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RE: wind on stair tower

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Thanks,

 

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 12.2.5.6 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.

 

Joe

 

 

Joseph R. Grill, PE

Verde Valley Engineering, PLLC

2220 Sky Drive

Clarkdale, AZ 86324

Ph. (928) 600-5459

Fax (928) 649-3659

email: VVEng(--nospam--at)cableone.net