How do you discern Flexible from Rigid Connections?

[flin8812]

Hi everyone,
I'm trying to understand how to differentiate the two connections from engineering drawings,
but had failed to distinguish the two.
Are there typical traits you could use to determine them?
Please use base connections and roof joints as examples if possible, and possibly some diagrams that illustrates
you're explanation and reasoning.

p.s. Is flexible= pinned?

Cheers

 

[flin8812]

Your

 

[dhengr]

Flin:
I’d say that’s a fairly open ended answer. Could you be a little more explicit.

 

[msquared48]

Yes, flexible is equivalent to pinned.

As to differentiating the two, one moves more than the other.

Pinned connections generally only resist vertical shear, while fixed connections resist the vertical shear as well have the capacity to generate a moment arm to resist applied moments.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[slickdeals]

I would like to clarify M^2 statement further by saying "rotates more than the other"

 

[msquared48]

Picky, picky, picky...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[kylesito]

A little more clarity would definately help...

For instance though, as was mentioned, a flexible connection or pin is often designed to only resist vertical load...shear for instance can be resisted by a single vertical plate or angle connection a roof joist. A rigid connection is designed to resist moments or rotation. The most common I can think of would be using a plate accross the top and bottom flange of 2 I beams framing into each other. You would still use a shear connection to resist the shear load but then use the top and bottom plate to resist the rotation through tension strength of the plates.

As for column bases, this gets trickier as it's tough to "fix" a base of a column. Often though, you would connect it to a base plate and then add stiffeners around the column to help restrict rotation at its base.

Sounds like this could be a homework problem...no cheating!


PE, SE
Eastern United States

"If a builder builds a house for someone, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death!"
~Code of Hammurabi

 

[TXStructural]

A cast-in-place concrete joint is rigid and fixed in all axes. <--- obligatory reinforced concrete plug

A simple shear connection using double angles on a wide flange beam is "pinned" to rotate in the plane of the web of the beam (or around an axis at the centroid of the connection, if you like). The beam can deflect under gravity and the bolts move a little and the angles deform a little, and a moment is not intentionally induced into the supporting member through the connection. Similarly, a plate or shear tab connection is flexible (pinned such that the beam can deflect with loading), but the axis of rotation is usually the centroid of the bolts.

It gets complicated when you have a flexible moment connection (Part 11 of the AISC Steel Construction Manual.) These connections act as simple shear connections for gravity loads and as restrained moment connections for lateral force resistance. These differ from fully-restrained moment connections (AISC Part 12).

 

[connectegr]

flexible vs rigid and pinned vs fixed are not necessarily the same.

In the 9th edition ASD manual. Flexible vs rigid related to the stiffness of the support, related to single plate connections. The connection material and detail of single plate connections are inherently stiff. Welded to a supporting girder web is considered flexible, since ductility of the girder web provides a degree of beam end rotation. Single plates welding to a column flange near the centerline, is considered rigid due to the stiffness of the support. These connections were designed differently relative to eccentricity. However research found that actual beam end rotations are minute, and the 13th edition manual changed the design procedure and considers the bolts in straight shear. This procedure did change again in the 14th edition. (it changes in nearly every edition)

Double (clip) angle connections provide end rotation do to the ductility of the outstanding angle legs. This flexibility is relative to the angle thickness. And therefore the degree of fixity varies. However if designed correctly this is considered a simple shear connection and a pinned support.

In general very few connection details are truly pinned in nature. An example is a seat connection.

Fixed connection refers to a moment connection. Connection details design to prevent or minimize beam end rotation.

http://www.FerrellEngineering.com

Providing fabrication and erection efficient structural design of connections. Consulting services for structural welding and bolting.

 

[BAretired]

Connections may be semi-rigid. These would be somewhere between flexible and rigid connections. The idea is that one part of the connection yields and can contribute nothing more to the required resistance. Moments and forces are then distributed to the parts of the connection which have not been stressed to the maximum.



BA

 

[shobroco]

Let's face it: almost no connection is fixed or pinned, every connection is somewhere between the two, & the reason you got a license to practice is that you can decide how you are going to analyse it & where you are going to say it falls. For flin8812, that translates into "it depends, & you'll figure it out but start conservatively & check everything more than once".

 

[paddingtongreen]

There are large moments and small moments. Look at the behavior under the actual loads. AISC used to have, and my still have, a rule that clip angles less than 5/8" thick were flexible and by inference, thicker ones resisted moment. Clearly, the in-plane moment resistance of a plate is high while the out-of-plane moment resistance is low, but there is some resistance. As shobroco says, it is your job to determine which is which (unless a code tells you otherwise).

Michael.
Timing has a lot to do with the outcome of a rain dance.