Analysis of an Industrial Building

[Jeevan1985]

dear all,

while analyzing an industrial shed,
if the whole frames are made of steel, then boundary conditions at both ends of the columns can be fixed.

suppose if i am having RCC columns and steel roof, what should be the boundary conditions.
can we assign the base as hinge.

the loading at roof is around 200 kg/sq.m including Live load and suspended loads.
the area of the hanger is around 32000 sq.m

regards,
jeevan

 

[JedClampett]

I don't understand your first statement.
"if the whole frames are made of steel, then boundary conditions at both ends of the columns can be fixed."
You have to provide a design that enforces those assumptions. A fixed base is difficult to design. There's a lot of anchor bolts, not to mention that the load has to be carried into the foundation. At the top, it's easier, but there's relative stiffnesses, sidesway, etc. to consider.
For a concrete frame, it's normally conservative and easier to implement, to assign a hinge at the base. But once again, you have to consistently consider this with respect to deflection, moments at the top, "k" factors for your columns, etc.
If this is your first large structure, is someone supervising you? These questions should go to him or her.
Your analysis needs to reflect your design and your design needs to reflect your analysis.

 

[Jeevan1985]

Thank you Jed,
yes this is my first industrial building.
unfortunately i don't have a good supervisor to monitor me or to take any firm decision.

initially it was discussed and the base was made hinge, that resulted in huge reinforcement in columns.
then my seniors told me to make the base fixed.
so, i am still in a confusion that, which is the better way to follow while analyzing an industrial structure using any analysis software.

 

[shin25]

If this is a single story portal frame building, then either the base of the column or connection between the roof and columns can be considered pinned and designed as such, the other end should be considered fixed. The structure will be unstable if both ends are considered pinned. You may also analyze and design the structure using some other system of frames, such as braced frame that may allow you to have pinned condition at both ends.

 

[AELLC]

If the columns are RCC, I don't see how their bases are pinned - they are doweled into whatever concrete foundations you have, and that should be designed as fixed base.

 

[JedClampett]

AELLC: It's done all the time. You provide the dowels and ignore them for fixity in the analysis. That way you don't have to design the foundations for the large moment. Also provides a good belt and suspenders approach.
Jeevan: Either way, you're just shifting around the reinforcing. If it's pinned, you could (but usually don't, as AELLC says) reduce the reinforcing at the base to a minimum and just design for shear. But that results in higher moments at midspan.
Jeevan, you need to understand where the forces are and where they're going. I'm a little afaid that you're using the analysis software as a crutch, not a tool. Even though it's a big structure, do some hand calculations and approximations before you throw everything in the black box. Make sure your results make sense. I feel bad for you if your supervisors aren't helping. That makes things lonely.

 

[AELLC]

Jed:

When I read RCC columns and steel roof, I interpreted that as steel beams not rigidly (fixed) connected to the columns.

If he has grade beams as part of the foundation, that will take care of the column base fixity.

 

[AELLC]

typo- steel beams connected to RCC columns, modeled as pined.

 

[hokie66]

Note that this OP says this is a "hanger", so I think he is talking about a hangar, and it is 32000 square metres. So this is going to be a huge, high, long span roof. Not something which a first time designer should be tackling without an able supervisor.

If it is concrete columns and steel roof, fixed base columns with pinned connections to the roof members would be my approach.

 

[Jeevan1985]

Hookie, this is an industrial building have multiple gable with column grid of 24x12, with plate girders supporting the roof.

I have grade beams only in the pheripery.
Jed, I have done some manual designs for the roof plate girders and then I am going with the software. As you mention, I am driven by the software and I am not able to visualize the behaviour of the structure.
now i can conclude that base can be fixed and connection at roof shall be pinned, which may also be economical.

 

[civeng80]

Its a portal framed structures spanning a huge area. So Propped portals.
Why are you going for concrete columns in the first place ?
And what material is the wall cladding ?

 

[haynewp]

I try to avoid fixed base columns unless they are not in combination with other lateral elements in the building. Cantilever columns are penalized for seismic. If the fixed bases are in combination with other LFRS elements then I put in a spring to compensate for the lack of true fixity at the base or model it as pinned which is conservative for the superstructure.

For hangars I have used bracing around the perimeter and between the hangar bays. Use a moment frame across the front of the hangar and braces at the sides of the front of the building (at the exterior of the building adjacent to the last bay of hangar openings) if needed. Typically they are all steel buildings around where I am with very large foundation uplifts.

 

[Jeevan1985]

RCC column is the client requirements.
elevation bracings are not provided,
metal cladding with girts are given.
Suppose i am making the base pin, then my reinf. In rcc column is shooting up.

 

[hokie66]

You can't have pinned base reinforced concrete columns and steel roof framing without the building being braced. Why would you even consider that approach? Seriously, get some help from an engineer experienced in this type of building structure.

 

[dik]

Your 24x12 module is a bit off... can you accomplish 24x24? or are you using 3" deck for a 12' span? Why concrete columns... likely the most expensive way to go and difficult to connect steel to with any certainty. Try steel with the bases pinned. Double fixity OK for theory, but not often done in the field.

Dik

 

[AELLC]

If this is not in USA, and not in a seismic zone, then RC columns fixed at base may make sense.

However steel plate girders in the roof doesn't make sense - why not steel trusses?

 

[hokie66]

dik,
Based on his first post, I think he is talking in metres, not feet.

A bit of a plan would help in understanding the intended structure. I thought he was talking about an aircraft hangar, but if so, it must be for small planes.

With only a bit of information in defining the problem, poor advice is inevitable.

 

[dik]

THanks hokie, says dik... wiping the egg off his face...

 

[paddingtongreen]

Jeevan,
You can rigidly fix a concrete column to a foundation, but if the foundation cannot resist overturning it is, effectively, still a pin, albeit with the pin in the foundation instead of on it. If it is a large foundation, you can, as others have said, call it pinned for the reasons they offer.

If columns are pinned the top and the bottom and if there is no bracing, it isn't a structure, it is a mechanism and under horizontal load it will fall down.

Making a two directional moment connection to steel framing at the top of a concrete column is difficult, the size of the connections tends to be out of proportion for the size of the column.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[civeng80]

Agree with Hokie helpful to have a sketch of proposal, so that better advice can be given.
A bit odd (but still possible) to have concrete columns and steel rafters with pinned cols connected to footings.
How do you propose to connect the concrete columns to the rafters with a moment connection?