Skewed Joint Lines in Conc Pavt

[WSUgrad]

I reside in Michigan; therefore, I am not entirely sure whether this is localized to MDOT or if it's similar in other places.

Specifically in the MDOT Road Design Manual it discusses skewed joint lines shortly and states that skewing the joint lines helps dampen resonant responses to traffic traveling over the pavement. It then discusses randomly spacing these joint lines @ intervals of 12'-13'-16'-15' etc.

My question is: What is the significance of randomly spacing the joint lines in this manner, and specifically is this aforementioned spacing the "best" or does it really matter as long as they are randomly spaced?

Thanks for any thoughts.

~Chris

 

[civilperson]

Why reinvent the wheel? Use their recommendations.

 

[WSUgrad]

Not re-inventing the wheel. This came to fruition because we were reading some standard notes on a set of plans for a Local Agency and the random spacing was different that what is specified in the MDOT Manual.

Hence, my need to understand it's origins in order to understand whether this is a mistake or whether it really doesn't matter as long as you don't exceed maximum joint spacing.

Thanks.

 

[ACtrafficengr]

I experienced an unsettling phenomenon once when the concrete joint spacing matched a harmonic in the suspension of the pickup I was driving. The back end started porpoising until it was hitting the bump stops. Not fun.

I would suspect that randomizing the slab length might reduce the probability of that occurring, since the shocks would have more time to damp out the motion before the vehicle hit the next slab of the 'wrong' length.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

 

[concretemasonry]

Try going over the Chesapeake Bay bridge span north out of Virginia Beach.

If you have poor shocks or struts the creep and uniform pier spacing effects are greatly magnified.

When it hit me,it took a while to get to the right speed to a avoid the porpoising (pun) since it could have caused accidents and some cars joining the rare dolphins below.

There might be something to the harmonics in a slab, but there is a lot of variable dampening. Also, michigan has some of the most unique heavy trucks and load limits.

Dick

 

[onyeuko]

Remember that resonance Occurs when a body is oscillating in harmony with a body that is supporting it. The wave form of the harmonic motion has equal spacing of crests and troughs. So the random spacing will not allow the supported body to attain this state as crests and troughs will occur at diferent points(not on regular intervals) on the lines linking the crests and the troughs, thus will not constitute a harmonic motion. This is the reason of randomly spacing these joint lines @ intervals of 12'-13'-16'-15' etc
Teddy

 

[WSUgrad]

This was very good insight. I appreciate all the help. I definately have learned something new here. Or atleast it's been put into terms that I grasp and seem logical. What seems to be important is the randomness in the joint spacing and not the specific randomness so much. This might be determined by others to be more effective or whatnot, but the important part is for it to be random so that vehicles do not start "porpoising". Very interesting I might add.

Thanks for all your help.

 

[GeoPaveTraffic]

Resonance is the reason that the spacing of the joints changes.

The joints are skewed so that heavy loads do not encounter the joint all at once, i.e. each wheel crosses the joint at a different time.

THere have been several studies done, not all of them find a benifit. However, how hard is it to skew the joint and change the spacing. Just remember to mark the dowel bar locations so the joints are sawed in the correct loctaions.