PT losses in pan live end Vs edge stressing live end

Generally speaking, it is significant enough that you usually have to do the extra calcs using the actual angle change of the stressing nose/s used on the pans to make the field measured elongations work out within the +/- 7%.

For short, panned-stressed tendons, these tendons often come up 'short' on elongation, sometimes making it necessary to re-stress. Or they get re-stressed with the 'pen' - ie fudged to satisfy the EoR.

This attached photo is NOT panned (slab edge stressed) but it is very short. The total length of ducting was 900mm (3 feet). Not an economic use of materials. It is a current project under construction in Sydney - engineer is one of the largest firms too.

VERY SHORT PT TENDON

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Bangaroo? haha.

Thanks for your reply, would you possibly be able to point me towards some reference docs to put some numbers to the differences between the live end anchorages?

Ingenuity,

Classic photo. Any idea what they were trying to achieve?

Trenno, no NOT Barrangaroo. A new car dealership north of the bridge.

I use the classic wobble and friction calc (quoted in most (all?) prestressed design texts), and for the stressing pans where segmented angled or curved stressing barrels/noses are used, I neglect any wobble through the nose (which is small anyway, in this instance). Angle of friction depends if a 2, 3 or 4 piece nose is used - a typical 3-piece 'curved'/segmented nose has about 30 deg of ADDITIONAL angle change. I usually ask the stressing personnel on how many segment are used and/or the angle change.

So the ADDITIONAL losses through the curved stressing nose are dependent on the friction value between the strand and the nose - if a new nose and new strand, then maybe mu is 0.15, if older strand and old nose then mu of 0.25 may be more applicable. See the attached file for some range of values.

This is a short-cut calc. You can get real fancy and calc the total friction and wobble loses in one calc by accumulating the angle change of the tendon (duct + anchorage + stressing nose), include the wobble effects for the ducting, etc, but given the assumptions we make on friction values etc, it is not worth the extra effort IMO.

Hokie66 - no idea what the engineer was thinking, but I assume if was a classic case of a consultant "design" that was a line across a drawing stating PT by others, and the engineer was sticking to his/her requirement of PT over the full extent, even though it was nonsensical to this depressed/folded area. Assuming 6.5mm/m of elongation (at 900mm effective tendon length that makes say 6mm), and 6mm of draw-in wedge loses) the net effect was about ZERO P/A! I guess the ducting will provide some shrinkage reo equivalent!

Ingenuity,

I like the detail at the left end of the top bars! A bit of development might have been useful! At least the bottom bars seem to have about 50% development with the cogs from the verticals at the left end. The top bars have zero.

As long as the very short tendons are grouted properly, they will act as very widely spaced bonded reinforcement and will be able to generate up to about 1000MPa with sufficient deflection (doubtful). A good example of not understanding design.

Agree on the approach for anchorage losses. The standard 4-5 strand anchorage has about a 2% loss due to the angle change in the anchorage. This is allowed for by good design programs. For pan stressing or any situation where a curved nose is required for stressing, the extra loss can be calculated from the angle change and friction factor, as they are for all other friction losses calculations.

rapt said:

The standard 4-5 strand anchorage has about a 2% loss due to the angle change in the anchorage.

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Good point about the losses through the anchorage.

Given the typical 5 strand anchorage/block arrangement (see attached PDF file), and assuming µ = 0.20 then strands 1 & 5 have approx 10° plan angle change (or 3.4% loss), strands 2 & 4 have 5° (or 1.7% loss), and strand #3 has zero (or 0.0% loss). So 'weighted' average loss is [2x3.4% + 2x1.7% + 1x0%]/5 = 2.04%

So RAPT SOFTWARE takes the 2% value as an average % loss over 5 strands through the anchorage - seems right to me.

Trenno said:

...would you possibly be able to point me towards some reference docs to put some numbers to the differences between the live end anchorages?

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In addition to the numbers I crunched above re pan stressing, go to ==> DOWNLOADS and in RAPT MANUAL PDF file go to section "T11.1 Immediate Losses" and it covers this topic well, in general form.