huangyhg

rc vs. steel
i am conducting a study about structural framing materials and i am focus on reinforced concrete (rc) and structural steel (wide flange section). my idea is to consider a frame model and design it as reinforced concrete and steel then compare the results.
i have a problem in generating my model, since in reinforced concrete, the ground floor columns are fixed to the pedestal going to the footing; but in steel framing system, the ground columns are connected by base plate to the pedestal and usually hinged. it seems that i dont have a common structure since for rc the ground columns are fixed and hinged for steel.
am i correct in my assumptions? is there something that i forgot to consider? i am only comparing the superstructure of the building.
i need to hear your smart opinion guys. thanks..
rlc
it depends of your intent. however the issue you are undertaking should be to most practical effects irrelevant. you can, if so wanted, get the steel baseplate connection enabled for moment, or simply, neglect the difference on the basis that you in doing so are just following standard practices. sometimes the most practical way of not seeing a problem is ignoring it; imagine the suffering of some aesthetically inclined man or woman seeing the imperfections at the huge amounts of detailing our human contraptions have; i can bet you understand we all have learnt to ignore them to some extent. i, for once, never should have delivered any works to use, for all them had standing imperfections at the ends of the works. and even for those that say theirs had none, i can assure you, if i was inspecting, i would find more than (it is clear) they are wanting to assume.
if you place nominal size starter bars(without any actual design for moment) then you may assume a pinned connection in reinforced concrete design, just as it is assumed for steel design, i.e. still use substantial hd bolts (and 4 of) but not designed for any moment in steel structures.
i have seen several similar comparisons made by industry bodies promoting either steel or concrete. i would look at these before reinventing the wheel. try searching for these comparative studies. in the end there will have to be some differences as the materials work differently.
ishvaaag,
my intention is to compare steel and rc as structural material for framing system considering cost, construction time frame and story drift. in doing so, i would like to meet standards in structural design and standard construction practice.
civeng80,
the common assumption in steel design is that the column is pinned connected to the base plate, but in reinforced concrete it is poured monolithically and usually assumed as fixed. please ilaborate your idea.
i was at a seminar a few years ago put on by the aisc. they made an argument for using springs at the bottom of pinned steel columns because they are not truly pinned. the equation they used was:
partial fixity = 0.4ei/l
this was based off the old k values used for column design. i don't know how applicable the equation is today. you would have to make that determination.
also, the pci design handbook has an equation for developing the spring resistance of a base plate based off the column size, base plate geometry/thickness, anchor bolt length/position/thickness, and footing size/soil sub grade modulus. i'm not sure if you can use a version of this equation in you analysis.

what brings to mind the meaning of standard. one can think as standard from some code, as practiced for most or as allowed by the technology itself. each is a kind of standard and you can place your comparison in any of the meanings.
i love ishvaaag's replies! like poetry
for the concrete building, it would depend on the foundation type.
if spread footings, i would never consider it to be fixed. depending on the ground conditions it may be possible to generate some fixity with footings into solid rock. otherwise, pinned.
if piles, it would depend on the filing arrangement whether or not fixity can be generated.
if it is a raft slab, you do not get full fixity. this should come out in the 3d analysis..
it is entirely up to the designer whether a pin or fixed connection is assumed in structural design and analysis and it is irrelavent whether material is steel or concrete.

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