investigate the system by FEA.

Additionally, some time later they will see a beam look like too strong (Option 2 scenario), and it will be additionally loaded without even looking at its load path. You should be ready and provide warning on your drawings for additional investigation for this kind of scenarios, perhaps written warning underside of the beam.

 

[NisD]

I will go to option 2 or 3 modified version

Option 3
Bolt 2 beams together, and do all around fillet weld, bevel if you have to, plus, some slot or plug weld, this way you combine the strength of the bolt and welding together at the cheapest cost

on option 2, you can omit the bevelling

 

[human909]

Thanks for the further responses.

Interestingly enough my initial calculations suggest the natural frequency of the floor is below the 16hz and I'm going to have to stiffen quite a few beams if I really want to escape that 16hz zone by a significant margin. And going back over some of my successful design installations, their calculated behaviour isn't vastly superior to this problematic install. So this certainly deserves more investigation before diving completely head first into a stiffening solution.

I'll be on site next week and be trying to chase the core problem(s). There are a few equipment support detailing that seems a little poorly though out so I'll be investigating that, and pretty much everything else.

I'll update this thread at some point, though depending on how things evolve that might not be until a successful solution is implemented.