shear force distribution to moment frame and shear core 2

[andrew17]

Hi Guys.

I have a question on shear force distribution to a steel framed building with concrete shear core.

The shear core is not at the centre of the rectangular slab. So the building has some torque. I put a number of moment frame and trying to balance the torque.

is there any simple hand calculation to determine how much shear will go to the frame and how much will go to the shear core?

A typical plan view of building is attached.

 

[asixth]

The stiffness of the core will be greatly stiffer than the frames and will attract the lateral load. Even the box section will resist torsion so not all of it will need to be rectified by the frames.

All about stiffness compatibility. Just looking at it I would say the core would take 90-100% of the total lateral force.

 

[slickdeals]

It makes sense to alleviate the torsion, but is it really necessary? If your core can handle the stresses and you are within code limits, I would think you should let the core do all the work. Doing a moment frame isn't cheap. How tall is this building?

 

[Guest262653]

I totally agree with @slickdeals comment. However, if you're still interested in assessing in a simple way (i.e., other than a full FEM analysis) the relative contribution of the central core and the moment frames, you could take a look at Karoly Zalka's books:

"Global Structural Analysis of Buildings" -

http://www.amazon.com/Global-Structural-Analysis-Buildings-Karoly/dp/0415234832

"Structural Analysis of Regular Multi-Storey Buildings" -

http://www.crcpress.com/product/isbn/9780415595735

Besides load sharing assessment, you'll also find methods for the simple calculation of displacements, vibration frequencies for the first translational and torsional modes, buckling factors, etc. They are great books.

 

[KootK]

Much depends on the height of your building.

In addition to Zalka's books, this one is an excellent old standby for lateral calcs done by hand: Link

I agree with others that the relative stiffness between the shear walls and the moment frames will probably limit the effectiveness of the moment frames. I don't think that the distribution will be quite so lopsided however. Here's why:

1) The concrete core that you've drawn as a box is really a three sided channel. It's got an elevator door opening on at least one side that takes up most of that side and makes coupling across the opening ineffective. A channel's torsional stiffness is drastically less than that of a closed section.

2) The center of rigidity of the system is likely pretty close to the core wall shaft. If the elevator door is on the north side, as is likely, the center of rigidity may be as far south as the south wall of the shaft. The effectiveness of a lateral resisting element varies with the square of its distance from the center of rigidity. That squaring is going to amplify the efficacy of that north wall moment frame.

If this is a low-rise building, doing a hand calculation for the distribution shouldn't be too bad. Estimating the stiffness of the moment frames is a cinch. Here are some pointers regarding the shear walls:

1) For a low-rise building, shear flexibility will govern the behavior of the shaft walls. Flexure and composite section torsion will fade to background noise. This is a big help as it will allow you to treat the shaft as three unconnected walls rather than as a complicated composite assembly. And you can only need to consider shear stiffness. Of course, you should do a quick check to verify shear dominant behavior for your building height.

2) Concrete wall stiffnesses need to be modified for cracking. Steel moment frames don't. This can be troublesome in mixed systems because the concrete cracking factors or pretty gross approximations and can skew the relative force distribution between lateral resisting elements quite a bit. You should play around with your numbers to assess sensitivity.

3) Torsion in the shaft eventually needs to become torsion in your foundations. Keep an eye on that.





The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[andrew17]

It is a 7 story building. I did run a dynamic response spectrum analysis on this building, and turns out only the shear core has the big reactions, all moment frame only has very little shear force...

I would just keep moment frames and play around the number to make them work. It is just moment frame, not braced frame. the cost difference between moment and simple connection isn't too big.

Thank you guys.

 

[KootK]

Unless you're going with bolted moment connections, the cost differential could be substantial. Just the inspection requirements can add up. If the moment frames aren't doing anything, ditch 'em.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[andrew17]

Hi Kootk,

Yes, i just do bolted one with an end plate. The simple shear connection requires 7 bolts and two L plates, as end plate moment connection just eight bolts and a big plate. Really don't think there are any different.

And in actual building shape, the rectangular slab is a little bit longer, so i still next to do a lots of paperwork for pleasing the peer reviews. Also take extra care to detailing the slab to core connection. Something I didn't tell is that this 7 story building will have a lots of shipping container attached to it. The 7 story container has its own braced frame. But the connection is 300mm below the slab level. This building has three different types of lateral force resisting system. Very easy to confuse people, and paper work has to be very clear.

The core foundation is a 9.5x9.5x1m pad, i have made them below the 800mm raft foundation, this will have extra fixity to the tower pad.

 

[KootK]

Thanks for the extra info Andrew. It sounds like an interesting project.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.