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RE: wind on stair tower

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I am back in the office so I can try to discuss this easier as typing on a Blackberry isn’t all that it is “cracked” up to be.  Also, I have received some more information from the fabricator.


I’ll attempt to explain the setup better as that is what is driving some of my questions.  The parking garage does wrap around two sides of the stair tower.  Exterior wall of garage then risers parallel to two columns, then two columns then another set of risers.  The stair tower consists of two HSS8x8 sections as columns and are spaced about 10ft apart.  The stairs wrap around the HSS sections with the landings at the ends of the two columns.  In another words a landing at one end of a column, climb up or down the 10ft length to another landing, walk around the column at the landing then again up or down the stairs.  There are two landings at the parking floors of the garage which I now know are two locations where the tower will be attached to the garage (pre cast concrete structure) along the side of the landing.  The tower will not be attached at the end of the landings (the second side where the building wraps around the tower).  Therefore the tower will only be attached along one side to the garage.  The stringers are channels with concrete pans at the stairs with metal deck and concrete at the landings.  At each floor level of the garage there will be HSS sections horizontally between the two HSS columns.  The screen material (kinda sorta like chain link) will only be attached to the HSS columns and the horizontal HSS sections between the columns. To look from one set of risers to a set on the other side you will look through the screen material.  There are a couple of beams at the intermediate landings that will span back to the garage wall.


You are probably still confused.


For wind loading I have now looked at a non building structure as sort of a trussed tower as then the exposure of the screening can be taken into account when wind loads to the lateral direction, perpendicular to the 10ft dimension between columns, is considered.  The resultant loads look reasonable.


For longitudinal loads (parallel to the 10ft dimension) I would like to use the HSS columns and horizontal HSS sections as a moment frame.  I think the connections will be pretty easy as fillets or partial pen welds as the loads shouldn’t be too large.  Lateral loads can run back to the structure at the landings, landing beams and beams at the intermediate landings.  O.K. for wind loads, however, I have now learned that the structure is in a Seismic Design Category D.  I understand (correct me if I’m wrong) that a moment frame of HSS sections would have to be an “ordinary” frame which is not permitted in a SDC D unless it complies with section of the ASCE 7-05.  This section refers to “single story” frames or a height under 65 ft with some DL considerations.  This works for the height limitations, but the single story thing?  Can you have 3 stories, 30ft, with a horizontal member, a frame at each story?  This section doesn’t make sense to me.  If I look at it as a non building structure, as I did with wind, where would it fall for seismic coefficient?  Foot notes for ordinary steel frame in SDC D say it is O.K. for pipe racks.  Can you use R=1.25 for “All other self supporting structures”?


Any help would be greatly appreciated.






From: Andrew Kester, P.E. [mailto:akester(--nospam--at)]
Sent: Friday, August 08, 2008 2:36 PM
To: seaint
Subject: re: wind on stair tower



We have done a few somewhat similar projects, such as a belltower for a church that had open portions but was mainly enclosed, free standing and not supported by the building.


This is my gut feeling without studying ASCE or your dwgs... You said it will be supported partially by the parking garage on one side, and then I assume at the outside by two vertical HSS sections that will make up part of a braced or moment frame? I would treat it as part of the building as far as any importance factor or anything is concerned, and your building department will probably like that also as it is a means of egress. But not for wind.


For wind loading you said it will be enclosed by parking structure on two sides, but you also say there will be a screening material? I am not sure I fully understand what this is all looking like, but I think the lateral pressures I would compare are that of a free standing wall and then that of a tower. As it will not likely make any appreciable cost difference on something this size, I would go with the worse case of the two. The stairs themselves will also have a vertical projection surface, probably more so than the rest of the structure.


If you wanted to get creative you could use the stair stringers and landings as a series of diagonal braces, but since these are probably supplied by the steel fabricator and engineered by them I think I would design this with X braces on the outside frame (if the arch OKs it). But sleep well knowing that your stairs will act as additional bracing (especially if these are steel stringers with steel pans filled with concrete)...