Bottom flange ties on prestressed bulb tees - Not needed?

[MIKE_311]

Working with a DOT on a current project, their prestressed beam details do not have flange ties in the bottom of the beam outside the anchorage zone (1.5h). Other surrounding DOTs I have worked with have a flange ties that are at a minimum placed with the web steel and tighter in the anchorage area.

My guess is that it may present a fabrication challenges with harped strands? If it's not really needed I can see leaving it out but I'd think you'd want some mild steel in the bottom flange for such a long area.

How common is this?

 

[STrctPono]

Yeah, outside of the anchorage zone, there are no specific requirements that I am aware of that require bottom flange ties. This then becomes a preference thing based on region and every DOT does it differently based on what they've tested and feel is a balance of cost and performance. Out here, we put them in. We also put mild longitudinal steel in the bottom flange. The bottom flange ties should not conflict with the harped strands as you can always change the shape of your tie to avoid this. See below. This is Post Tensioned and does not have harped strands but essentially a similar scenario as the ducts may conflict with the ties at some point. Anchorage section has different shaped ties than the typical section.

Then on this job, the tie geometry is changed since there is a 3rd PT duct lower and would conflict with a bottom flange tie with a hat.

 

[Ingenuity]

STrctPono:

From the girder photograph, is the anchorage end shown a fixed-end anchorages for all three tendons, or stressing-end anchorages with no strands installed to date?

 

[STrctPono]

Ingenuity,

Future stressing-end anchorage without the tendons installed. This is a spliced girder with match-cast ends that will be stressed on site.

 

[Ingenuity]

Thanks, STrctPono.

Often dummy-strands (temporary, then removed) are installed during casting to avoid a collapse/damage of the metal spiral ducts (overzealous vibrator, concrete drop etc).

Was SCC mix used on this girder?

 

[STrctPono]

I remember reading something about them awhile back... I've never seen them used, however. Perhaps we've gotten lucky on all our jobs, or we just are clueless that the duct is in fact getting destroyed. It's never been reported that they've had issues pulling the tendons through afterwards. I suppose this gets into a gray area of whether this is Contractor's means and methods or if the EOR does have the right to request it.

The precaster (you know who) has gotten much better QC over the past few years and their quality of girders has gone up significantly. For awhile, we were getting girders from another state it was so bad.

I'm not sure if an 8" slump is enough to be classified as a true SCC but it certainly has many qualities of an SCC since vibrating below the ducts is so hard. It's a good mix that never gives us issues with voids or honeycombing after they strip. 0.34 W/C, and contains VMA and Super P.