Bridge Jacking - Determining Vehicular Live Load 8

[JHighfield]

Hello Everyone!

Coming here to see if I can get some help from someone with experience in bridge jacking design. I'm currently working a project where we're jacking up a bridge, span by span, in order to perform repairs
(I am a structural engineer for a contractor). I'm currently trying to determine a vehicular live load for the structure, but the AASHTO LFRD Bridge Design Specifications are confusing me, so I'm not sure how to proceed. The bridge will need to remain open to traffic for the duration of the project.

Any help on this would be greatly appreciated!

 

[SlideRuleEra]

I never had to deal with jacking of spans. For a temporary trestle (H20 or HS20 loading) would use 640 lb/ft, distributed over a 10' lane width plus a concentrated load, every 50', of either 18 kips for moment or 26 kips for shear.

http://www.SlideRuleEra.net

 

[STrctPono]

You are performing some spooky work here. Are you stamping this? Certainly doable and something that I have done before but very specialty work for sure. I don't mean to sound harsh but based on your question it seems apparent that you are probably in over your head if you are the one designing and stamping this as it doesn't sound as if you are very familiar with AASHTO code. What is confusing you about the AASHTO LRFD Bridge Design Specifications? You design for either an HL-93 load which consists of a design truck and design lane, HL-93 Truck Train with lane, or a design tandem truck. You need to know how to run a moving load analysis but since you are really only concerned with reaction forces at the piers/abutments you need to figure out where to position the truck to produce those maximum effects. Multiple presence factor, dynamic load effects, and live load distribution factors are also all terms you will need to be familiar with. Yes, it is quite confusing unless you use the specifications regularly, hence why this sounds like a spooky endeavor for you.

You should definitely NOT be allowing live load on the bridge during the jacking process. I have never done this nor have I ever heard of this. You stop traffic, jack it, then shore it, then you can allow live load back on.

What kind of jacks is the contractor going to use? Do you have enough clearance from the soffit of the girders to the top of the pier cap/abutment cap? Do you have enough horizontal clearance from the edge of the cap to the bearing. Sometimes not, which then requires a innovative design of providing shoring on the face of the pier cap. If the bridge isn't too tall, you may be able to post down to the ground. Be careful where you place your jacks. It's not as much a concern for a prestressed concrete bridge but for a plate girder bridge with stiffener bearings there is a serious concern about web crippling.

How big is this bridge that you are working on to give us a size of scope? Obviously a big bridge carries a lot more risk.

 

[Ingenuity]

I have done a few bridge jacking projects - most on design-build basis. From replacing bridge bearings, hydraulic lifts for concrete repairs, and one was a large 200+m span balanced-cantilever where we used 8,000 kip total capacity flat jacks (4 each x 36" diameter).

The hydraulic systems (rams, pumps and gauges etc) is rather simple, but you need to consider redundancy of the hydraulics - be it due to leaks, blown seals, pump failure etc. Need to think about locking-collars, wood dunnage, steel shims, back-up rams etc. Lots to consider.

If you wish speak to a company with real-world expertise in the US on the West Coast, call CH Bull Jacking Solutions in California (Link) and speak with Stan Sheppard. Stan has a lot of experience with jacking all sorts of bridges - mainly in California - and a real nice guy too. CH Bull also do load calibration with capacity up to 1,000 kips.

If you are on the East Coast, consider Richard Dudgeon Co (Link). They have been around for 150+ years, and have a very large inventory of jacks, pumps etc, but not sure if they provide site-based services.

From a loading perspective - be conservative in determining your span reactions and loads to the hydraulic jacks. Keep your load paths as simple as possible. Extra hydraulic lift capacity is a good thing BUT when lifting you need to have full control (via manifolds, check valves etc) on where the hydraulic jack forces are going/distributed. There are horror stories where multiple rams connected to single pump systems have gone awry because they were not synchronously controlled.

As STrctPONO stated, give us a bit more info on span/scope so we get a sense of scale of your project.

 

[Lomarandil]

Great advice from STrctPono and Ingenuity.

I'll agree that none of my jacking projects have ever had live load during the operation (or hydraulics left "live"). Lat night temporary shutdowns to jack and shim are standard.

----
just call me Lo.

 

[bridgebuster]

@JHighfield - if the client would permit you to use the Standard Specifications, it would make life so much easier. You could be conservative and use HS-25 loading.


30+ years ago I was resident engineer for a rehab project - a mile long viaduct on I95 in New Haven CT. We had something like 300 bearings to replace.The plans didn't allow jacking under live load. These were simple spans and the plans called for jacking part of one bearing line in a lane closure and putting down some asphalt to eliminate the bump. After the job started ConnDOT directed the contractor to jack under live load to save time and eliminate the paving. Under the change, the contractor had to perform simultaneous jacking of both lines of bearings at the pier; only one pier at a time. I think the total lift might have been 1/2" max; I'd have to dig out those plans. I recall doing the first jacking operation at 4:00AM when traffic was light. Everyone was a little on edge; the contractor used locking jacks from Dudgeon. The contractor would jack 1/8", lock everything, make sure things were OK, then repeat the process until the maximum lift was reached. Eventually, everyone was OK with things and they would jack to 1/2" in one shot. They never jacked when traffic was heavy. In NY, the DOT specs say jacking under live load is only permitted if shown on the plans. However, I've never seen it permitted on any NYSDOT project I was involved with.

 

[Guest090822]

Not sure who the client is, but some states give the lifting loads on the contract plans, generally the dead load and live loads are given. You could always have the contractor send in an RFI requesting the loads from the owner. They should have the information on file given that a load rating exists for most bridges.

 

[JHighfield]

Okay, a lot to answer here, but here goes: For this project, we’re jacking up spans of a 50+ year old two-lane bridge, with the longest span being 110’. As the contractor, we are proposing a jacking system to be used, with the city’s resident Engineer reviewing. I do not believe that we are stamping – my idea will be vetted and designed by the engineer-of-record. That being said, I do need to flesh out my concept a little and these comments do provide a lot of help. In the drawing I’ve attached you can see a section of the bridge; the longitudinal jacking girder will be furnished by us and will be bolted to the girder (we’ll use this for each girder-girder span). Not shown (as I haven’t drawn it) I propose to use a 100 ton pancake jack with an HSS member to jack up the bridge via the girder. We’ll then shim it once it’s been jacked.

STrctPono: Spooky work indeed! Yes, I recently started here and I don’t have a whole lot in terms of guidance beyond some concepts. And you are correct – I’m not very familiar with the AASHTO specifications. What you said makes sense now that I’ve read the chapter 3 a few times. So, the state we’re operating in does allow for vehicular traffic to be present on the bridge during jacking – in addition to the dead load we need to incorporate HL-93 loading with an impact factor, and the DOT specs also provide LRFD factors.

Ingenuity: Thanks for the advice, and for the contacts. I think I’ll reach to Richard Dudgeon, see what they have to offer. In fact, I’ve been meaning to reach out somewhere, but I can’t find a lot of info online about who to go and talk to – any further advice would be appreciated (my concept is above).

Lomarandil: Indeed, great advice! Thanks for your comment as well, and yes I may need to talk to the PM about doing that.

BridgeBuster: It’s a DOT job, so we’re beholden to the state’s and AASHTO’s spec (required by the state spec). And yes, I’m going to look into Dudgeon. Probably we’re going to have to shut down the bridge temporarily and shim it, as that seems to be the safest bet.

Therick109: While not on the plans, I agree it’s a good idea to reach out to the EOR for that information, which should be easy to get.

 

[StructEngBrah]

You probably need to look into the project specifications to see what is required by you, the contractor. if I had to bet, the specifications probably state the contractor to provide a sealed jacking plan. That does not mean the EoR for the whole job is going to seal your jacking plan for you. The EoR will likely review your jacking plan to see if it passes the sniff test and look for big things that are out of whack. It is typically on the contractor to seal the jacking plan either with an inhouse engineer, or engage a consultant to do the design and seal the jacking plan. This provides some flexibility for the contractor to use a system they want, and prevents the EoR of the job from dictating means and methods.

 

[STrctPono]

JHighfield,

Thanks. That's nice that Ingenuity provided some contact points for you with contractors who specialize in jacking. I think having them come up with the conceptual plan is the best bet if you guys are going to be hiring them anyway. I say that not only for a liability reason but also because they very well might trash any idea/work you have come up with anyways and start from scratch. Either way, getting a preliminary tonnage for your jack size is correct. You can calculate dead weight of the structure easily. This would be steel girders, cross bracing, concrete haunch, concrete deck, AC overlay, sidewalk, barriers, and utilities. Multiply all the dead weight by 1.30 (AASHTO 3.4.3.1) but multiply AC overlay and utilities by 1.50 for load factor. Live load is hard if you've never done this with AASHTO before. Conservatively, Take your tributary girder spacing and multiply it by 64 psf x half your bridge length for the lane load effects. For truck load, assume 1.0 LLDF and put the whole 72kip reaction at each girder bearing. Multiply the truck load by 1.33 for the dynamic allowance and then multiply both the lane and truck load by 1.75. (This may be about 40% overly conservative with truck reaction load but it will be conservative)/ Add your dead and live load reactions together and you have the weight that needs to be carried by each jack.

Make sure that the jack that you are choosing can fit between the soffit of the girder and the cap/abutment shelf. Pancake jacks are nice for this since they can be used where headroom is minimal but check to make sure the diameter of the jack can also fit between the edge of the shelf and the bearing. If not, you have a problem. Your structural engineer should be able to help you check your shoring member design and make sure that by changing the centerline of your reaction load you are not negatively affecting the column, abutment, or foundations. This last part is something that is not going to be obvious to you but should be apparent to any competent Engineer. This is less of a concern if the bridge were closed to traffic since it's just dead loads but with live loads this could be a real concern.

Essentially, work with your jacking sub to come up with the plan and have your structural consultant check the proposed plan and design the shoring system.

 

[Lomarandil]

STrctPono, will jack sizing be done using AASHTO strength combinations, or service? IT's been a little while, but I remember most manufactured products being sized for service/working loads (and then their own factors of safety against that)

----
just call me Lo.

 

[STrctPono]

Lo,

You are absolutely correct. I misspoke, for jack/cylinder sizing the manufacturers typically only deals with service loads and apply their own industry accepted safety factors. The strength level loads would still be applicable for everything else of course. If you follow AASHTO 3.4.3, "The design forces for jacking in service shall not be less than 1.3 times the permanent load reaction at the bearing.... Where the bridge will not be closed to traffic during the jacking operation, the jacking load shall also contain a live load reaction consistent with the maintenance of traffic plan, multiplied by the load factor live load."

JHighfield, if you remove the (1.25, 1.50, and 1.75) load factors you will be left with Service Level loads. This is what you should be using to size your jack.