provisions for future vertical expansion

huangyhg

provisions for future vertical expansion
i am designing a 4-story steel building (100'x100'). the first phase of the construction will only build the first 3 stories.  what type of provisions should i make for the future story? it is unknown how far in the future the 4th story will be added.  thanks.
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i would think you'd have to design the frame for both the 3 story condition and the future 4th, checking for whichever controled the various parts of the total design.
at the tops of the columns, make provisions for a future splice.  usually, the detail involves extending the columns above the roof approximately 3 to 4 feet (to allow a future welder good access to the splice).  this can be worked in with the architect (hopefully) with perimeter parapet walls.  interior columns would extend above the roof, and would already be flashed and waterproofed with the roofing.  so when the future construction occurs, the splicing and framing can proceed without digging into the existing roofing materials and compromise the user spaces below.
the 3rd floor roof/future floor would most likely be concrete so you would satisfy safety issues for occupants below during construction of the fourth level.  
after the upper, final roof is in place, the existing 3rd level roofing can be removed and finish work could proceed.
to add to jae's advice the reinforcement which projects above the roof level should be encased in concrete and waterproofed as jae said. i would suggest that this concrete is a weaker mix to allow for breaking out later for lapping the next column lift reinforcement onto it. it serves no structural purpose but protects the projecting reinforcement.
carl bauer
something that will make the future expansion design much easier is to clearly document the design intent.  provide a thorough narrative of the loads and structural system (vertical system, lateral system, diaphragms, foundations, etc.).  explain any unusual conditions or assumptions.  in a few years, your memory may not be as clear as you would like.  and it will be especially helpful if someone else is trying to figure out what was done previously.
all good advice, with concrete structures i usually cast a small 'cap' of low strength concrete at the top and extend the rebar.  the rebar is covered with insulated enclosure and flashed to the small cap.  the small cap is removed and the addition simply cast above.
with steel structures, i generally provide for an opening at the location where the columns are to be added if a deck is cast.  often don't include for the the concrete deck to be added at the time of construction (many proposed additions never happen).
use tapered insulation to achieve the roof slopes.
taro's comment with respect to documentation is of particular note and i usually provide a good outline on the drawing itself for methodology and design loads assumed, not the project design note file.  in addition to including info on the additional design loads, also include information on lateral loading.
for horizontal expansion, and sometimes for vertical expansion provide detail of connection.  i've encountered many projects where the only provision for future expansion is an increased load capacity for columns and footings.
they may forget about future expansion after 10 years, because building codes increase lateral loads so bad, that framing below and foundations desinged to todays codes will not support it. (unfortunately, this is from my personal experience.) in 10 years you will need to reinforce all framing. good luck.