Holdfast Training Services Trial Blog

I recently had the pleasure of a week of night shift on my project (fortunately it will be my only one), during which I was responsible for overseeing the installation of the first half of a 90T beam on which a gantry crane will run to lift sections of the Tunnel Boring Machine (TBM) from the surface into the portal structure prior to launch.

Fig 1. Highlighted gantry crane section

I had the night before been thrown a hospital pass and asked to install the bearing plate for the beam which sits in a recess in the capping beam, but that’s another story and it did eventually go in, see Fig. 2 below.

Fig 2. Installation of bearing plate

The installation of the gantry crane beam was quite simple, as you can see from the following photos, and the whole operation went quite smoothly;

Fig. 3 Gantry crane beam install

As part of our other works the crew were also busy prepping the bearing plate on the opposite capping beam and the second section of gantry crane beam was due to be installed the following day, once our high early strength (HES) grout had gained sufficient strength. I decided to do a couple of check measurements to see exactly how the second beam would sit and to make sure we installed the bearing plate correctly. I knew from  the drawings that we were looking at around 80mm of clearance from the back of the capping beam recess, but my initial tape measurements suggested it was more like 270mm. I managed to grab the surveyors before the end of their shift and they confirmed my measurements.

Having had a discussion with the foreman, and mindful that there is a risk associated with every lift, we decided to do a dummy run of installing the second beam to see exactly how it would be positioned in relation to the capping beam and the bearing plate.   Having taken some photos when the beam was flush against the first section, it was evident that the “shoe” on the bottom of the gantry beam extended some 20mm beyond the edge of the capping beam, 195mm short of where it needed to sit on the bearing plate as outlined below.

Fig 5. Gap between capping beam and beam

Fig 4. “Shoe” extending beyond capping beam

Fig 6. Location of beam in relation to capping beam recess and bearing plate

This news was not very well received in the morning and the issue appears to be the result of the the gantry beam design not being reconciled with the wider cut and cover design, though the senior project engineers are being a bit cagey with the information on this…

Clearly some remediation works is required and I suggest there might be two ways forward;

1. Remediate the capping beam.
   1. Break back the concrete in and around the recess.
   2. Tie in to the reinforcement and extend the capping slightly to give enough bearing for the gantry beam.
2. Fabricate an intermediate section of gantry beam.
   1. Install the gantry beam so that it has the necessary bearing on the existing capping beam.
   2. Get an intermediate section fabricated to fill the gap between beams. (a rail will run on top of the beams so potentially a ~200mm gap not too hard to bridge?)
   3. Get new splice plates fabricated to extend across the gap in the beams

(Note: the design of the bearing plate means that relocating the one already installed would not be sufficient to fix the problem)

My initial thoughts are that to remediate the capping beam would be too complicated with potential issues due to the fact that the load on the beam may be supported partly on a cantilever and not transferred through the capping beam directly onto the piles below.

Fabricating an intermediary section with new splice plates seems like it might be a more viable option.

Thoughts from the wider forum?

The Chosen Solution

So in the end despite my cynicism, it would appear that the error in fabrication length of the gantry beam was not related to the engineering team and the blame lies with the steel fabricator in Singapore. The team did not want to draw attention to the issue so that a speedy resolution could be found on site with the upper echelons of the project wading in. It was decided that the most effective way to resolve the issue was to make changes to each of the bearing plate to allow the “boot” of the gantry beam to fit. The side already installed had fabrication work done on site , while the other was sent away to a worshop;

You’ll see from the image above that the front edge of the recessed plate was removed and an additional section of steel welded to the plate to bring it out level with the front edge of the capping beam. Both gantry beams have now been installed and at the headwall the excavation has reached level 2 of the props so waler brackets and walers beams are about to be installed. With only 2 weeks until the site closes for Christmas, the excavation will not reach base slab level as hoped but the engineering team are being driven hard to deliver. This is my last day on site, so unfortunately I won’t see level 2 going in.

I have elbowed my way into the team that have been deciding on how to ensure that the design levels will be within tolerance in the finished state and remain within the movement and tolerance specifications as per the NSSS, as well as determining how to optimise this process. This is to be done through pre-setting baseplates and connections and jacking up columns a particular stages.

Within this team, which includes BHL designers, myself and the Mace project engineer, we discussed how to cater for the uncertainties inherent in designing for movement and the change in construction methodology will impact on the pre-set strategy. The the various causes of  movement considered are:

Foundation settlement

Column shortening

Elastic elongation of connections

Deflection in beams and trusses

The pre-sets resulting from column shortening are to be accommodated by adding packing plates to splice and base plate locations. Foundation settlement was accounted for by elevating the columns with base packs on the single piles and by elevating the steel comb-cap pile caps.

Deflection of level 5 transfer deck trusses. The original plan was:

1. Install Truss with pre-camber to accommodate self-weight of frame above, SDL of frame above and cladding above.
2. Construct Levels 6 and 7 (including slabs).
3. Jack between transfer truss and level 6 to push levels 6 and 7 back up to level
4. Construct Levels 8 and 9 (including slabs).
5. Jack between transfer truss and level 6 to push levels 6, 7, 8 and 9 back up to level
6. Construct Levels 10 and 11 (including slabs).
7. Jack between transfer truss and level 6 to push levels 6, to 11 back up to level

This is illustrated below:

This incremental jacking aimed to avoid high stress build ups in slabs. However it is a very time consuming process so we instructed to BHL carry out further, more in depth analysis with a view to simplifying the process. This showed that the expected dead load deflection of the transfer trusses is around 20mm, which is significantly reduced from the potential movement allowed for during the preparation of their original report and which the slabs would be able to withstand. As a result we have updated the method to install pre-sets at level 5 and jacking is planned only as a mitigation measure as part of an observational method that would likely occur when slabs at all levels are installed as per image below.

A I will explain under the next title, the steel handover level at L5 will be somewhere between design level and full L5 elevated level. This value and level must be calculated to determine the handover level for level 5 and all other levels above and supported off level 5.

So we need to come up with a plan of monitoring the deflection to ensure that it is as per the designers’ expectations. To do this I will survey the levels of L5 connections before they have any load in them, i.e before steel is erected and again when fully loaded. This will show if the trusses are deflecting as expected. If movements stay within the tolerances of the levels predicted by the designer then no jacking will be required. If the movement at level is not as predicted i.e. the level 5 trusses move more or less than expected, the columns will be jacked at level 5 and lifted, once disconnected from the trusses the base shims will be added to or removed to lift or drop the structure accordingly.

Settlement due to altered construction sequence.

An alteration to the programme involved speeding up pouring of slabs at lower levels, this meant that there would be higher loads earlier on, with associated settlements, deflections and elastic shortening.

The mitigating jacking method was based on the optimised and instructed construction sequence agreed at the time the construction analysis was carried out. It was expected only two levels of slabs would be poured at the time steel erection commenced above L5. With this amount of fabric in place the designers believed that the fabricated pre-set would not have dropped significantly. This allowed us to confirm that handover levels would be level 5 design level + full pre-set. However the changes meant that the level would be somewhere between design level and level 5 elevated level at level 5 handover.

We need to know what level to build the steel to above level 5, taking account of the movement that will occur earlier than expected, in order to know if it is within tolerance. To accommodate this I have collated the as built survey levels of column base plates, and have organised a survey to take place again when all floors have been poured up to level 5. This will show the amount of settlement that has actually occurred. The steel supporting the slabs above level 5 should be set at level = design level + pre-set – actual movement below level 5.

Intro

I’ve recently started my phase three attachment with Arup in Sydney and I’m working within the structural team on the Metro Martin Place project which involves:

• demolition of existing buildings
• tunneling for new underground rail line,
• integration with existing underground rail line
• excavation for 5 underground station levels
• construction of two towers with link beneath existing structure (current Macquarie Bank HQ).

A project which Macquarie Bank offered to deliver for NSW government in return for an unsolicited proposal.

Figure 1 – Metro Martin Place Project

Structural Load Take-down

One of my first tasks has been to conduct a manual load take-down for the Northern tower in order to both; produce output which will inform design and, to verify the output of the ETABs model which will be used for further analysis.

Figure 2 – Northern Tower

Dynamo – http://dynamobim.org/learn/

As part of my load take-down I have been asked to use software called Dynamo to provide a code script which will automatically calculate tributary areas for columns on all floors (save lots of time and enables quick auto change to calcs should design/model change) by taking data from Revit, computation in Dynamo then export to excel . Currently working with another Arupian to try figure it out as it has previously been attempted but not finished. Its blowing my mind at the minute as I’ve just started looking into it – if anyone is secretly a king coder and think they can do it then there is a wham bar in it for you…..

Figure 4 – Indication of current complexity of script map to compute something relatively simple

We are about to install a section of beams to support a Mezzanine floor level. The supporting structure is made from expLORe pre-cast walls (Laing O’Rourke’s own brand of twin wall). In the photo you can see the pre-formed pockets where the steel beam will bear into the 8m high wall at mid-height.

The pre-cast walls have a 16-week lead time for manufacture and so detailed installation considerations had to be made several months ago. In order to develop this, our on-site Digital Engineer modeled the installation angles the beams would need to achieve whilst on the crane hook. This check identified the need to oversize one of the pockets so beams could be ‘maneuvered’ into place.

The point of this blog is to showcase 3D pdfs. I had never seen this. Download the file on from this link.

3D pdf model

It allows you to send 3D models to those without expensive software – an ideal method to communicate information where 2D drawings lack detail.

A very useful tool for the generalist RE when developing RAMS maybe…?

On my Phase 2 at Victoria Square, Woking quality sits second on our client’s priorities after health & safety but above programme.

A lot of my time here has been spent addressing quality issues and it’s management. Having not long heard of this incentive on the project I wonder if anyone else has heard about it? And if so how our respective projects and offices across Ph 2 & 3 and the world are spending/investing in the World Quality Day?

At VSW we’re getting a presentation from the Project Manager and some light nibbles, well for the office staff anyway. Posters and graphics on the rolling screens in the welfare for the site teams… Priorities arguably a little off.

I did spot this on site however – a useful incentive to learn more than just swear words from the guys on site!

All,

I’m currently on the PET(C) course in phase 1. A few of us are looking for a placement in London or the South East for Phase 2. Is there anyone currently on London placements who can put us in touch with contacts for work for next year?

My email address is alastair.bramson@googlemail.com. Any help finding a placement would be gratefully appreciated.

Thanks,

Al Bramson