×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Posting a load sign on an existing bridge

Posting a load sign on an existing bridge

Posting a load sign on an existing bridge

(OP)
Hi,
The is an existing bridge (steel girder and concrete deck), 87ft span and 14 ft wide.
There is no load-rating on the drawings for this bridge, however I am very sure that was designed for HS-20 truck.

1-When I contacted the consulting that designed the bridge, they said at that time (1978 to 1982) there was no requirement to state the bridge load rating on the drawings or specs !!! Does anybody it is true?

2- Will I be correct to tell the owner to put a sign of 72000 lbs. on the bridge ?
An Hs -20 : 8000+32000+32000 = 72000 lbs.
Is that a correct sign and load rating for the bridge?

Thank you for your comment and direction. I am not a bridge engineer and my practice is regular structural engineering. I apologize to all bridge engineers and don't mean that I am stepping on their toe. This request just came to me to assist the bridge owner.

Sincerely,
SKJ

RE: Posting a load sign on an existing bridge

Why stick your neck out? Along comes a heavy load and down she goes. Next day you get a summons to fight a claim for damages.

RE: Posting a load sign on an existing bridge

It does seem like an unnecessary risk, especially if it's not your area of expertise.

Surely the original designers should be providing this information.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Posting a load sign on an existing bridge

If you are in the US, your state will have a procedure for analysis and rating (and posting) of existing bridges. Check with your local DOT for their manual.

----
The name is a long story -- just call me Lo.

RE: Posting a load sign on an existing bridge

In the US, I don't believe there are any requirements to post a bridge if it is adequate for the standard legal load trucks. It would be a rare case for a bridge designed for an HS-20 truck to not be adequate for the standard legal load trucks. That said, load rating bridges, when required, should be done by an engineer with the training in evaluation and experience load rating of bridges. It's not something I'd advise taking on by yourself without an experienced mentor to guide you.

RE: Posting a load sign on an existing bridge

(OP)
Thank you all for your comments.
Bridge is located in Union, Kentucky, USA.
The bridge is not a highway or State road and is in a private facility/factory, not a public road so I don't know if rules of highway bridges or as "Lo" stated State rules apply.

I appreciate it if possible also answering to the two first questions I had since yet I am unclear on the answers to those questions.

Respectfully,
Skj

RE: Posting a load sign on an existing bridge

I think this is one of those areas of engineering where if you have to ask how you’re practicing outside your expertise. Learn from a competent mentor until it’s second nature.

If something goes wrong that will be a horribly difficult lawsuit to defend. You’d be taking on any/all design and construction errors that are yet to rear their ugly heads. Nice to be helpful, but there should be limits.

RE: Posting a load sign on an existing bridge

(1) Not sure not my region sorry.

(2) If you don't have access to the original signed off calculations or the loading isn't stated on the drawings then the bridge needs to be assessed to the relevant design standards. How else can you know that the bridge was designed to take X vehicle?

Sounds to me that the ball is in the court of the original consultancy.

RE: Posting a load sign on an existing bridge

Quote (Skj)

The bridge is not a highway or State road and is in a private facility/factory, not a public road so I don't know if rules of highway bridges or as "Lo" stated State rules apply.

Only if the owner chose to have the bridge designed to highway standards. They may have their own standards which could be either lower or higher than typical highway bridges. Find out how the bridge was used when it was new, that will give you an idea of the original design requirements.

www.SlideRuleEra.net idea

RE: Posting a load sign on an existing bridge

The state guidelines may not legally apply, but they will be considered as the standard of care if liability ever comes into play. Plus, many states actually put out a good set of instructions to walk you through the rating process.

I've done bridge rating and posting before, major structures down to rural private bridges. There's no way that I'd consider putting any sign up (originally HS-20 or otherwise) without getting eyes on the bridge myself, or several dozen quality photographs by someone who knows what to look for. If you go forward, make sure you do this.

So to answer your questions:
1) Very possibly true.
2) No, this would not be correct, for the reasons mentioned in this thread.

If you want to really assist the bridge owner, it sounds to me that you should probably recommend an experienced engineer.

----
The name is a long story -- just call me Lo.

RE: Posting a load sign on an existing bridge

After 40 years, I would not rely on the original design loads without verifying there has been no section loss or other damage. You should hire an experienced
bridge engineering firm to do an inspection and load rating. If it hasn't been inspected, it's due.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. - http://xkcd.com/319/

RE: Posting a load sign on an existing bridge

I agree with AC, especially if this is a one-off proposition. You'll spend way more time getting up to speed on the process than someone who's done it before, and unless it's a simple span, the analysis will be very difficult without the correct software. Load rating is typically done for several trucks of different configurations, so there are multiple sets of moving live loads to consider, even after determining the magnitude of those loads.

The important thing would be to verify the design load. If the bridge was actually designed for an HS-20 truck, and it's not suffering from significant deterioration, it should be adequate for any truck that would be allowed on the public roads without a overweight load permit. That's the design load used for highway/interstate bridges for decades. I've done load ratings on many of those, and have yet to see one that that wasn't more than adequate for all of the legal load configurations.

RE: Posting a load sign on an existing bridge

(OP)
SlideRuleEra (Structural), Thank you for your comment. I checked with the owner and they don't have any standards.

Lomarandil (Structural), Thank you for your response and care. I totally understand the concern in the existing condition of the structure for load rating. My question is if found that the structural is in good condition (which has been inspected recently by professionals)and no significant damage and knowing that the bridge was designed for an HS-20, would that be correct to call the bridge rating 72000 lbs or 35 Ton? or typically they reduce this number? I understand 72000 lbs is just total weight of truck but unsure in sing posting they go by the total static dead weight of the truck or else?

HotRod10 (Structural), Thank you for your response as well.

I appreciate any and every comment and points.

Sincerely,
Skj

RE: Posting a load sign on an existing bridge

"...knowing that the bridge was designed for an HS-20, would that be correct to call the bridge rating 72000 lbs or 35 Ton?"

That would normally be the low end of the rating for an HS-20 notional load. However, the load rating for trucks in other configurations may be more or less. The HS-20 is a minimum of 14 ft between the rear axle and the 'drive' axle. Some trucks have axles spaced less than this distance. The load effects generated by a truck with 32,000 lb axles spaced 6 ft apart would exceed that of the HS-20.

RE: Posting a load sign on an existing bridge

SKJ25POL - keep in mind an HS-20 truck (72k) is not a real truck. The load posting signs you see are based on real trucks. There could be a load rating somewhere, assuming the owner complies with the FHWA inventory requirements.

RE: Posting a load sign on an existing bridge

(OP)
HotRod10 (Structural)and bridgebuster (Civil) Thank you so very much for helping me to understand the subject.
So,the question is if the bridge is designed to HS-20 ASSHTO truck (which is not a real truck according to bridgebuster (Civil)) and the load rating of the bridge is 35 Tons:

1- Is the sign on the bridge shall read 35 Tons conservatively(low end according to HotRod10 (Structural)) ?
2- The 35 Tons weight limit not sufficient for allowing trucks of 35 Tons or less to pass the bridge and axles number and distances also has to match HS-20 ASSHTO truck to be complied with HS-20 load rating?
3- What a correct sign for a HS-20 ASSHTO truck designed bridge would read?

The bridge is in the State of Kentucky if the geographical location matters in the sign selection.

Thank you,
Skj

RE: Posting a load sign on an existing bridge

I don't believe the bridge should be posted at all. If for some crazy reason the owner wants the bridge posted, a load rating will need to be performed for whatever the legal load trucks are for Kentucky (hint: the HS-20 isn't one of them). The HS-20 is a notional load, an imaginary composite used to approximate a range of common trucks on the road.

RE: Posting a load sign on an existing bridge

(OP)
HotRod10 (Structural), the owner wants to know or have a sign when trucks come to the facility know that if they can cross the bridge or not allowed to do. Knowing that the bridge was designed based on HS-20 how can we communicate this?
If he asks me such and such truck is coming to the facility and if it is okay the truck goes on the bridge how are we checking this? So if no sign each time they need to call me and ask if this and that truck is okay to cross the bridge? Is that how practically needs to be done? And then I have to check the total weight and axle distance that if it matches to HS-20? If matched I say the truck is okay to cross over?

Thank you,
Skj

RE: Posting a load sign on an existing bridge

If the owner wants to know what trucks can go over the bridge, a load rating will need to be done for the legal load trucks that the state uses. So long as the ratings for those trucks is acceptable using the same criteria (load factors) that the state uses, it's load carrying capacity is at least equal to the state-owned bridge across the state. Anything beyond that loading level would require an overweight load permit from the state to travel on the public roadways in the state. Assuming the load ratings are as good as the state's, it should satisfy the owner's concerns, since the states are very careful and conservative in protecting their bridges from damage.

RE: Posting a load sign on an existing bridge

(OP)
HotRod10 (Structural) and bridgebuster (Civil) thank you very much for your effort to help me. I am very grateful.
I finally got a little bit information and want to may be ask this question for the truck in below pic with total growth weight of 44,300 lbs, is the existing bridge (designed for HS-20)okay?
I mean is the below truck below or within the designed load for the bridge?



Thank you for your continuing help.
bridgebuster (Civil), thank you for the attachments. They were so informative.
HotRod10 (Structural), thank you for your great comment. I couldn't find legal load truck for KY. Seems this state is working on it. I believe HS-20 can be considered legal load truck !!!

RE: Posting a load sign on an existing bridge

Quote (OP)

I couldn't find legal load truck for KY.

You didn't look very hard then; found this in about 30 seconds: https://drive.ky.gov/motor-carriers/Pages/OWOD-Leg...

Here's their bridge design page: https://transportation.ky.gov/StructuralDesign/Pag...

HS-20 is a design load that should exceed or meet the worst case of legal loads. For the weight/truck you indicated it's difficult to tell unless you know how much load is on that tandem rear axle.

Based on the questions you are asking I feel you shouldn't be undertaking this design and should consult with a qualified bridge engineer. I'd also reach out to KYDOT as they likely can assist you in the legal requirements you must undertake to load rate and post the bridge.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries https://americanconcrete.com/

RE: Posting a load sign on an existing bridge

I can't say for sure whether that truck would rate as well as the HS-20. Technically, it would depend on the weight distribution and the length of the bridge. What I can tell you is that it appears very similar to our Type 3 legal load rating truck, which typically rates over 50% higher than the HS-20 for our bridges. I can't see that changing much except for maybe a bridge with very short spans (less than 30ft).

I can also tell you that somewhere around half of the public highway bridges in service in the US were designed for HS-20 live load. Those hundreds of thousands of bridges are used by millions of trucks of every legal configuration on a daily basis. There are maybe a few with major damage or severe deterioration that may be posted, but the vast majority are not. If the DOTs in all 50 states are not concerned about that, why is the owner of this bridge?

As I said, if the owner wants a load rating, my suggestion is refer them to an engineering firm that does load ratings. They have the expertise and the software to do it efficiently. Unless you plan on doing alot more ratings or start designing highway bridges, you're going to overcharge them or short-change yourself, due to the time it would take you to learn the process and the code requirements, and what it would cost you to buy the software package.

RE: Posting a load sign on an existing bridge

Or, you could take this approach...

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

eBook - The Future of Product Development is Here
Looking to make the design and manufacturing of your products more agile? For engineering and manufacturing organizations, the need for digital transformation of product development processes just became more urgent than ever so we wanted to share an eBook that will help you build a practical roadmap for your journey. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close