Discussion: View Thread

Hi, 

I'm new to ASCE association and hope it's "ok" to ask questions related to other structural standards besides say ASCE 7 for example, but something I'm currently working on is bridge repairs and I'm doing basically some load rating calcs for the first time. 

I noticed the AASHTO spec only considers the steel section properties for bridge beams (girders? - I typically associate girders as a member supporting other members like joists, etc but seems various terminology is used across the board), when calculating section properties. I see there's a Kg which I guess accounts for the stiffness of deck slab (which looks like a modified version of the parallel axis theorem), but it occurred to me that given the composite section and assuming in reality the neutral axis shifts towards top flange of steel beam (probably right around the top flange), then your positive moment capacity for that beam goes DOWN, given the "c" distance to outer fiber goes up. 

So, how does AASHTO account for the "true" flexural capacity by only considering the steel section properties.  I did take a bridge design course in college and I know there's methods for taking an effective section of concrete deck and transforming it into an effective "block" but thought it was interesting how AASHTO does these calculations and would like some greater insight into where/how the equations were derived.  Do the equations account for all this in some way, or is that what the Kg factor is doing? 

Any insight would be greatly appreciated (and if I'm outside the domain of this discussion board then please let me know). 

Thanks,
#Other

------------------------------
Daniel Foltz EIT, A.M.ASCE
Engineer
Williamsburg VA
------------------------------

Daniel,

Thanks for posting your question here. Please refer to AASHTO LRFD Bridge Design Specifications for the definition and background of Kg and AASHTO Manual for bridge evaluation for additional info and examples. 

You are more than welcome to post questions on ASCE 7 and other ASCE standards.

------------------------------
Huajie (Leo) Liu, PhD.,S.E., P.E., M.ASCE
Engineering Manager
LCRA
Austin TX
------------------------------

Original Message:
Sent: 10-26-2022 07:01 AM
From: Daniel Foltz
Subject: AASHTO Bridge Design

Hi,

I'm new to ASCE association and hope it's "ok" to ask questions related to other structural standards besides say ASCE 7 for example, but something I'm currently working on is bridge repairs and I'm doing basically some load rating calcs for the first time.

I noticed the AASHTO spec only considers the steel section properties for bridge beams (girders? - I typically associate girders as a member supporting other members like joists, etc but seems various terminology is used across the board), when calculating section properties. I see there's a Kg which I guess accounts for the stiffness of deck slab (which looks like a modified version of the parallel axis theorem), but it occurred to me that given the composite section and assuming in reality the neutral axis shifts towards top flange of steel beam (probably right around the top flange), then your positive moment capacity for that beam goes DOWN, given the "c" distance to outer fiber goes up.

So, how does AASHTO account for the "true" flexural capacity by only considering the steel section properties.  I did take a bridge design course in college and I know there's methods for taking an effective section of concrete deck and transforming it into an effective "block" but thought it was interesting how AASHTO does these calculations and would like some greater insight into where/how the equations were derived.  Do the equations account for all this in some way, or is that what the Kg factor is doing?

Any insight would be greatly appreciated (and if I'm outside the domain of this discussion board then please let me know).

Thanks,
#Other

------------------------------
Daniel Foltz EIT, A.M.ASCE
Engineer
Williamsburg VA
------------------------------

Hi Daniel,

Great question.  I'm not a bridge engineer, but composite design (steel beam/girder engaging a concrete slab in compression) typically assumes general yielding of most or all of the steel section rather than triggering failure at initial yield of the extreme compression or tension fiber.  Plastic section design doesn't punish a large "c" distance at initial yielding, because the yielded extreme fiber continues to provide capacity even as stress plateaus at Fy across more and more of the section.  Below are some section stress states at ultimate to illustrate the point.

I have seen rare instances in fully elastic design (for pile foundations, for instance) where adding structure reduces the capacity due to a larger "c" distance, but I'm not aware of any such cases for plastic design.  As for your specific questions about AASHTO, hopefully someone with expertise in bridge engineering can step in to answer.



------------------------------
Christian Parker P.E., M.ASCE
Structural Project Engineer
Washington DC
------------------------------

Original Message:
Sent: 10-26-2022 07:01 AM
From: Daniel Foltz
Subject: AASHTO Bridge Design

Hi,

I'm new to ASCE association and hope it's "ok" to ask questions related to other structural standards besides say ASCE 7 for example, but something I'm currently working on is bridge repairs and I'm doing basically some load rating calcs for the first time.

I noticed the AASHTO spec only considers the steel section properties for bridge beams (girders? - I typically associate girders as a member supporting other members like joists, etc but seems various terminology is used across the board), when calculating section properties. I see there's a Kg which I guess accounts for the stiffness of deck slab (which looks like a modified version of the parallel axis theorem), but it occurred to me that given the composite section and assuming in reality the neutral axis shifts towards top flange of steel beam (probably right around the top flange), then your positive moment capacity for that beam goes DOWN, given the "c" distance to outer fiber goes up.

So, how does AASHTO account for the "true" flexural capacity by only considering the steel section properties.  I did take a bridge design course in college and I know there's methods for taking an effective section of concrete deck and transforming it into an effective "block" but thought it was interesting how AASHTO does these calculations and would like some greater insight into where/how the equations were derived.  Do the equations account for all this in some way, or is that what the Kg factor is doing?

Any insight would be greatly appreciated (and if I'm outside the domain of this discussion board then please let me know).

Thanks,
#Other

------------------------------
Daniel Foltz EIT, A.M.ASCE
Engineer
Williamsburg VA
------------------------------

Thanks for your response. 

I'm afraid I "jumped the gun" and fired off a question due to a lack of understanding. I was following an FHWA design GoBy calc for some repairs, and they did not consider the composite section of bridge girder, only steel girder properties, and so I assumed that was how AASHTO designed their steel girders, only to find out they definitely do consider composite action (and there's plenty of more info. I have yet to learn) and for some reason the GoBy calc I was using didn't explain why they were only considering steel section, but in any case I guess it's conservative to only consider the steel for carrying design loads. 

Anyways, long story short, I know what I didn't know and will "proceed with caution" when asking further questions :)

------------------------------
Daniel Foltz EIT, A.M.ASCE
Engineer
Williamsburg VA
------------------------------

Original Message:
Sent: 11-07-2022 05:12 PM
From: Christian Parker
Subject: AASHTO Bridge Design

Hi Daniel,

Great question.  I'm not a bridge engineer, but composite design (steel beam/girder engaging a concrete slab in compression) typically assumes general yielding of most or all of the steel section rather than triggering failure at initial yield of the extreme compression or tension fiber.  Plastic section design doesn't punish a large "c" distance at initial yielding, because the yielded extreme fiber continues to provide capacity even as stress plateaus at Fy across more and more of the section.  Below are some section stress states at ultimate to illustrate the point.

I have seen rare instances in fully elastic design (for pile foundations, for instance) where adding structure reduces the capacity due to a larger "c" distance, but I'm not aware of any such cases for plastic design.  As for your specific questions about AASHTO, hopefully someone with expertise in bridge engineering can step in to answer.



------------------------------
Christian Parker P.E., M.ASCE
Structural Project Engineer
Washington DC

Original Message:
Sent: 10-26-2022 07:01 AM
From: Daniel Foltz
Subject: AASHTO Bridge Design

Hi,

I'm new to ASCE association and hope it's "ok" to ask questions related to other structural standards besides say ASCE 7 for example, but something I'm currently working on is bridge repairs and I'm doing basically some load rating calcs for the first time.

I noticed the AASHTO spec only considers the steel section properties for bridge beams (girders? - I typically associate girders as a member supporting other members like joists, etc but seems various terminology is used across the board), when calculating section properties. I see there's a Kg which I guess accounts for the stiffness of deck slab (which looks like a modified version of the parallel axis theorem), but it occurred to me that given the composite section and assuming in reality the neutral axis shifts towards top flange of steel beam (probably right around the top flange), then your positive moment capacity for that beam goes DOWN, given the "c" distance to outer fiber goes up.

So, how does AASHTO account for the "true" flexural capacity by only considering the steel section properties.  I did take a bridge design course in college and I know there's methods for taking an effective section of concrete deck and transforming it into an effective "block" but thought it was interesting how AASHTO does these calculations and would like some greater insight into where/how the equations were derived.  Do the equations account for all this in some way, or is that what the Kg factor is doing?

Any insight would be greatly appreciated (and if I'm outside the domain of this discussion board then please let me know).

Thanks,
#Other

------------------------------
Daniel Foltz EIT, A.M.ASCE
Engineer
Williamsburg VA
------------------------------

No worries; I'm glad you found the correct reference.  That's what ASCE Collaborate is for!

Best of luck,

------------------------------
Christian Parker P.E., M.ASCE
Structural Project Engineer
Washington DC
------------------------------

Original Message:
Sent: 11-08-2022 04:11 PM
From: Daniel Foltz
Subject: AASHTO Bridge Design

Thanks for your response.

I'm afraid I "jumped the gun" and fired off a question due to a lack of understanding. I was following an FHWA design GoBy calc for some repairs, and they did not consider the composite section of bridge girder, only steel girder properties, and so I assumed that was how AASHTO designed their steel girders, only to find out they definitely do consider composite action (and there's plenty of more info. I have yet to learn) and for some reason the GoBy calc I was using didn't explain why they were only considering steel section, but in any case I guess it's conservative to only consider the steel for carrying design loads.

Anyways, long story short, I know what I didn't know and will "proceed with caution" when asking further questions :)

------------------------------
Daniel Foltz EIT, A.M.ASCE
Engineer
Williamsburg VA

Original Message:
Sent: 11-07-2022 05:12 PM
From: Christian Parker
Subject: AASHTO Bridge Design

Hi Daniel,

Great question.  I'm not a bridge engineer, but composite design (steel beam/girder engaging a concrete slab in compression) typically assumes general yielding of most or all of the steel section rather than triggering failure at initial yield of the extreme compression or tension fiber.  Plastic section design doesn't punish a large "c" distance at initial yielding, because the yielded extreme fiber continues to provide capacity even as stress plateaus at Fy across more and more of the section.  Below are some section stress states at ultimate to illustrate the point.

I have seen rare instances in fully elastic design (for pile foundations, for instance) where adding structure reduces the capacity due to a larger "c" distance, but I'm not aware of any such cases for plastic design.  As for your specific questions about AASHTO, hopefully someone with expertise in bridge engineering can step in to answer.



------------------------------
Christian Parker P.E., M.ASCE
Structural Project Engineer
Washington DC

Original Message:
Sent: 10-26-2022 07:01 AM
From: Daniel Foltz
Subject: AASHTO Bridge Design

Hi,

I'm new to ASCE association and hope it's "ok" to ask questions related to other structural standards besides say ASCE 7 for example, but something I'm currently working on is bridge repairs and I'm doing basically some load rating calcs for the first time.

I noticed the AASHTO spec only considers the steel section properties for bridge beams (girders? - I typically associate girders as a member supporting other members like joists, etc but seems various terminology is used across the board), when calculating section properties. I see there's a Kg which I guess accounts for the stiffness of deck slab (which looks like a modified version of the parallel axis theorem), but it occurred to me that given the composite section and assuming in reality the neutral axis shifts towards top flange of steel beam (probably right around the top flange), then your positive moment capacity for that beam goes DOWN, given the "c" distance to outer fiber goes up.

So, how does AASHTO account for the "true" flexural capacity by only considering the steel section properties.  I did take a bridge design course in college and I know there's methods for taking an effective section of concrete deck and transforming it into an effective "block" but thought it was interesting how AASHTO does these calculations and would like some greater insight into where/how the equations were derived.  Do the equations account for all this in some way, or is that what the Kg factor is doing?

Any insight would be greatly appreciated (and if I'm outside the domain of this discussion board then please let me know).

Thanks,
#Other

------------------------------
Daniel Foltz EIT, A.M.ASCE
Engineer
Williamsburg VA
------------------------------