How to manage too much wind load on staircase tower
How to manage too much wind load on staircase tower
(OP)
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.
---- 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.






RE: How to manage too much wind load on staircase tower
Carl Bauer
www.bauerconsultbotswana.com
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
---- 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.
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
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
www.bauerconsultbotswana.com
RE: How to manage too much wind load on staircase tower
Also, for wind case, does the code allow for overstress?
RE: How to manage too much wind load on staircase tower
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.
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
---- 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.
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
My guess is that in anything but the lightest breeze, the tower will sway so much people will feel uncomfortable using it.
RE: How to manage too much wind load on staircase tower
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.
RE: How to manage too much wind load on staircase tower
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.
RE: How to manage too much wind load on staircase tower
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.
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
---- 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.
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
---- If I want to do a conservative design for the stair tower, should I pin the supports with the adjacent structure.
---- A pin would mean that the stair tower would get resistance in the other direction also, which may not be fully correct. So, should I release translational degree of freedom along the non-supporting direction.
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
The green symbols are the RISA model boundary conditions (i.e. supports). These occurred at the bottom of the model (at the footings) and near the top where the cross beam was connected into the main building. With boundary condition reactions I could then check the existing building members to verify that they could adequately take the lateral forces from the tower into the building diaphragms.
RE: How to manage too much wind load on staircase tower
Thanks to all of you for your time and the valuable suggestions you have given in the past few days. Looks like my staircase tower can be reasonably designed, and the anchor bolts carry reasonable tension.
Will look forward to your help in future too.
RE: How to manage too much wind load on staircase tower
Why can't the building support the stair tower in both directions?
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower
RE: How to manage too much wind load on staircase tower