Shoring concrete floor system

[DEL2000]

I asked a question the other day about getting loads on a crane outriggers . . . well, now I am shoring for that crane to drive out onto the slab it is going onto, as well as shoring for the outriggers.

I haven't done a lot of shoring, so I don't know if I am missing something obvious . . . it seems impossible to shore the slab and use the slab at all to distribute the load if it doesn't have negative reinforcement. I have one way slabs spanning 13', so any line of shoring added in the middle of that slab won't have negative moment steel (The negative moment steel is cut off at the quarter points of the slab as that is all that is required for the building in its completed state).

What are the options here? Does the contractor have to put steel in the top of the slab now? Or do you have to put enough shoring in there that the concrete is in straight compression, transfering load straight to the shoring? This second option would seem too problematic that I haven't even suggested to the contractor, but maybe it is better than adding reinforcement in the top of the slab and having to worry about the detailing of the rebar and all of that added cost.

We mentioned limiting where he could drive the crane on the slab and provide shoring directly beneath where he was going to drive . . . they rightfully didn't want to be limited in where they could drive the crane if they get out there and had to move the crane this way or that way, they didn't want the whole job shut down.

Just looking for some ideas from somebody who may have come across this situation.

 

[DTGT2002]

Using a crane on an elevated concrete deck that was not designed for such a load is quite a challenge.

I (in conjuction with my supervisor and mentor, as I am not yet a PE) faced a similar problem earlier this year and will be heading out this week to reinspect the shoring system in use as the crane will be in use on the deck again shortly.

My situation is a bit different, though quite analogous. Please describe the problem a bit further.

My job required the placement of several cranes on an elevated precast roadway for the erection of several tower cranes in the middle of an expansive jobsite with many structures in place, limiting access only to elevated precast roadways.

One of the first disjoints in logic I found was in my impression of the weight of the crane. Initially, I took the weight of the crane as given by the weight on the manufacturers ground pressure program. This weight is appropriate for the crane when equipped to make a crane pick, but does not necessarily represent the weight at which the crane may travel. The GVW of the crane in my case was ballpark 300,000 lbs at max load, but about 100,000lbs rolling down the highway. By driving the stripped down crane to the location of the pick and assembing with a helper crane, shoring the full travel path was not required in my case.

I recieved a lot of misinformation in the beginning of my research and planning for the shoring system and it was justified. No one in my area had any experience in what we needed to do, so no one ever assumed we would need to use a stripped down crane to drive onto the roadway, then be assembled in place by an additional crane with multiple setups.

In the end, my concept involved a strong regulation of the locations of all outriggers and the location of staged materials. Additionally, I specified clusters of reshores to be engaged through substantial sills and blocking at each outrigger location.

Short of supplying the massive amount of shoring you have seen required to allow the slab to act only in compression, I don't see a good way to allow the crane to be located on a whim.

This issue is best solved through communication. In retrospect, I wish I could have met with the crane setup crew even earlier in the process which would have provided insight into many ore simple issues that took me days to discover and resolve, one being the ability of the crane to "walk" or tilt on its outriggers, allowing higher sills or dunnage to be installed versus what the spec sheets call out as ground clearance.

Additionally, the operator of the crane has serious resevations aout what he was being asked to do, but once on board with the plan, helped execute the plan to his satisfaction, as it was he who would stand the greatest risk in the event of disaster.

Please discuss further and provide any and all details.

Are you the EOR of the structure to be driven over? Do you have access or a good working relationship with the EOR?

Good luck!
Daniel Toon

 

[SlideRuleEra]

It is not unusual for a contractor to request a design change to the permanent structure to accomodate his construction methods. Added reinforcement steel sounds like he "cleanest" way to go. Of course he should pay for the added design work, materials, labor AND the time it takes for the Engineer of Record to ensure that there is no adverse impact on the final structure.

If he does use some type shoring instead of design changes, perhaps you can require that he provide an "engineering report", prepared at his expense, describing exactly what he will do and why it will not harm the permanent structure.

 

[DTGT2002]

With regard to the slab, designing the slab to support the load might be a reasonable possibility. how much additional steel would be required to give the needed capacity?

Had we not stripped down the crane, we had considered a topping slab of enough substance to remove the need for dunnage where the crane would be driven.

 

[DEL2000]

Thanks for the responses . . .
we are actually not the engineers of records on the building, but have been asked by the contractor to do the calculations for the shoring. My supervisor previously worked at the Engineer of Record's office, so we should be able to discuss with them what we are planning to do.

This job will probably only be a single Rough Terain Crane that weighs approximately 100,000 pounds, so decreasing the moving weight is probably not an option. Mine definetly doesn't sound as rigorous as DTGT2002 had to do. But we plan on locating shores directly beneath the outriggers once the crane gets located to make the pick.

The slabs would require quite a bit more steel to drive the crane onto them, and a redesign would be required. The good thing is that there are only 3 or 4 different types of slabs, so redesigning them wouldn't be terribly difficult. I am beginning to think that a slab redesign in conjunction with shoring would get the job done.

 

[DTGT2002]

The slabs in my case were 4" precast with a 5" to 8" topping. The steel was far from adequate to carry the outrigger loads we required to our grid of reshoring.

I think you are heading in the right path. If the slab can be removed as the limiting factor, the shoring can become more reasonable.

Depending on labor rates in your area, it may be economical to pack in a forest of shoring instead of adding the steel. Do you have a relationship with a shoring contractor?

Other thoughts that do come to mind are paying attention to the micro loading once the marcoloads are taken care of. Aligning anything witht he crane exactly is very difficult. Give a grid with some room for error and dunnage to help spread your load out into the gird of shores. Do make sure all assumptions are clearly stated and understood by those performing the work. If there is a region of the slab that will not be suitable to drive on make it obvious. This operation will likely be one worth getting onto the site to oversee in some capacity.

if the crane you will use is to be a linkbelt crane, most of their new models have literature on the internet. I was told we would be using such and such crane, i asked if it was the rough terrain crane with the same capacity and was told yeah. The difference in the yeah and the real one to be used was around 30,000lbs of weight and a very different outrigger configuration.

The biggest consequence of the outrigger configuration is that I assume the persons placing the shoring will want to have it inplace long before the crane arrives and with plenty of time for you to inspect it. If the crane comes onsite with a completely different configuration than expected, it will take some time to relocate the shoring below the outriggers.

Also, it was well worth my time to go out to our crane subcontractor and view the cranes planned to execute the work. Most spec sheets are pretty close to reality, but many crane models get revised over the years and the spec sheets do not always match what is in the yard.

Good luck and stay safe!

Daniel

 

[shepherd]

Del200,
I have worked for a bride/marine contractor in the past and we were faced with similar problems. We had available (in stock) many rated crane runways (fabricated and tested in house). The runways were usually beams with diaphragms welded between topped with adequate timber decking. With these runways you could acutally bridge the crane to your concrete beams (block runways at beam locations, spanning over the slab), and shoring would only be required at your beams. If the contractor does a lot of this type of work, it may be benefical for him to fabricate some temporary runways like this? It may also be cheaper than all the temporary shoring to begin with? Just a thought. Hope this gives you some ideas.

Shepherd