How to manage too much wind load on staircase tower 1

[Chandan321]

I am designing a steel staircase tower 4m x 6m in plan area and 63m in height, divided into 2.4m high flights. The following are the details of the structure
---- At each landing level, there are 4 steel beams all around.
---- The steel staircase tower is supported on 4 pedestals.
---- I have used k brace at all elevations in both directions.
---- Basic wind speed is 50m/s.
---- I have applied wind load as per standard Indian codes.
---- I have divided the height of the structure into 4 groups as per wind pressure zones.
---- I have applied wind load on all beams, columns and bracings in each direction separately
---- I have used a shielding factor for farther frames
---- Dead load = 1.2KN/m2, Live load = 3 KN/m2

I am getting too much tension in the anchor bolts at the top of the pedestals. I know from previous project information that 8 bolts of 1.5 inches would be sufficient. But, my tension is way too much high.

Guys, I want your suggestions on how to reduce the wind load on the structure so that I can reduce the tension in the anchor bolts.

 

[phuduhudu]

Surely the staircase structure accesses a building at each storey and can be stabilised off the building. I can imagine that you would get massive tensions of a 63m high tower with a plan width of 4m. If the structure is stabilised off the building then you will only need locating bolts.

Carl Bauer

http://www.bauerconsultbotswana.com

 

[hokie66]

Instead of assuming that your wind loading is incorrect, I would first check whether your "previous project information" is correct or not. What uplift force are you getting at each leg? Do you mean a total of 8-1.5" bolts (2 at each corner)?

 

[csd72]

Use a higher grade anchor bolt.

 

[Chandan321]

Thanks to all of you guys for your prompt suggestions on my problem. Here are my views:

---- Carlbauer, I started off using your assumption of stabilising off the building, but my lead has specifically asked my to design the stairway independently.

---- Hokie66, my uplift force is nearly 2200 KN for each pedestal. I have 3 bolts equally spaced along each side of the base plate, totalling 8 bolts for one pedestal.

---- csd72, no one has ever used more than 1.5" bolts for such structure. So, I want to be very sure that there is no other alternative before I increase the size of the bolts.

 

[csd72]

I am saying a higher stress grade rather than size, you can get anchors up to 120ksi.

 

[phuduhudu]

I would expect a lot of uplift for a tower those dimensions but 2200kN sounds like too much. That is a base moment of 17600kNm for the whole tower about the short side giving an equivalent uniform load of 560kN over the height of the tower. That is 1.5kN/m2 of tower on the long side. That would be too much even for a solid clad tower. You may be adding too many wind loads in by double counting members.

The other question is, if it is independent there is going to be considerable sway at the top. How will you detail the link to the building it is serving such that it doesn't load it?

Carl Bauer

http://www.bauerconsultbotswana.com

 

[kslee1000]

If you have such high uplift, obviously you will need huge/heavy concrete footing to counter the overturning effect. Can you embed the columns directly into the footing?

Also, for wind case, does the code allow for overstress?

 

[JAE]

I recently designed a similar stair tower (similar size).

We used diagonals at every level and tied the tower to the building at its roof and let the vertical framing serve as trusses spanning between base and roof.

While you may be able to somehow bully your design to be strong enough, I doubt you will be able to design it to be adequatly stiff enough to avoid it banging against the building.

Just my 2 cents. I'd connect it to the building - laterally only.

 

[civilperson]

The shielding that is assumed is probably not effective for members more than 4 diameters/thickness behind the shielding member. Try adding diagonals between the columns at the lower half of the staircase. Thus the force is resisted by a 4 meter deep truss.

 

[Chandan321]

Thanks to all of you once again for the valuable insights.

---- csd72, I appologize for misreading your post. Actually, the client has already specified the permissible tensile and shear capacities for bolts, thus fixing the grade.

---- carlbauer, your summary calculations were very helpful in getting the bigger picture. I checked the loads to make sure I did not apply them on duplicate members. To give you an idea of the magnitude of the wind loads, my wind load varies from bottom to top as below:

Columns: From 0.34 KN/m at bottom to 0.46 KN/m at top
Beams: From 0.68 KN/m at bottom to 0.93 KN/m at top
Bracings: From 0.27 KN/m at bottom to 0.37 KN/m at top

The above loads are applied at the windward frame, and shielding factor of 0.7 is used while applying them on leeward frames.

I have defined all the bracings as truss members, released moments at the ends of beams where they meet columns. Also, I have released the moments at the lower end of the steel column where it sits on top of the concrete pedestal.

The sway at top and link to the building was a very good observation. I really did not think of it at this point, but will think about it and let you know.

---- kslee1000, my steel columns rest on 600 mm square concrete pedestals (750 mm high). The pedestals are embedded into a huge mat foundation below. However, my problem is with designing the bolts for tension. The code does not seem to address the issue of overstress.

---- JAE, good to know that you have designed a similar structure. I have the following points:
: By diagonals, do you mean X bracing?
: Have you provided the X bracings in both East-west and North-South directions?
: Could you elaborate on "let the vertical framing serve as trusses spanning between base and roof". I mean, what special member specifications did you define in the software to achieve this, like defining truss members or releasing moments etc.

 

[Chandan321]

---- civilperson, by "adding diagonals", do you suggest me to remove the K bracings and replace them with X bracings.

 

[civilperson]

Model the vertical frames as trusses, connect the joints of beam/column with diagonal members which could be X-braces or simply full length diagonals, (receiving both tension and compression force). Do not forget 30, 45, & 60 degree wind direction which has components in both axis with NO shielding. Wind also has a vertical direction,(not all wind is horizontal), adding force to platforms and platform/stair supports. (A minimum of 15 degrees off of horizontal is assumed in tall structures used in refineries).

 

[lkjh345]

Echoing others, I really question try to cantilever a tower this tall off of the foundation without supporting it (laterally) back to what ever structure it is serving.

My guess is that in anything but the lightest breeze, the tower will sway so much people will feel uncomfortable using it.

 

[kslee1000]

Globally speaking, for free standing tower, I will get ride of the concrete pedestal, and bring the columns directly to the mat foundation with proper corrosion control measurements.
Or do as JAE suggested - analyze the tower as a truss spanning in between the building roof and its own base.
If the stair is to serve the building, it will be somehow connected together on the top. By proper detailing/design, you maybe able to live with the original anchor bolt pattern, or even less.

 

[JAE]

Chandan321,

My apologies - I went back and looked at the model and discovered that we used moment connections between all the beams and columns.

The attached jpg shows the model. But the idea was the same - span from foundation to upper level, with the entire framework of 4 columns and beams working like a large vertical tube.

 

[sdz]

I did a quick hand check and an uplift of 2200kN per column sounds about right for the figures provided. Changing the bracing and beam to column connection detail will have little effect on that.
I agree with JAE that deflection (sway) will likely be a limiting requirement. Trying to cantilever a tower of these dimensions is a bad idea.
Perhaps you should print or email this thread to whoever gave you the instructions to design it this way.

 

[hokie66]

If the stair tower is to serve a building, certainly connect it to the building as others suggested. I thought from your OP that it may be an observation deck or similar. I think you have the idea that the tower will sway if free-standing. But that may not matter too much if it is a fire tower or something, as the users would not likely brave it in high winds, and if they did, they would be accustomed to the movement.

 

[Chandan321]

Guys, thanks to all of you and your suggestions. Backed up with all your arguments, I talked to my lead and he agreed to take support from the adjacent structure at 2 points. With my revised model, I could reduce the tension in the bolts to around 500 KN, which is very reasonable. After reading all of your posts, I have a few questions:

---- civilperson, If I want to analyse the structure as a truss between the roof and the base, should I define all my columns, beams and bracings as truss members in my modeling software STAAD. Or, is there any other way to define moment releases. If so, which member ends should I release moments at.

---- JAE, thanks for sending the jpg file. What are the green symbols at the top of the structure.

---- slkjh345, kslee1000, dz and hokie66, thanks for your arguments which helped me take support from the adjacent structure. Now, the adjacent structure can only support the staircase in one direction only. Is it OK if I assume pin joints at the the points of support with the adjacent structure.

 

[kslee1000]

You can provide direct connections in direction that the building can support, and provide diagonals to stabilize the stair tower in the other direction. The support can be treated as pin connection, or spring, depending the geometries and stiffness of the support/bracing members.