CONTINUOUS BEAM OR SIMPLY SUPPORTED? 2

[milldude]

I need to design a steel beam that is in total 80ft long.
It will be supported at each end, and at 20ft spacings by
steel columns underneath the bottom flange. I plan to put
bearing plate caps on the tops of the columns, and weld the
beam, and plate caps to the columns. My question is this:
Should this beam be treated as a simply supported beam at a
20ft span, or should it be treated as a continuous beam with
supports at every 20 ft? Also, would these supports be
considered statically as rollers, pins, or fixed connections?
Any help here would be much appreciated.

Milldude

 

[Ginger]

If the beam is supplied in one single length then it is a continuous beam on knife edge supports. If the beam is supplied in 20'lengths and you are not connecting them together in order to transfer the moment then the beam would be simply supported. However, beware of the connection to the column. If you are only welding the bottom flange to the column then this should not be a problem. Too much rigidity in the joint will encourage moment transfer into your columns and you will have a new set of problems to deal with.

 

[prex]

It is largely up to you.
If you treat the beam as 4 separate beams, you'll be conservative, but your beam will be heavier. If it is a single piece of steel (or full penetration welded sections or moment connected sections) you can treat it as a continuous beam.
For the columns the way of reasoning is similar: if you treat the joint as pinned, you are on the safe side, otherwise you need to make a frame calculation, accounting also for column flexibility and checking columns with the end moment, and also checking the strength of the beam to column connection under moment. prex

http://www.xcalcs.com

Online tools for structural design

 

[ghghghgh]

Seems like an odd situation.
I'm interested to know why are you spannning over the columns rather than framing into them? What type of structure is this?

 

[JAE]

Be sure to weld vertical stiffener plates to the webs and flanges at each column location. This eliminates the potential for lateral roll-over on the column. See AISC Section K1.5, Web Sidesway Buckling for the numerical check. We always provide these stiffeners on beams at columns despite what K1.5 says as I've seen a lot of failures where this occurred.

 

[LPPE]

I'd like to see an 80' continuous piece of steel arrive on site.

 

[DaveVikingPE]

Moment distribution! Rock on!

 

[curvbridger]

IN the bridge industry we routinely have field sections with lengths of 100 to 120 feet (not to mention 10 depths). 80 feet continuous is not necessarily a problem, unless it it to be installed under roof somewhere.

It is not always conservative for the column to assume a pinned joint at the top. Introducing moment at the top is significant. Also, the negative moment section of the beam across the columns should be checked, even if you choose a shape based on simply supported span moments.

Model what you build, and build what you model!!!!

Curvbridger

 

[milldude]

Thankyou all for your input. In response to "ghghghgh", the
reason I am spanning over the columns rather than in to the
columns is that the axial load on the columns was at a max
of 26 289 lbs. I checked into an HSS 102x102x6.4 and it
has more than enough capacity to hold up this load. Because
of the wider dimension of the flanges of my beam (W310x45)
which is 166mm, I decided to have the beam bear above the
columns, rather than frame into it. (The reason for the beam
and columns is to provide a ledge for roof trusses that will
be supported by the beam; the other side of the trusses will
be supported by the existing structure behind my new construction.
In response to JAE, I like the idea of putting stiffener
plates in the beam above the columns. I will check it out.
I'm getting a little concerned about the introduction of
end moments at the top of my columns which has been mentioned by Ginger, Prex, and CurvBridger above. Am I to
then check the column as a beam-column with end moment and
axial load? Incidentally, the total factored load on this
beam is 1150 lb/ft.

 

[chicopee]

Use bolted connections between the beam and columns which shoud be welded with end bearing plates predrilled with slotted hole for bolting. Reinforcement with stiffeners may be advisable. There should be little moment transfer with bolted connections. Consult your AISC manual for design criteria on the columns. Consider wind and seismic for shear on the bolted joints. Field welding should be not considered due to the inherent hazard to the welder plus the fact that erection time is reduced with bolts and nuts.