×
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

pinned connections in trusses

pinned connections in trusses

pinned connections in trusses

(OP)
Its been a while since I've had to do structural detailing and from my dim memory of truss design in college I seem to remember that if the centrelines of all the members at a joint intersect then the joint has no moment capacity - i.e is a pinned joint. Is this right?

RE: pinned connections in trusses

The orientation of the intersecting members has nothing to do with pinned vs. moment connection characteristics.  The orientation simply determines whether eccentric axial loads will develop at the joint.  

The type of connection at the joint determines whether they are pinned or fixed (moment).

RE: pinned connections in trusses

I think a truss analysis assumes pinned connections.

RE: pinned connections in trusses

truss analysis assumes that the members carry axial load only, that each strut is a simple two force member.  this theoretical state is only achieved if the members join with a single pin (well greased?) and (if you're getting picky) on the neutral axis of each of the members.  

in reality the connection is a multiple pin arrangement (therefore capable of reacting moment) and offset from the neutral axis (of at least some members) therefore inducing bending in the strut (the load axis isn't aligned to the strut).  in reality these two effects are usually small enough to be neglected.

to answer the OP, the strut axes may meet at a point and the struts can, or cannot, react moment depending upon the attachment between the struts (whether it is a single pin or a multiple pin).

RE: pinned connections in trusses

If you are using a computer program to analyze a truss, you woud have to make sure that you release the end moments at the right plce to create the pin connection. For example, your top chord may be continuous but have many diagonal members attaching to it. In this case, you should release the end moments for each diagonal member only.

RE: pinned connections in trusses

It is typical in structural engineering to neglect the rotational capacity of the web members in design.  In certain cases, long span, or welded connections, at some times to include the moment effects of the web members.  While theoretically to include web member rotational restraint typical practice neglects the moment capacity of the connection in design even if it has multiple bolts in the connections.  Almost all, I would say 90% or more, of civil structural trusses utilize multiple bolted connections and truss action is assumed in analysis.  This has been successfully practiced long before any of us were engineers.  If you have eccentricity at the connections (centerlines not lining up at the panel points) then you need to include this effect in your analysis.  If you choose to analyze the truss utilizing rotational resistance of the web members, which is valid in some cases, then you have a whole different set of detailing and analysis items you have to deal with.

RE: pinned connections in trusses

If your members are not deep, should be able to neglect eccentricity.

RE: pinned connections in trusses

Dag I need to read my post before I send them!  Above should say, (Sentence 3) "Analysis including web member rotational restraint is theoretically correct typical..."

Sorry about that.

RE: pinned connections in trusses

If a "typical" truss were analyzed on a computer as a frame it's my guess that the bending stresses would be not be significant, assuming of course that centerlines and work points intersect.  When member sizes become relatively large then secondary stresses would need to be considered, but I don't remember any rules of thumb that would be helpful.

Regards,
-Mike

RE: pinned connections in trusses

kxa,

In your post I would say instead that "if your members are not deep" you should be able to neglect secondary stresses due to moments.  I would however be very careful with any eccentricities caused by centerlines not intersecting at a point.

-Mike

RE: pinned connections in trusses

It all depends on the way the truss is configured and how it is loaded. You could have significant negative moment over the joints for your continuous members.If every single member is pinned at the joints, then the bending moment depends on the load each member sees.

RE: pinned connections in trusses

mrMikee, I agree. I was concerened with joint/connection design. I guess you can always model a dummy rigid member to carry the centerline to the surface of the member at the connection/load transfer point.

RE: pinned connections in trusses

lvkb - I have a worked out, pinned truss problem (zipped PowerPoint file), in the Miscellaneous portion of my web site, link below. The title is "Truss Analysis With Point Load". This may help to refresh your memory, since the problem seems a lot simpler than it is.

www.SlideRuleEra.net reading

RE: pinned connections in trusses

What someone above seems to miss is that the moments developed in the elements of a truss (with intersecting neutral axes) need not be negligible (and often they aren't) to be neglected in the analysis.
In fact those moments give rise to secondary stresses (as they are called in pressure vessel analysis). Such stresses are not load controlled quantities, instead they are deformation controlled quantities: the distortions they cause will tend to decrease them, and even if they reach the yield condition, they will stay there and no more increase (whilst a primary stress, if the external loads increase, will increase till the structure collapses).
Our grandfathers didn't know the theory behind that, but that's why the truss method of analysis works so well.

prex

http://www.xcalcs.com
Online tools for structural design

RE: pinned connections in trusses

(OP)
Thanks for all your posts! They are all helpful.

RE: pinned connections in trusses

Having designed many trusses and observed their behavior, I think that anayzing trusses with pinned ends reasonably approxiamtes real life UNLESS there are significant deflections or the truss is made of very slender members.  The definition of significant is of course relative to each truss and the connection designs.  When deflection under load become significant then the members begin to "feel" the end restraint.  It can also be said that for certain trusses, the member end fixity determines the resistance to deflections.  So, it's not simple unless your truss is designed, loaded and behaves in a certain way.  Fortunatly, this tends to be the most common scenario.

RE: pinned connections in trusses

I agree with the sentiments expressed in the above posts if a conventional truss arrangement (like a pratt or warren type truss) is used.

However if a Vierendeel type arrangement is used then the members should be designed to resists axial load, moments, and shears.

VB

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