Pedestrian Bridge 12

[medeek]

Technically I'm not suppposed to engineer this bridge. They will be replacing the main members and deck exactly as they are currently constructed so as to avoid having to bring it up to code:

My responsibility on the project is to simply document the process, draw an as built of the bridge and provide a scope of work (what members are being replaced etc...) The pilings were inspected and approved by some state agency and were deemed acceptable so those will remain.

In the process of replacing the main members the owner would like to do some minor upgrades:

1.) Provide strapping to secure PT glulam beams to pier system directly below.

2.) Provide strapping between abutting PT glulam beams for continuity and so beams have less chance of "walking" off of their bearing points. This pedestrian bridge services the golf course as well as pedestrian traffic so vibration from vehicles (albeit small) is something to consider.

3.) Provide some form of anchorage between the abutments (concrete) and the end terminal beams. Currently they are just bearing on a concrete ledger without any securing mechanism, see image below:

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[hokie66]

So what is the question?

If the "main members and deck" are in bad shape, what about the piling?

 

[medeek]

Technically I am "not to engineer" the bridge, which I find a bit odd, but that is the situation. I've never even looked at a pedestrian bridge before or any type of bridge for that matter. My question has to do with the items listed above. The owner wants some form of "recommendation" as far as straps or anchorage for his minor upgrades. All other repairs are simply to replace the bridge main members, deck and railing as it is currently constructed.

Being familiar with residential work and conventional light frame construction my first inclination is to turn to my Simpson manual. Whether this is a reasonable solution or not is somewhat of a question to me so I want to put it out there for others to weigh in:

1.) Secure beams to piling system with CS14 and 10d stainless nails
2.) Tie beams together at ends with strap nailed to one side of beam, CMST14 and 10d stainless nails
3.) Either a couple of DTT2Z into the side of each beam and a 5/8" all thread epoxied into the concrete abutment or some form of angle bolted into the bearing ledge and then into the beam.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[hokie66]

Strange. My first reply was to a post which had only the first picture and the writing above the picture.

I think what you are describing would be considered design or redesign. I don't like to use "engineer" as a verb.

 

[medeek]

Agreed, this one is a little bit odd for me. I originally looked at the design of the bridge and he wanted to redesign it with 3 beams per span instead of 4. However, the city inspector came back and told him that would require a re-design of the bridge and an engineer's stamp. I then gave him a proposal for the re-design and engeering, the additional cost for re-design, engineering and possible retrofitting to current standards offset any savings he might have realized with going to a 3 beam design. He then proposed to the city that he just leave the design as is and remove and replace the rotted out main members and decking/railing. The city came back and said that would be okay and not require an "engineered" design or a stamp. Then a couple days later they send him an email requesting that a engineer or design professional at least provide a formal scope of work and as built drawings for the bridge and its repairs since they had nothing on file (bridge was originally built back in the 1960's with no drawings or plans).

Of course he wants all this done by tomorrow and I am the only one in the local area that is willing to take it on with such short notice. So that is how I'm stuck with this one. Drawing up the scope of work and as built is a no brainer, I'm just a little uncomfortable recommending additional repairs/upgrades when I am not actually applying any numbers to it.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Notice how rotted the main beams are, clearly evident in the second picture above. All sorts of "fix its" have been applied to this bridge to try and keep it serviceable over the years. Before Christmas I came out and did my first quick survey of the bridge and was amazed that they were still driving golf carts across it considering its condition. A quick letter to the owner of the golf course and to the city had them shut it down to all traffic, which then instigated a full scale repair or replacement.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[OldBldgGuy]

Sorry, I don't see anything in the pictures or the posts that makes me feel good about any of it. You agreed to the as-builts, stick to that with disclaimers hanging on every thing you touch or hold out for an actual design assignment.

 

[rb1957]

i think your client is trying to have the best of both worlds ... save the expense of bringing the bridge up to code, by replacing like with like, but also fixing some other issues with the original design (presumably to save on maintenance costs in the future).

pick one or the other.

another day in paradise, or is paradise one day closer ?

 

[medeek]

@rb1957

I think you've hit the nail on the head.

I just met with him about an hour ago to review a few last minute items to add to the scope of work. Specific design flaws in the original bridge construction caused the bridge to rot out (main members). This time around he will be taking measures to avoid those previous mistakes, so realistically there are some design changes but the hope is that these design changes are minor enough that it does not trigger a "new design" by the building department.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Just to show the condition of the bridge I thought I might post this picture:

Notice the end of the smaller beam, it has rotted to the point it is collapsing down on to itself, kind of scary to think they were just running 1,000 lbs golf carts across it a couple weeks ago.

Of course the major problem with the original bridge construction is the use of non-pressure treated glulams in a very wet environment (olympic peninsula, WA) and it is within a 1/4 mile of the beach.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[dhengr]

Medeek:
You ought to be applying some real numbers to the problem. This isn’t just the IRC in its lightest version, where you follow the cookbook and ask a bunch of questions on a forum, to form some engineering determination. If you haven’t got the experience to do this job, maybe you shouldn’t be doing it. Have you read that part of the engineering registration and stamping criteria which says you should only practice in areas where you have experience? Someone has to make an engineering judgement about the foundation of this bridge, its condition and the profile of the pile caps, or you’ll end up with about the same thing you have now, but with newer floor beams and decking. The City Bldg. Dept. is being chicken shit too, in saying replace it in kind or you need an engineer to stamp it. In their case, do they want someone with good engineering experience to evaluate the entire condition of the bridge, P.E. stamp or not, or are they only looking for a cheap-fee engineer who is really acting as the insurer of last resort, through his insurance coverage. There seems to be incredible misunderstanding, partly intentional in today’s world, about the duties, responsibilities and obligations of a registered engineer’s involvement in a project. Why don’t they go directly to a reinsurer if they aren’t willing to pay an engineer for his knowledge, experienced, time and judgement in bettering the outcome of a project? Hell, just go to the big box store, they probably have a program to sell material which will design this for them.

 

[hokie66]

Like dhengr, I think you may be in over your head. If you work on this project, no matter the extent of your involvement, you will have bought the liability for the design, because there is no one else to blame if something goes wrong. Mr Simpson is not going to be of use in that regard, even if some of their hardware is applicable, which I doubt.

If I were doing this, the first thing I would want is to see the inspection report and approval by the "some state agency" who gave the pilings their blessing.

 

[medeek]

I get that I am over my head with a re-design, hence my proposal included the additional cost of involving one of my mentors who would then peer review all of my work, I wasn't about to tackle this one all on my own. Prior to my submittal of the proposal I actually sat down with said mentor and we discussed all of the issues with the bridge and what additional work might be required to bring it to current code. The lateral bracing of the pier system was definitely part of that discussion as was the quality of the abutment system. In the end they would probably be replacing the entire bridge, and so the proposal was rejected outright due to the high cost of repair/replacement/retrofitting.

My responsibility now is to simply provide a scope of work for the replacement of the bridge main members, deck, and guard rails. I am also to provide an as built drawing of the existing bridge showing the location, size and length of each beam. Both the drawing and the letter will have a large disclaimer that there has been no attempt to provide any engineering or re-design of the bridge and I am not making any declaration as to is suitability or fitness for use of any purpose. Furthermore I cannot make any claim that the bridge meets or complies with any building or bridge codes (ASHTOO, IBC, ASCE etc...), in fact I will categorically state that the bridge in all likelihood does not comply with current code as it relates to lateral wind and seismic loads. Technically anyone could draw up these documents it does not need to be an engineer but the city official has required that a registered design professional provide them in order to issue the permit.

I appreciate your advice and insight @dhengr and @hokie66 on the liability and politics of the situation. I see your point where the city is probably trying to leverage some liability at some level from me even though I'm not working on this as an "engineer", at the same time appease the owner by giving them an easy out. Thank-you for helping to clarify this. Even with all of the disclaimers in place does it protect me from the liability if the bridge were to fail in the future? I guess the question is can an engineer really take a non-engineering role? I will consult with my mentor first thing tomorrow morning and make a determination on how best to proceed.



A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[hokie66]

That is the issue. As you are a Professional Engineer, you will be taken to be providing services of Professional standard. The city official has a reason for requiring documents by a Registered Professional, and the reason is not to direct work your way.

 

[PEinc]

Time for a new bridge!

http://www.PeirceEngineering.com

 

[rb1957]

golf carts, you say ...

i'd vote to replace the bridge with a couple of "ski-jump" ramps ... just gun it !

another day in paradise, or is paradise one day closer ?

 

[oldrunner]

There is a lot of work for you and your co-consultant to do here and a lot of liability that may arise. Make sure you get a good fee to cover so that you can afford your E&O for years to come. If the client doesn't want to pay a fair fee, then walk away. If you do perform this project, I suggest you do the inspection also. The building department might even suggest or require special inspection of some sort. It might even be better to design a complete new bridge.

 

[bridgebuster]

If I'm reading this correctly your client want to replace the superstructure. If that's the case you're obligated to design the superstructure to current standards. I imagine WSDOT has some requirements to this effect for a non-state owned bridge.

Use the AASHTO Guide Specifications for Pedestrian Bridges either the Standard Specification version or LRFD version, depending upon the requirements of the AHJ. I would use the Standard Spec version if the substructures are to remain. The guide spec gives you the loading and vibration requirements; nothing mysterious about it. The design requirements for the timber would be based on the AASHTO Standard Specs, which more or less taken from NDS.

Railing is railing although AASHTO has a variety of loading cases to account for bicycles and pedestrians.

Tie downs are tie downs; use the AASHTO wind loads or even ASCE -07.

The USDA Forestry Service has a variety of publications and plans for timber bridges.

 

[medeek]

I've informed the client and the building department that any further involvement or documentation of the project would require a re-design that brings the bridge to current standards. Essentially I've had to walk away. It is up to the building department now to determine the direction this takes.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[dhengr]

Medeek:
You claim to be a Professional Engineer, although as a Mech.E. you are kinda working outside your educational area and probably even your professional experience area, so you better start acting like a real P.E. You have to be able defend and explain this experience/knowledge situation/gap if the issue comes up, and it will if anything goes wrong with that bridge. Assuming you only do some measurements, drafting and comments about member condition, your work will still be judged as having been done by a P.E. (not just some dumb draftsman) you should know enough to sound the alarm if anything about the whole design does not measure up. You must know enough about the foundation conditions to do the design correctly. If the owner wants to save a buck, by not doing the whole thing right, he puts you in a pretty precarious position if anything goes wrong. But, don’t worry, all they can take is your house, your car, your savings, and your engineering licence, they can’t take the other tools of your trade. That is, your pencil and.... It isn’t the Bldg. Dept. which has a decision to make, it is the bridge owner who must determine that he will spend the money to make it right, and not leave you alone, hanging on a hook. The Bldg. Depts., these days, always swings a big stick, and assumes very little responsibility, despite their ignorance about anything other than the exact code verbiage. But, you have to guard against the owner trying to save some money by putting all of the responsibility and liability on your back.