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Vertical Bracings takes a considerable amount of the gravity load

Vertical Bracings takes a considerable amount of the gravity load

Vertical Bracings takes a considerable amount of the gravity load

(OP)
when modeled a multistory steel building In SAP2000 or any FEM Program the vertical bracing takes about 35% of the gravity load. I don't Like that; I like designing the columns for the whole gravity load. I want to know what do people do? Do they really design the brace for its share of the gravity load and the columns for the rest?. If not how they model it

RE: Vertical Bracings takes a considerable amount of the gravity load

Columns take all the gravity load. Bracing is for lateral loading.

RE: Vertical Bracings takes a considerable amount of the gravity load

(OP)
Hokie66, how you modeled in SAP2000 or what ever program so that the whole gravity force will transfer to the columns?

RE: Vertical Bracings takes a considerable amount of the gravity load

I don't know about SAP, but just leave out the braces, and the load will go to the columns.

RE: Vertical Bracings takes a considerable amount of the gravity load

(OP)
This means I have to make two models one for the gravity load with no vertical bracing and one with the vertical bracing for lateral loads. and even in the last case with the load combination ( D + L +lateral) the vertical bracing will take again part of the gravity load

RE: Vertical Bracings takes a considerable amount of the gravity load

Oh, the problems of using computer models.

RE: Vertical Bracings takes a considerable amount of the gravity load

Pin the braces and make them tension only.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Vertical Bracings takes a considerable amount of the gravity load

(OP)
Thank You All. I think, yes, the idea is in making the braces to carry tension only.

But I have two other Questions:
do people really do this when modeling a multi-story building?
The other question why not design the columns and bracing to their share of forces especially this is what really should happen in reality; that is the brace will take part of the gravity loads

RE: Vertical Bracings takes a considerable amount of the gravity load

How many storeys are we talking about? What type structural system? What are the typical column and brace sections?

And to answer your question...structural engineers use a wide variety of modelling techniques, including the good old pad and pencil, especially in the preliminary design phase. When it gets to the final design, many of us design the floor system first, run the loads down for the columns, design the columns, and only then refine/design the lateral system. Sometimes there have to be changes to the gravity system to accommodate loads imposed by the lateral forces, but we don't revise the bracing/lateral system to take gravity loads.

RE: Vertical Bracings takes a considerable amount of the gravity load

(OP)
I am talking about a six floors refrigerator building with total height of 25meters. the building dimensions is 7meter width X 22meters length. the system consists of secondary beams simply supported on the main frames which spans the short directions. the lateral stability is achieved through main frames in the short directions and vertical bracing in the long direction between the columns. the vertical bracing configuration is X-Bracing , V-bracing and sometimes a single diagonal; this is because piping is going in and out and the bracing have to avoid them.

most of the columns sections are HEB500. the Vertical bracing are 2channels UPN240 Forming box section
thanks for your quick response

RE: Vertical Bracings takes a considerable amount of the gravity load

In that case, I doubt that any practicing engineer would assign gravity load to the diagonal braces. When we model building structures, we don't try to figure out exactly what path the forces take, we just make sure there is a reliable path. Often, the structure is smarter than us.

RE: Vertical Bracings takes a considerable amount of the gravity load

Not smarter Hokie, just more stubborn.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Vertical Bracings takes a considerable amount of the gravity load

Because sap2000 method of analysis it is inevitable. Your structure is undetermined and the loads be shared between all member in a joint. It is a real event. But in analysis and in vertical loading case, if you don’t like axial force create in bracing if your bracing system is X you must design structure (columns) without bracing elements in vertical loads case and then design all of structure for vertical and lateral load cases.

RE: Vertical Bracings takes a considerable amount of the gravity load

I agree that developing or purposely defining a competent load path can work for some structures but if you have heavy X-braces in a multi-story structure, the X-braces DO take axial load - in reality.

If you imagine a pair of columns with a floor beam below and above, with an X-brace - if the upper two joints move relative to the lower two joints then the X-braces MUST take axial load. There is no getting around this. The question is whether you must consider this axial load in your design (i.e. define a competent load path per hokie66 above).

I think the concern I would have is that if you have a multi-story building with fairly rigid braces which soak up some axial load, perhaps significant axial load, then under a seismic event or wind load condition your brace would have additional axial force where you would have somewhat less capacity due to earlier buckling.

RE: Vertical Bracings takes a considerable amount of the gravity load

If there is a load path for the load to go into the brace then I design the brace accordingly. I have designed plenty of bracing like this. I am currently looking at designing some connections for a fabricator (US procedures) where I suspect the engineer designed the bracing for gravity load (greater compression forces than tension forces). The EOR even specified that no welding is done on the bracing until the slab is poured (good luck with that one when pouring a suspended slab in the winter) to try and reduce the loading on the connections.

You can always adjust your columns (by hand) to carry the full load w/o the bracing. How difficult is that?

RE: Vertical Bracings takes a considerable amount of the gravity load

I see no problem with designing the structure how it will behave, which should be captured by a properly built FEM. As SteelPE said, you can always overdo the columns if you want to sleep a little easier.

RE: Vertical Bracings takes a considerable amount of the gravity load

I design the braces to take their share of the gravity load under gravity+lateral combinations because they will. I also design the columns (and beams if you are in a chevron or x type configuration) to be able to handle 1.2D+1.6L gravity only load cases as if the columns aren't there as a point of personal preference.

As far as having to create two different computer models in order to do the above, boo hoo, it takes 10 extra minutes, try doing it with moment distribution...

RE: Vertical Bracings takes a considerable amount of the gravity load

err...replace columns with braces in second sentence above.

RE: Vertical Bracings takes a considerable amount of the gravity load

(OP)
Thanks Everyone

RE: Vertical Bracings takes a considerable amount of the gravity load

This is one of those strange cases where both are correct. Designing for vertical loads going into the bracing is clearly correct, but experience tells us that designing for only lateral load works. The answer is perhaps in the different member sizes produced by the designs. I tend to think that if a bracing designed for lateral only is ever hit by full lateral load plus its theoretical vertical load, something yields, bolts slip etc., and so adjustment is made.

That's the only explanation I have for the fact that both designs work in practise.

It would bother me to put in columns that were light because some of the D & L loads were in the bracing.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Vertical Bracings takes a considerable amount of the gravity load

If you can define the connection so that it is welded after a significant part of the dead load has been placed (for example, instruct them to install the braces after the concrete has been poured on the floors) you can reduce some of the load into the braces. But they would still see part of the subsequent dead and live loads.

RE: Vertical Bracings takes a considerable amount of the gravity load

There are two ways to do an analysis on these types of buildings and their are two types of programs.

One school of thought is to model everything together (i.e. the ETABs way) and solve it all at once. The other school of thought is to separate out the gravity solution from the lateral solution (i.e. the RISAFloor or RAM method).

Both methods will have their own pros and cons. But, I'd say this thread does a good example of explaining why this 2nd method is relatively popular. If you've got braces that would take significant load out of columns (or beams in a Chevron type frame) under pure gravity loading, then many engineers would prefer the 2nd method over the ETABs way.

RE: Vertical Bracings takes a considerable amount of the gravity load

I think it would be unconservative to completely ignore the compression braces under gravity loads since this will lessen their capacity to take lateral.

RE: Vertical Bracings takes a considerable amount of the gravity load

I don't think I have seen it mentioned in this thread and may have no bearing on the OP's problem although it is relevant. I will inject into the conversation that if there are seismic requirements when using R > 3, then for V and inverted V braces the code AISC 341-05 specifies the beam should be designed assuming the braces provide no support of the dead and live loads.

RE: Vertical Bracings takes a considerable amount of the gravity load

I only skimmed through this post, but I usually make all my vertical braces "inactive" for the gravity cases to be sure that the vertical braces don't rob too much gravity load, then, I run another set of combinations with the braces "active" and design for the worst case.

I would think that the only gravity loads you'd want the braces to be able to withstand would be from column shortening.

RE: Vertical Bracings takes a considerable amount of the gravity load

Haynewp -

If you solve your gravity only load cases (or gravity + notional loads) in your lateral model then the braces will be designed for their share of the gravity loads. That

RE: Vertical Bracings takes a considerable amount of the gravity load

I meant generally speaking. It may be that whatever software you are using can account for the additional compression in the braces from gravity in a lateral model and combine the results. But only solving gravity cases is not going to get you the combined effect on the compression braces.

RE: Vertical Bracings takes a considerable amount of the gravity load

if the bracing takes 35% of the gravity load, then design the columns for 35% more load.

the analysis isnt wrong... load follows stiffness. so your bldg wont fail. your emgineering judgement is right. i'd have the same concerns as you. you just need to go model crazy. just upsize the columns by a reasonable amount and be done with it

RE: Vertical Bracings takes a considerable amount of the gravity load

you DONT need to go model crazy. just add some extra load to the columns

RE: Vertical Bracings takes a considerable amount of the gravity load

Depending on what jurisdiction governs your design you may end up with special loads in the bracing/columns anyway. A jurisdiction that I do a bunch of work it requires the designer to comply with section 8.3 of the AISC Seismic Provisions when selecting a R=3 system. I had to be a little cunning to figure out how to check this inside of the software. Kind of a pain but that is the cost of doing business today.

RE: Vertical Bracings takes a considerable amount of the gravity load

If you assume that the lateral braces are a load path for gravity forces, won't only the top level of columns be undersized? I know a bunch of assumptions come into play (^ bracing, uniformly distributed loads that are equal on all levels, etc.), but how much force really gets taken away from the columns?

"We shape our buildings, thereafter they shape us." -WSC

RE: Vertical Bracings takes a considerable amount of the gravity load

MJB315, A lot of it depends on the angle of the braces (wide, more horizontal vs. steep/more vertical braces).

RE: Vertical Bracings takes a considerable amount of the gravity load

Good morning all,

This one seems to have brought lot of attention and I think points out some limits to consider in the total reliance on computer model results. Most programs in a conventional analysis mode complete the formation of the structural system in a zero gravity environment and then turns on the gravity switch once all the stiffness present (not the way a building is constructed in the real word). This approach will tend to be less accurate for the amount of gravity load present in braces, particularly for upper stories. In fact in high-rise structures it's not unusual to see computer model columns on upper floors go into net tension due to column shortening and load redistribution in the bracing system. Doing a sequential loading model or using a parallel model with reduced brace axial stiffness can be helpful to bracket the solution for the column/brace forces, as others have suggested.

Also if you chose to go with an design that rely on a large portion of the gravity load being carried in the bracing I think you'd have to question the potential ductility and performance of that system. Where does the gravity load go when the brace buckles?

regards,
Michel

RE: Vertical Bracings takes a considerable amount of the gravity load

Hi,

I would design the X-bracing as a rope tension only and the V and single diagonal braces to take their share of the gravity load under gravity+lateral combinations.





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