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.

 

[Chandan321]

Thanks kslee1000 for the suggestions.

---- 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.

 

[kslee1000]

If the direction (not supported by the building) is the LONG (strong) direction of the tower, you can try, maybe the results are tollerable, to release the support in that direction. Also, fix the tower base (moment connection) would be a big help.

 

[JAE]

Chandan321,

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.

 

[Chandan321]

Thanks kslee1000 and JAE for the help.

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.

 

[hokie66]

Chandan,

Why can't the building support the stair tower in both directions?

 

[Chandan321]

hokie66, my main structure spans North-South longitudinally. My staircase tower stands adjacent to the south end of the main structure, with the flights running East-West. The landings are connnected to the main structure in the North-South direction. Thus I am planning to take support from the main structure in the North-South direction. Is there any way that you think I can take support in the East-West direction also?

 

[hokie66]

I see. I would probably do it your way, with that southern wall of the stair tower diagonally braced, but you could consider diagonal braces horizontally outside the landings on each end at the levels where you are bracing the tower north-south.

 

[Chandan321]

hokie66, thanks for the suggestion of horizontal bracing. Do you think X bracing has any advantage over K bracing in these type of structures.

 

[civilperson]

X bracing loads the members with axial load. K bracing induces moments. No superiority either way, just added calculations for combined stresses.

 

[Chandan321]

civilperson, thanks for the tip about bracings.

 

[hokie66]

Just to be clear about your bracing method, K bracing can mean different things to different people. To me, K bracing goes from node to node, and the members are designed for tension and compression. I call bracing which goes from node to beam chevron or vee bracing, depending on its orientation. X bracing, well that is obvious, and can be either all tension or T-C.