Support condition of a connecting member to existing structure

[Enhineyero]

Hi All,

I am just curious what support condition will you have if for example you will add a beam to an existing structure and it will be supported by an existing beam. usually i do pinned at both ends because its hard to develop moments when drilling and putting in dowel bars on the existing beam. but in a sense it should not be pinned right? because my support will actually deflect due to application of loads.

same goes when adding an additional floor, i will use steel and place it on top of an existing column, but i think using a pinned assumption for this case is a bit realistic because the settlement of the column will be very small, except of course that it will sway when lateral loads are applied.

I am just wondering how others assign their support condition in those two situations.

replies will be much appreciated

 

[Ahmmed123]

it depends on the connection that you are going to use. are you going to weld them or bolt 'em? if they will be bolted then it depends on the number of bolts. if you want to calculate the deflection then if you pinned it you will have a big deflection than fixing it. I would checking both conditions and run a FEA on both to make sure it will pass. also you may wanna check the foundation of the existing column before you add anything!

 

[delagina]

it should be spring with stiffness based on your calc. i usually assume pinned though if i got your question right.

only time i use spring support is for piles.

 

[delagina]

you may need to be careful as pinned attract more loads than spring. there maybe case when it makes more sense to assume certain stiffness to distribute the loads.

 

[Ahmmed123]

also, I would assume it spring for bridge piers, and thats due to the bearings. but if I get his question correctly, he was trying to make a column-beam connection?

 

[Enhineyero]

Thanks for the replies

I usually encounter this on additions on existing structure, like mezzanines or expansion of an existing structure.

@ Delagina /ahmmed

For example I have a steel frame connecting in an existing RC structure. so that my new Steel beam will be supported by an RC beam on one end and a steel column/beam in another end. If i assumed the point of support on the RC beam to be pinned, theoretically that would be incorrect right? because the RC beam will have some displacement, so i have to make my support as a spring. how will you calculate the spring stiffness of that support?

 

[JedClampett]

IMHO, if you need to worry about deflection of your supporting member, you're putting too much load on it.
It's a pretty theoretical discussion. Assuming similar supporting conditions on each end (usually the case), the deflection will be similar and you'll end up with a pinned connection on each end.

 

[delagina]

i would assume spring stiffness by putting a unit load of 1 kip then check it's deflection. if deflection is 0.5 then your spring is 2kip/in. i dont do this though, i just assume pinned support.