×
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

Post tensioned duct void ratios
2

Post tensioned duct void ratios

Post tensioned duct void ratios

(OP)
Hello all -

We've run into a slight problem on a bridge job. The contractor mis-read a P/T note on our cross beam plan. Our beam had 4 rows of duct runs (4 duct in each row - 16 ducts total, with the Pj for each row given 21,,092 kN. The contractor read this as 21,092 kN total for all rows, and sized the ducts accordingly. Our reviewer missed this on the shop drawings. One row of ducts (4 total) was to be left empty for later use for a proposed bridge widening.
Since we fortunately have more ducts to work with, the idea is to stuff the ducts with more strands (go from 12 each to 24), which will reduce the desired void from 2.5 minimum to slightly less than 2.0. I'm guessing that the min void area is for constructability, but want to make sure I'm not getting myself in trouble by stuffing more strands in the ducts. We can adjust our grout a bit for easier flow, and fortunately, the strand trajectories are simple parabolic drapes. Is reducing the void ratio an OK thing, or should I reduce the strands and use my empty (widening) ducts?

Thanks

Steve R. Yates
City of Portland Oregon
Office of Transportation Engineering

RE: Post tensioned duct void ratios

2
The desired void ratio requirement is due to the constructability and practicality of the tendon installation and grouting.
Both LRFD and ASD AASHTO specify desired duct area as at least twice the net area of prestressing steel, with the requirement further increased to 2.5 for the pull-through method in the AASHTO LRFD.
As the pull through method is typically applicable for tendons exceeding 400’ in length, this is definitely not required for the crossbeam, which is much shorter.
In my opinion it will be OK to reduce the void ratio to slightly less than two, and adjust the grout mix for the easier flow.
Using the spare ducts right now will make future widening of the bridge difficult, if not impossible.

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