PSLRFD - PennDOT's prestressed concrete beam design software

[timmyo131]

I am new to bridge design and am using the PennDOT program PSLRFD to design a simply supported, adjacent box beam bridge. My spans are only 40' long. My deck is only 16' wide out-to-out, so I only need 4 box beams. There is no skew and no elevation differences. Pretty straight forward in general. When I do a design run, I receive this warning message:

%WARNING: **THIS MUST BE APPROVED BY CHIEF BRIDGE ENGINEER**
Distribution Factor Applicability Check Warning
For Beam Designation <4817>
Number of beams is not within acceptable limits
Number of beams, Nb = 4

Range of Applicability Lower limit = 5
Range of Applicability Upper limit = 20
The above applicability checks for span 1 DESIGN, 1 lane loaded
MDF1 distribution factors have failed.



The program seems to be giving me reasonable results despite the above warnings. Should I proceed to take what the design run has given me and just rerun it in the analysis mode to confirm? Or can a serious problem result from these warnings?

Also, I've run the design for one span. My bridge is actually 8 spans long, but they are all simply supported. The PSLRFD user's manual says that if several spans are entered, the program will assume they are continuous. Am I approaching this correctly, only dealing with one span since they are all simple spans?

 

[Qshake]

With box beams, can you achieve or did you expect to achieve contintuity? I have seen details that allow continuity for multispan precast box girders but most are simple span.

The issues are obvious in that if the program assumes continutity for either the dead load or the live load, you will have less positive moment acting on the girder than you actually have and this is dangerous.

As to the number of girders required, the distribution factors are often conservative so if you don't meet the exact requirements, you can reduce the inherent conservatism in the design. Also, while you might only need 4 girders for everyday loads, sometimes policies are written to cover the period of construction or maintenance for the bridge and so you need to think about how many girders are necessary to keep traffic on one side of the bridge while the other side is removed.

Good luck.

Regards,
Qshake

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