huangyhg

how to manage too much wind load on staircase tower
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.
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
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)?
use a higher grade anchor bolt.
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.
i am saying a higher stress grade rather than size, you can get anchors up to 120ksi.
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
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?
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 strongfff"> enough, i doubt you will be able to design it to be adequatly stifffff"> enough to avoid it banging against the building.
just my 2 cents. i'd connect it to the building - laterally only.

the shielding that is assumed is probably not effective for members more than 4 diameters/thickness behind the shielding
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

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