CCB – 8607 Renovation
Work has been distinctly ‘bitty’ so far with the proposed work on the Fort Lee Training facility still being just that. A proposal. This is mainly down to IT issues which have now been sorted, thanks to a bit of a whinge to my line manager. To fill the void I have been working on the other renovation project I mentioned in my last post.
ATFP / Progressive Collapse
The output required is the technical specification and tender documentation, which is being produced by a number of different USACE departments (electrical, mechanical, architectural, structural etc) with support from the costing department and PM section. The structure in question is being transformed from an old barrack block into office and administrative space. The main considerations for the structural section have been to do with the anti terrorist force protection and progressive collapse system. The ATFP considerations are required because the costs of the renovation are more than 50% of the total value of the building, which is one of several potential ‘triggers’ in the Unified Facilities Criteria (UFC) documentation. UFC is suite of standardised Department of Defense building codes applicable to all branches of the military. There is currently no progressive collapse or ATFP installed because the construction of the original structure pre-dates the current requirements. The ATFP requirements are classed as minimal, limited to such things as stand off and observation requirements. These are fairly cheap and easy to achieve and can be designed in early provided the designer is given enough real estate to play with. In this case this not a problem and as long as the car park is sufficiently far enough away and no trees get planted with branches lower than 5’ all should be ok. The progressive collapse requirements, triggered by the fact that the structure is three storeys and will be a ‘primary gathering area,’ are more onerous. The UFCs say that for this category of building either Tie Force measures or Alternative Path design is acceptable. This renovation is part of an ongoing programme and both methods have previously been used in similar buildings. The preference is to use a tie force method which will see a large steel beam placed in the roof of the building supporting hanging steel rods in tension at the column locations. These tie into each floor slab, supporting it in the event of column removal. When I say ‘preference’ I mean this is the USACE preference, not necessarily the client’s (the client doesn’t really care how the building is made compliant with the UFCs, just that it is). This is likely because its been used before and is simpler practically than a retrofit alternative path solution. To this end the UFC requirements have been quoted, but only examples of tie force have been provided. But why not just state outright that tie force is preferred? The technical spec seems somewhat to be leading the eventual tenders to arrive at a pre-determined conclusion but is still allowing some room for design innovation. One of the things required for the project, which will not form part of the invite to tender documents is a cost estimate. This allows USACE to get a feel for what the incoming tenders could / should be. I provided a list of quantities of materials I expect will be required for the ‘preferred’ progressive collapse system to the costing team who will build the cost estimate. USACE policy dictates that if incoming tenders are within 10% of the government estimate there is no requirement to go through a negotiation phase to consolidate the difference. So in leading the designers to use the same system then perhaps the cost estimates will be in the same region, potentially speeding up the tender process. Time is, from what I have seen so far, the predominant driving factor on most USACE projects. In allowing some discretion however some cost savings may be realised.
Existing Capacities
The client’s brief will contain a list of requirements to turn the accommodation building into the required administrative and office space. From looking in the UFC I anticipate that this will involve interpreting the requirements and allocating enough space for conference rooms, partitions, storage and general office space. All of these have minimum loading, much like you can find in EN1991. The designer will presumably be trying to find a combination of the above space to meet the client requirements whilst trying to do as little structurally as possible. To help both us and the designer understand how much, if any, work will be involved I’ve provided an annex to the documentation highlighting the existing capacities of the floors.
These are formed from cast in place concrete T sections; 5” beams supporting a 2.5” thick 26” on centre one way slab spanning 24’. The beams have 2 #5 rebar top and bottom (#5 being 5/8” dia.) and the slab has a mesh of undetermined diameter, therefore assumed not to exist. Destructive testing was carried out at numerous locations and most of the above has been confirmed, except mesh was only observed in a few locations. I calculate* that the existing system has a capacity in the region of 17 kip. ft [23kN.m] moment capacity (positive moment) and 16 kip.ft [21kN.m] negative moment capacity and about 4.5 kips [20kN] shear capacity. Using a computer system called Enercalc structural library (if anyones heard of that?) I applied various loading conditions and assess that the designer can do almost nothing without the requirement to increase the capacity of the existing structure. Why is this important, and what have I learned? Its important to know as a client how much you expect your requests to cost. Like taking your car to the garage, you’re much less likely to get ripped off if you know what the likely costs of replacing your brake pads will be. You can either research this yourself or get someone else to do the research, perhaps because you don’t know what a ‘brake pads’ is. In this case USACE is doing the research on behalf of the client so the government department requesting the work won’t get ripped off and can appropriate enough funds from senate to do the work. I learned that clients can be a nuisance. They have yet to identify an end user, but have a few in mind. One of these few has requirements for Sensitive Compartmentalised Information Facility space which sent the electrical and mechanical team into a tizzy. I learned that technical specs and tender documents can provide more than just a bus route for Steve if they are considered properly. They can also do more by saying less in a good way as much as a bad way. I also learned that youtube has lots of really boring videos.
In other news
The snow has gone and I paid $550 to Nissan to fix something on my car. I don’t know what it was called but the flashing light on my dashboard has stopped flashing.
* using a mixture of class notes, text books and a helpful youtube video.
Holdfast Training Services Trial Blog 
Brad
There’s a bit of confusion in the robustness description. Peripheral ties are usually formed form existing structure with the tie forces not being counted as ‘in addition’ to the normal structural element forces but as ‘available’ form the structure. No thought is given then to how the tying will work, but it is assumed that it will with limited collapse. The verification forces are found in civilian codes EN 1992 and 1993 , for example and tends to be quite low .
Alternative load path is where elements are removed and the structure is then checked to see if it can still ‘hang there’ ( limited damage is allowed but it should be noted that, in a multi storey structure the ‘lost floor area is applied to the floor below in the robustness calc. IN the alternative path case strategic elements (say a column ) are removed to see what the damage is. Since you describe an alternative hangar, I assume that you are describing alternative load paths; what was compression in a column ( for example) being supported in tension from the a structure hand above ( notwithstanding the load must still travel safely to the foundation)
Of the second note; I am confused. As with above it would have been clearer if a sketch had been proffered. I think you describe a ‘t’ bema of 200 mm depth ( including the slab) and at 550mm c/c spanning 7.3m
All sort sof queries. IF you ignore the flange and just use the rectangular section ( I’m assuming 200 flange width to accommodate 5 x H16 bars with side cover) ; you get a minimum MoR of about 650kNm ( as I say without flange) . This has to be compared with a BS EN 1991 factored actions
If I take if another way and use your MoR over the 7.3 m span then I get a udl of only 3.6kN/n – this would barely include the concrete self weight and finishes and services let alone a variable floor load?
Again in desperate need of a sketch!
John – quite right. The suggested solution is an AP method. I have included a photo of a similar scheme from an adjacent building into the original post. As you can see it is a fairly substantial construction. Tying into your second point, yes the floor capacity is very low. I’ve added the scanned calculations I did to estimate the capacity. The computer analysis agrees fairly closely with my numbers. As you note in order to add even the smallest live load an increase in the slab capacity is required by the contractor therefore I think that the slabs will likely be demolished and re-built and so it is more likely that the contractor will select a Tie force method in this instance, installing as the floors are constructed.