Chevron, Inverted V, X-bracing 2

[chluk2425]

I am currently working on a steel design project which involves designing bracing system for a multistory building. The all elements and pin-ended and lateral load is resisted only by the bracings. I am now deciding what type of bracing I should use and have got a question on it:

(Refer to the attached file)
For a X-bracing, which across two storeys, the intersecting point of the two diagonal bracings will connect to the beam at the storey in between, basically to reduce the effective length, but for such arrangement, practically will the be a difference to an inverted V + chevron type bracing? I mean, say two big X-bracings across L1&L3 and L3&L5, what's the difference between this and a: chevron in L1->L2, inverted V in L2->L3, then chevron in L3->L4, then an inverted V in L4->L5?

Thank you

 

[ishvaaag]

These things depend a bit on the case, if the structure is small better do the two examples and decide. Except by forcing tension-only etc it is difficult the braces not to alter the vertical allocation of loads to columns, and you may prefer some columns be better more loaded or the braces, or you can operate on such things to get a more uniform and buildable structure.

 

[ToadJones]

Usually the things you are asking about are dictated by where openings are required (in my experience). Saying what is the best way is hard to do.
Also be careful with your force transfer through the inter-story beam if you decide to connect your "x-brace" at this location.

 

[chluk2425]

thanks for the replies!

i am not actually worrying about the openings, well not particularly worry; i am more interested in the deflection, ultimately i need to calculate the total deflection of the building the virtual work method (total deflection = shear deflection + bending deflection)

I initially thought of using big X bracings across cause that's the most efficient way but due to the bay arrangement, having a cross bracing with the angle of 27degree diagonal doesn't look efficient... then i thought of using chevron bracing and a big X across two storeys, but basically this two things are the same wouldn't it? in terms of deflection

 

[ishvaaag]

Quite likely the one that has a V at the lower level must have a bit more drift, since the lower columns are not then helped by the diagonals to reduce their tensile or compressive stress and then at the root of the cantilever you will have bigger rotation, and -since the more influential zone for drift- more drift.

 

[steellion]

As a slight aside, I generally do not assume that the brace reduces the unbraced length of the beam. Think about how it will be attached, and what if the brace is damaged in a severe load case? There are other ways to brace the beam, as long as you do not have a floor opening on each side of the frame.

 

[sandman21]

Is seismic an issue, AISC 341?

 

[jdgengineer]

This question largely depends on whether or not you need to follow the Seismic Provisions of AISC 341. If you do need to follow these provisions, and you are designing the brace frame as a special concentric braced frame (SCBF), then there are differences between the configurations. The chevron brace and inverted V will behave similarly, but the beam that the braces meet at will need to be designed for the unbalanced loading from the braces. See AISC 341 for further information, (It's essentially 100% tension force and 30% compression force). The two-story X frame does not have this requirement as there are no unbalanced forces on the beam.

When given a choice, it's my opinion that the two-story X frame is always the better choice. However, a lot of the time this is not architecturally feasible, and one of the V braces are necessary in order to fit in doors/windows.

I expect the differences in the drift between the three configurations to be minor, although I haven't personally compared them. From what I remember when I had to do virtual work in school, the brace is the only thing contributing to the deflection as all the other connections are designed as pinned (unless you have fixed column bases). Although, a simple SAP or ETABS model would allow you to do alot of these calculations quickly. Any reason you are avoiding this software?

 

[paddingtongreen]

The top chevron induces moments into the roof beam.
The joint directly below may put moments into the columns.
If the bracing is a tower (between only two columns)those columns will strain more than the rest because only they resist moment from the horizontal loads.

I would not worry about the flat angle, they resist a horizontal load so it is efficient that way.

If possible, I would stagger the bracing in different bays at different levels to spread the overturning moment out to all the columns.

If you must have a bracing tower, see if you can brace all bays across one level, high up in the building, this will transfer some of the overturning to all the columns and reduce lateral deflection.

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

 

[chluk2425]

ishvaaag: the screen shot is just part of the structure, it ends with an inverted V instead of a V type bracings

jdgengineer: thanks for your detail reply.
Seismic is not an issue. you mentioned chevron and inverted V will have unbalanced loading on the beam, but isn't a two-story X frame behaves the same? or do you mean it spans across two levels without connecting to the beam, so effectively the bracing is slightly offset from the building face?

I am actually comparing the values from hand calculation with ETABS, and from what I get in hand cal is about twice I got in ETABS, that's why I raise the question here on the actual behavior to understand whether I had model it wrong or not.

paddingtongreen: all connections are pinned, so there shouldn't be any moment in it; for flat angle, I thought the angle will affect the shear rigidity? staggering the bracing at different is not quite a choice, cause the spanning on one side is 14m and the bracing will be very long and slender if doing that.

 

[paddingtongreen]

chluk2425,
"for flat angle, I thought the angle will affect the shear rigidity"

Rigidity is adversely affected when the short direction is parallel to the resisted force; in your case, a vertical leg longer than the horizontal would tend to decrease stiffness.

Try a simple example for yourself, it's the only way to be sure.

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

 

[ishvaaag]

I was just pointing that likely -everything else being equal- the scheme with the magenta braces would be showing more lateral deflection.

 

[chluk2425]

thank you for all information gave