Difference between Transfer Length & Develop. Length in Prestressing 1

[GalileoG]

What is the difference between transfer length and the development length in prestressed concrete?

Silly question yes, but I thought at the end of the transfer length my strands would be developed for my prestress force, 0.7% fpu * lossess. Why then would I need the development length in any of my strength calcs if I'm designing for an effective prestress force?

Danke

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[civilperson]

Development length is a term I apply to rebar. When used with strands, it has the same meaning as transfer length at the ends of members. The procedure is different for grouted/bonded strands vs. greased ducts and tendons.

 

[bnickeson]

This is a topic that can be somewhat confusing, so it's not a silly question by any means. The transfer length of a strand is the length required to transfer the effective prestressing force (f_se, or prestressing after losses) to the concrete member. It is commonly given as (f_se/3)d_b

The development length is the length required for the strand to reach it's full design strength/stress (f_ps). The development length is alway longer than the transfer length, as that equation is given by the transfer length plus the difference between your f_ps and f_se times the strand diameter. If you have the PCI design handbook, there is a nice diagram on page 4-41 that shows this.

 

[GalileoG]

It appears that transfer length is for servicibility calculations whereas the development length is for strength calculations. This is different than what civilperson posted.

Clansman

"If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[bnickeson]

Essentially, yes, that is my understanding. It's been awhile since I've done any prestressing calcs, but a general rule of thumb I've tried to remember is that when you are calculating stresses in the concrete at the service level (such as at release according to 18.4 in ACI), you are to use f_se, which applies to the transfer length. When you are calculating nominal strengths and stresses in the steel strands, you use f_ps, which applies to the development length.