Column Brace For Additional Loading 4

[mountaineers19]

I am trying to find alternatives to increasing the capacity of this crane without messing with the foundation if I do not have to. I know the foundation is good for the current loading and according to my model this brace would handle the additional load and help disperse it on a larger effective area on the foundation. Does this seem like a logical fix or would the load just go straight down regardless?

 

[KootK]

Interesting concept. Usually we're trying to get braces to not absorb gravity load.

In my mind, this all comes down to how stiff the full height column is. If it flexes laterally a lot at the top of the brace then the brace won't pick up much vertical load.

So I'd be concerning myself with things like whether or not the top of the full high brace ought to be restrained laterally by a spring that reflects diaphragm stiffness etc.

In principle, I think this could work.

Any chance you could install a similar brace on the other side?

 

[haynewp]

My inclination is to reinforce the column instead of trying to load share with the brace, especially if this is on shallow foundations as it sounds like it is. Even if you have to do something to the foundation.

 

[human909]

I would suggest that you improve your model. Don't just trust computer output. Your assumptions are quite incorrect. (Eg your rigid ground restraints are unrealistic in such a scenario when tiny changes in deflection hence strain will massively change the load path.)

The column is likely to be at least an order of magnitude stiffer than the brace both due to lateral effects and minor settlement effects. Even have 0.5mm of settlement would likely completely unload the brace. Likewise bolt hole movement can unload the brace (though this somewhat overcome by friction bolting or preloading).

Even for a new build this configuration can be challenging to suitably distribute loads. For a modification it becomes significantly harder.

I suggest you use a different approach though there is insufficient information provided to suggest much.

 

[human909]

Here is a quick example. 4 scenarios. Just slightly changing the boundary conditions in each one to become more realistic. It progressively gets worse until I add in vertical springs for the footings. But all this is assuming new construction without existing consolidation and without connection slip or yielding. Both would reduce the load on the brace.



The is a reasonably likelihood your existing foundation is good unless there is a massive increase in capacity. (Which by your proposed solution might not be the case.) If your column is under capacity then there is a heap of easy options to improve that.

 

[mountaineers19]

Here is a quick example. 4 scenarios. Just slightly changing the boundary conditions in each one to become more realistic. It progressively gets worse until I add in vertical springs for the footings. But all this is assuming new construction without existing consolidation and without connection slip or yielding. Both would reduce the load on the brace.

View attachment 2000

The is a reasonably likelihood your existing foundation is good unless there is a massive increase in capacity. (Which by your proposed solution might not be the case.) If your column is under capacity then there is a heap of easy options to improve that.

The column is indeed under capacity

 

[271828]

If the brace attracts axial force then that axial force has a horizontal component. What resists that?

 

[mountaineers19]

Interesting concept. Usually we're trying to get braces to not absorb gravity load.

In my mind, this all comes down to how stiff the full height column is. If it flexes laterally a lot at the top of the brace then the brace won't pick up much vertical load.

So I'd be concerning myself with things like whether or not the top of the full high brace ought to be restrained laterally by a spring that reflects diaphragm stiffness etc.

In principle, I think this could work.

Any chance you could install a similar brace on the other side?

Unfortunately no due to spaces requirements for aisle ways.

 

[SWComposites]

human909 - yay, at least someone understands the limitations of FEA and the significant effects that can result from subtle boundary condition changes. I see all too many people blindly believing FEA results when they don't get any error messages. Stars for you!​

 

[HTURKAK]

mountaineers19 said ;​

The column is indeed under capacity

You are trying to find alternatives to increase the capacity of the crane without messing with the foundation and your first option is the bracing of the columns . My initial thought is you have problem with bending capacity of the column rather than axial load . Is this true ?
Can you provide more info . ( sizes, no. of bays ,crane loads , bracings..) ?
What about the horizontal bracings at roof level along the bldg?

 

[mountaineers19]

human909 - yay, at least someone understands the limitations of FEA and the significant effects that can result from subtle boundary condition changes. I see all too many people blindly believing FEA results when they don't get any error messages. Stars for you!​

Apologies I just threw that together real quick for visual purposes.

 

[mountaineers19]

You are trying to find alternatives to increase the capacity of the crane without messing with the foundation and your first option is the bracing of the columns . My initial thought is you have problem with bending capacity of the column rather than axial load . Is this true ?
Can you provide more info . ( sizes, no. of bays ,crane loads , bracings..) ?
What about the horizontal bracings at roof level along the bldg?

No issue is the increase in axial load transferred from the column to the foundation. 50 foot runways and and roughly 47.5 feet wide.

 

[mountaineers19]

If the brace attracts axial force then that axial force has a horizontal component. What resists that?

There are plates at the top of column and top of runway braced back to the building column to resists axial loads from the crane.

 

[human909]

No issue is the increase in axial load transferred from the column to the foundation. 50 foot runways and and roughly 47.5 feet wide.

And your solution doesn't appear to meaningfully decrease the load on the foundation (Even if you model suggests it does.). I start by crunching the numbers on the foundation. (If you don't know the size then find out.)

There are plates at the top of column and top of runway braced back to the building column to resists axial loads from the crane.

Yes. Many of us could see that and assumed that. But how stiff is that column and it's lateral restraint? Have you modelled the entire structure? The lateral stiffness of the both columns and the entire structure heavily influence the load taken by the brace. As my text and my example shows subtle changes in the boundary condition change the result very easily in this setup,

My advice is that you do not proceed with this approach. It very hard to make work due to the multitude of reasons already mentioned. And unless you can grasp and understand and account for every single one of them then you won't achieve the outcome your software package is telling you.

 

[271828]

My advice is that you do not proceed with this approach. It very hard to make work due to the multitude of reasons already mentioned. And unless you can grasp and understand and account for every single one of them then you won't achieve the outcome your software package is telling you.

I'll second that. There's little or not chance the column / brace combo will behave like your model.

 

[mountaineers19]

No issue is the increase in axial load transferred from the column to the foundation. 50 foot runways and and roughly 47.5 feet wide.

I think I may have been confusing with my wording for my question. The steel for this system is good for the load increase but the foundation is not. I'm in a situation where doing concrete work is to be avoided if at all possible so my thoughts are to somehow increase the effective area for the increased axial load to transfer to the spread footer. It is a 25% increase in weight and my main concern is the soil bearing capacity. This was a crane system installed over 20 years ago.

 

[SWComposites]

Put a 6 inch thick steel plate under the existing column to spread the load.

 

[human909]

I think I may have been confusing with my wording for my question. The steel for this system is good for the load increase but the foundation is not. I'm in a situation where doing concrete work is to be avoided if at all possible so my thoughts are to somehow increase the effective area for the increased axial load to transfer to the spread footer. It is a 25% increase in weight and my main concern is the soil bearing capacity. This was a crane system installed over 20 years ago.

All that was understood in your first post.

But your approach to 'spreading' the load is simply not workable.

You computer model is WRONG.

You are FAR FAR more likely to squeeze 25% more bearing capacity out earth under a 20 year old foundation that has had 20 years to settle and consolidate. It is quite unlikely that that 25% increase in load is going do much. Maybe settle a couple more mm over the near 20 years....

(EDIT: Of course I don't know your foundation conditions so maybe you can't. But it seems you are fixate on non workable solution and have a "belief" in your model. This belief without understanding is a little scary, especially because you don't seem to have taken onboard the feedback from this thread.)

 

[mountaineers19]

All that was understood in your first post.

But your approach to 'spreading' the load is simply not workable.

You computer model is WRONG.

You are FAR FAR more likely to squeeze 25% more bearing capacity out earth under a 20 year old foundation that has had 20 years to settle and consolidate. It is quite unlikely that that 25% increase in load is going do much. Maybe settle a couple more mm over the near 20 years....

(EDIT: Of course I don't know your foundation conditions so maybe you can't. But it seems you are fixate on non workable solution and have a "belief" in your model. This belief without understanding is a little scary, especially because you don't seem to have taken onboard the feedback from this thread.)

Dude I know computer models are not end all be all. Reason I posed the question was to get feedback or other alternatives. I never said I believe this will work 100%, I wouldn’t be posing the question if I was. I really do appreciate your feedback. I was more pointing towards the question for one scenario this seemed to do something and wanted to see if anyone had any experience in the matter. Don’t have to get all condescending.

 

[KootK]

@mountaineers19 : some questions that may allow us to better serve you.

1) Is your foundation issue one of punching shear?

2) Are we talking about a discrete, pad footing? Or a raft?