V Inverted Bracing in Crane Buildings 1

[TekEngr]

I am designing a shed which includes 10 ton capacity crane so to resists the longitudinal force (6 KN) due to the crane movement I am providing the V inverted bracing up to almost half length of the columns as shown in attached picture I cannot add bracing up to bottom of the column due to some clearance restrictions so my question is does this type of bracing will transfer the loads to the foundation properly however I check the drifting in SAP model which is under the allowable.
Or I should provide the portal frame under the inverted bracing,please suggest regarding this matter

Thanks in advance

Regard’s
Ammar

 

[KootK]

So long as it's not a seismic application meant to be highly ductile, I see no reason that this couldn't be made to work. It's essentially knee bracing taken to the extreme. If you could install a horizontal member connecting the bottoms of the two braces, that would definitely be an improvement.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bowlingdanish]

The tension/compression in the struts from lateral loads will need to be transferred to the foundation through the columns, in shear, bending and axial forces. If the columns have the capacity to resist those internal actions, I can't see an issue with that arrangement. There may well be a better way to achieve what you want though, if you provide more information about the full structure.

 

[HoboMoe]

If you could install a horizontal member connecting the bottoms of the two braces, that would definitely be an improvement.

Why? In my opinion the unbraced length would not be changed = 3.5 m. And the moment in the base of the column depends only on this. Diagonal member from the base of the column to the bottom of the brace would be much better.

 

[TekEngr]

thanks for your kind reply
i am attaching the SAP Model for your better understanding please review
some where i read that we should provide the bracing from top to the bottom of the column so that the forces transfer into the foundation through base plate.
can i use portal frame with this type of inverted bracing for the better result and for proper transformation of loading ???

 

[KootK]

Why? In my opinion the unbraced length would not be changed = 3.5 m.

With the horizontal member:

1) the frame would be much stiffer laterally.
2) the frame would be much stiffer vertically.
3) the demand on the roof beam would be much less.
4) the columns wouldn't have mid height, transverse loads due to gravity.

In my opinion, the effective length situation is rather complex. KL would be in excess of 7m for pinned bases and would be greater without the horizontal member than with it.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[HoboMoe]

can i use portal frame

Unfortunately, I don't have SAP, but I think portal braced frame is exactly what you need. See the C-type on this picture

 

[HoboMoe]

With the horizontal member:

Yes, you are right, thanx. I have never used such type of frame like shahg123 showed. Now I see it much more clearly.

 

[KootK]

I have never used such type of frame like shahg123 showed.

Me neither really. I've just been drawing parallels to knee braced systems.

some where i read that we should provide the bracing from top to the bottom of the column so that the forces transfer into the foundation through base plate.

That's generally the most efficient way to go about it. It's not the only way to get the job done however.

can i use portal frame with this type of inverted bracing for the better result and for proper transformation of loading ???

Could you post a sketch of what this system would look like? I'm having a tough time visualizing it.

In my experience, many knee braced frames can be constructed more economically using conventional moment frames with bolted beam-column connections. There are exceptions, of course, and this may well be one of them. Still, I'd give some consideration to conventional moment frame.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ajh1]

One f the down sides of knee brace type framing is that they can be fairly soft in terms of lateral deflections under load. This could be particularly problematic for such a system taking the longitudinal or lateral forces from a top-running crane system. A rigid portal made up of a beam and two columns with the columns oriented in their strong axis for resisting the forces is going to be considerably stiffer than a knee brace type of system. Not as stiff as diagonal bracing to the ground, but better.

 

[KootK]

A rigid portal made up of a beam and two columns with the columns oriented in their strong axis for resisting the forces is going to be considerably stiffer than a knee brace type of system.

I'm surprised to hear this. I've always considered a knee braced frame to essentially be just a moment frame with really good beam-column connections (the knee). Does extra flexibility accrue in the knee connections etc.? I'm assuming the same beams and columns and the same orientations for the two systems.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAGW]

@K
I think unbraced length for the column with the knee brace is going to be L=4.2m for the shorter column and L=5.0m for the longer column. K is always equal to 1.0 if direct analysis method is used. So KL will be 4.2m for the shorter column and 5.0m for the longer column. I have always assumed members to brace each other when they are framing within 45 degree angle. Knee braces as shown in the diagram seems to frame into column at approx 45 degrees and can be assumed to brace the column irrespective of whether there is a horizontal beam or not at that location. The only other thing I would verify is, if the horizontal component of the knee brace capacity is greater than 2% of axial force expected in the column at that location.
I completely agree with you with addition of horizontal beam, the system behaves better.

 

[hokie66]

Where is the crane runway in your picture? Is the runway supported on brackets off these columns? If that is the case, the columns may need some help torsionally.

 

[KootK]

K is always equal to 1.0 if direct analysis method is used.

I agree. However:

1) We don't yet know if DAM is being used and;

2) Without making reference to the effective length method (K), we really have no basis upon which to discuss stability considerations. K is still very useful for that.

I think unbraced length for the column with the knee brace is going to be L=4.2m for the shorter column and L=5.0m

It sounds as though your are talking about the upper portions of the columns. I was speaking of the lower portions of the columns where KL > 2.0 x 3.5 m > 7m.

I have always assumed members to brace each other when they are framing within 45 degree angle.

An important to remember about these "braces": the far ends of them move. That's important as, normally, the far ends of braces stay put. If one placed a pin at the center of the upper beam, the effective length of the columns would be their full height, even if the top of the frame were restrained from moving laterally.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TekEngr]

i am attaching the different bracing system please review that and suggest me which option is more suitable to resist the longitudinal displacement in my case ?
crane runway beam is resting on the bracket of the main frame column however my crane bracket level is 5500 mm from the column base plate.
what your suggestion about the level of the portal frame ?? in my opinion the level of the portal frame beam and runway beam should be same so it can resist more in longitudinal displacement what is you suggestion ??

 

[ajh1]

I've been away from the forum for a couple of days off on some business travel.
In reference to KootK question to my original quote: A lot of it depends on how large the knee brace dimensions get. If they are minimal and you simply have a "large knee", then your limiting criteria from a deflection standpoint is going to be the stiffness of the column below the brace. Typically for this sort of layout that column may well be oriented in its weak axis for the brace forces. If the brace dimensions get too large you are working over into the K or inverted V bracing system. Depending on the stiffness of the beam a full inverted V brace will certainly give better results than any sort of a moment frame. The primary benefit of the rigid frame approach in this area is that you are working with the strong axis of the columns and can get a lot of stiffness there. In addition, if you had to take it there, putting fixed base plates on the column would further enhance the stiffness and deflection characteristics.

 

[KootK]

Thanks for the clarification ajh1.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.