×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Reaction forces at any point

Reaction forces at any point

Reaction forces at any point

(OP)
Hi,

I'm trying to calculate the reaction forces on a vertical continuous beam (seawater caissons on offshore platforms if anyone's interested). I'm doing this in Excel. So far I've been able to calculate everything up until deflections if the beam was a cantilever but I'm not sure how to calculate the deflections when they're fixed at the supports annotated in the diagram (red arrows). Any ideas on how to go about doing this in Excel?

RE: Reaction forces at any point

(OP)
Sorry I mean how to calculate the reactions, not deflections.

RE: Reaction forces at any point

One way is as follows:
Consider the beam simply supported at the upper and lower supports. Calculate deflection Δ at middle support due to the applied load. Calculate end reactions.

Under the action of a unit load, calculate deflection Δu at middle support due to unit load. Calculate reactions due to unit load.

Calculate reaction at middle support, R2 = Δ/Δu.

Adjust end reactions for R2.

BA

RE: Reaction forces at any point

(OP)
Thanks for your help. I used your idea but I split my beam (3 spans) into 3 individual beams, fixed at both ends. Then I was easily able to find the reaction forces at each end. Here's the output from STAAD.Pro and Excel. The scales are exaggerated but the actual forces turned out to be roughly the same.

RE: Reaction forces at any point

assuming fixed at the middle support in probably unconservative, and fixed at the top ... sounds odd.

you could analyze as a beam, cantilevered at the base with two supports (doubly propped cantilever). This is doubly redundant, but the singly propped cantilever is a well known book solution, so then it's "only" a matter of using displacements of the singly propped cantilever to determine the reaction at the second prop (unit load method). you can solve your structure two ways to check your solution. This solution allows the beam to develop it's "correct" internal moment, as a continuous beam, probably less than your fixed assumption.

Quando Omni Flunkus Moritati

RE: Reaction forces at any point

I suspect that none of your support points is fixed against rotation. The assumption that they are fixed against translation may be wrong too. You have to determine the boundary conditions before the problem can be solved.

BA

RE: Reaction forces at any point

(OP)
The caissons I'm analyzing are fixed at multiple locations along its length using guides. These guides are assumed to fix the caisson in place and I'm modelling it assuming that no rotation is allowed at the guide locations. So according to that, the spreadsheet outputs a similar solution to the software I use for comparison.

RE: Reaction forces at any point

Well, if all support points are fixed, you can treat each beam as a fixed end beam and solve it easily. The continuity of the beam is not a consideration.

BA

RE: Reaction forces at any point

how does the picture, showing reactions at specific points, line up with the thread title "reactions at any point" ... maybe just an ESL thing ?

I think BA's point is that nothing is truly fixed, but then it seems you're doing a hand calc to verify a FEA ?

Quando Omni Flunkus Moritati

RE: Reaction forces at any point

(OP)
Yes I'm doing a hand calc to verify the FEA.

I should've been a bit more clear, those three points were arbitrary points and depending on the situation, there can be more or less of them. I just put them there as an example.

RE: Reaction forces at any point

that sounds confusing to me, but then i'm not seeing the bigger picture ...

Quando Omni Flunkus Moritati

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources