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Rules of Thumb - Building Construction

07/01/2010 6:09 AM

Gents

As a Project Manager for large industrial plants ( and not a major in Civil Engineering!), I am always being confronted with various options for most economical civil designs ( piles, foundations, steel structures). When making estimates for preliminary project costing, the most economical design needs to be considered.

Can any of you in the forum guide me on how to form preliminary judgements on most economical designs for piles, foundations, steel structures, etc, i.e all the elements that are common in industrial plant buildings?

Rules of thumb as a guide would be most helpful.

Many thanks for the help.

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#1

Re: Rules of Thumb - Building Construction

07/01/2010 9:56 AM

If such rules exist, there would be many of them on the shelf and the appropriate ones would be selected only on project/site specific input. I would want to know the approximate loading and have a soils report before selecting the foundation system. It could be spread footings, steel piles, wood piles, concrete piles, drilled piers etc. etc. Similar info is needed for the selection of the structural system.

The only way I can think of is to put a data base together of your other jobs, by type and cost, for comparison with the new project. This way you might get a reasonable upper and lower bound for the cost.

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#2

Re: Rules of Thumb - Building Construction

07/02/2010 1:00 AM

Absolutely not, that animal has not yet been discovered. If you would attempt to do what you present here, I can tell you that whoever you work for cannot afford you.

Engineers design foundations to last for given periods of time as well as for anticipated loads and life expectancy for the work. Ex: Engineer provides a specific design mix for 5000 psi concrete with a 1" slump. You wish to save money and estimate a locally standard ready mix that will equal the psi and slump requirements, for a 30% cost savings. The engineer has designed a foundation that will last for 50 years. Your changes will only last for 25 years.

If the Developer/owner decides to accept what you have done he is entitled to recourse, usually in the form of a reduction in the value of your contract. If not, then you must remove the work that you have done, and replace it with that which is originally specified. You will eat all cost overages, and be liable for time lost penalties.

How did you get to become a project manager?

TMF

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#6
In reply to #2

Re: Rules of Thumb - Building Construction

07/02/2010 12:15 PM

Hmm that seems a bit expensive, since most civil-geostructural engineers I work with don't consider the life expectancy of a foundation just its strength plus standard safety factors for things like seismicity and wind loads. I guess you could go through and account for the probability of various seismic events, probability based wind loading and other probabilities impacts on the longevity in probabilistic terms. You could not design a foundation for just 50 year life span, but rather a 50% probability of remaining functional for 50 years. Generally, there are standard factors of safety that must be employed in which longevity of a structure is implicitly built in as part of the factor, things such as structure usage factors or importance factors.

also a 1" slump would be extremely stiff and unworkable, and a 5000 psi mix somewhat stronger than typical. So it might provide a better cost to go back towards standard materials like 3000 psi mix at 3 inch slump, since concrete plants would have that material available on hand and the concrete contractors are familair with working with such materials (thus no need for special mixes, special testing protocols, and specialty contractors at premium fees). This would not be the same psi, but could generally be re-designed to enlarge the foundations to gain the necessary strength.

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#7
In reply to #6

Re: Rules of Thumb - Building Construction

07/02/2010 1:02 PM

There are places where your run of the mill, locally produced, batch plant will not generally produce concrete, specifically purpose designed. For example, airport runways and ramps, Driveways entering and exiting trucking facilities, bridges and elevated highways that are part of the interstate highway system. These and foundations, along with retaining walls are classic examples of places that concrete designed for specific lifespans and is generally placed with the use of vibrating equipment, and the slump is specified by the design engineer. The slump generally called out in plans and specifications for this kind of work is limited to 1" and drum revolutions specified. These restrictions along with time limits for the wet batching to placement are the responsibility if the engineer. I worked, supervised, estimated and owned my own construction for a total exceeding 40 years and did work in 7 different states. Regarding the geological issues, the foundation must be designed to carry specific loads. In some cases support must penetrate down to bed rock and in cases such as underground transportation where the carriers ride on rails and vibration is present, the foundation must be designed to accommodate the resulting shock waves for a specific given period of time and this is generally calculated in the numbers of load/cycles per given period of time. The estimated cycles/per are then extrapolated out for a given number of use/years.

Cheers,

TMF

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#8
In reply to #7

Re: Rules of Thumb - Building Construction

07/02/2010 1:56 PM

Actually, most every batch plant that has sufficient qualifications for use on a project make standard mix designs called for by Caltrans, which most other DOTs use as a standard. Harsh mixes, while rare, do occur and are mechanically formed (all concrete placement is vibrated for consolidation as a common practice, except smaller non-professional rsidential and landscaping). I actually have to specify the slump and mix design for the specification I have to develop for CalTrans projects and major water structures. We try to specify typical mix designs, and design projects based on the standard mix design strengths, sicne it reduces cost, allowing flexibility for the contractor to choose suppliers and specific details of the mix design. The idea is to specify a mix design, steel, pipe sizes, etc, based on industry standards. Specifying something different, causes problems since the DOTs standard are based on typical accepted materials. to do something different requris special approvals from the DOT and then is going to cost more for both the permitting process and the contractor bids. The time limit is an inductry standard set by ACI, 90 minutes from the moment water is added until placement. Any engineer that modifies this is hugely open to liabilities (if extended) or client/contractor/DOT engineer complaints if substantially reduced (as it adds cost). Most of these things, while specified in the docuemtns are drawn directly from industry quality standards from organizations such as ACI, PCI, DOTs, AASHTO, AWWA, etc.. substanitial cylcic active loading or activities do provide for life cycle analysis, road ways, bridges, water lines can provide some life cycle considerations (though they are based on industry studies and standardized), though the life expectancies are industry standards. However, for more static structure like buildings in seismic zone for instance, the seismic forces for a specified probability event along with the use and importance factors of the building will typically substantially over ride any other loading considerations, in these cases life expectancies are just probabilistic guess work because the safety factors are huge.

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#9
In reply to #6

Re: Rules of Thumb - Building Construction

07/02/2010 3:28 PM

Where possible, using locally procured materials will always be less expensive and no one is challenging your engineering ability. However the FDOT and FAA could care less what materials you may be able to provide from local batch plants at "affordable" costs."

Jumbo jets make one hell of an impact on touchdown. This is why design specific concrete is called out in the specifications. Here in sunny Florida, most batch plants use crushed and screened lime rock as it is readily available. It is suitable for residential driveways, sidewalks, curbs, building slabs, footings,piers ,etc.. In fact it is used in most all high rise construction. Sometimes the steel in the concrete will weigh nearly as much as the concrete binder and will be placed so closely that difficulty is occurred when attempting to place and finish the mix. But this locally available material isn't used for the construction of International Airport Runways. The granite aggregate is shipped in via rail car to local batch plants. Further we don't make the binder here either. It typically is shipped in by barge or rail car. AND; yes it may be that a driveway for a trucking facility can be constructed with locally procured materials and placed to the same specifications. However I have personally removed a number of these concrete materials because they simply couldn't withstand the lateral forces they endured from the churning tires under standard loads v. cycles. This is a classic example as to the purpose of using materials called out in the Specifications provided by the Site Planning Engineer, and not winging it while estimating ones bid. Of this I can guarantee, not all Engineering firms are equal, regardless of their qualifications. This is an example as to why not all Engineering firms can qualify to offer services on State and Federally funded projects.

I have lived and worked in California. I know what goes on there. There you follow the UBC. here we follow the SSBCC. They are similar in many ways, but where they differ, they can be very different. My quotes are based on personal experience derived from the hands on placement of materials to the supervising of the same and the pre biding, estimating and submission of bids for the work. My experience comes from having worked in N.J, S.C., Fl., Tex., Cal., Nev., and Idaho. I long ago learned not to get creative. Bid the documents as they are provided. Leave the engineering to the guys who make the big bucks and carry huge liability responsibilities.

TMF

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#10
In reply to #9

Re: Rules of Thumb - Building Construction

07/02/2010 3:45 PM

Actually, most jurisdictions, including California have begun transitioning over to the ICB, there are no more publications of the UBC after 1997. Plus the UBC is usually only really applicable to buildings, with some exceptions we use for risk management. For CalTrans projects, and many other States that just use CalTrans Standard verbatim or reference them explicitly, we use the Caltrans standards. For local Public works, well we write those Standards, so.. But we do reference those Standards to other large agencies like LA or San Jose, or to standards groups like AWWA, API, ACI, ASTM, etc..

Airport runways are obviously different case, they are highly specialized work, far more than highway bridges. In those cases FAA and USACE would frequently prevail as standards. Both of those agencies have a very explcit and detailed sets of standards, there really isn't much independant decision-making for the engineer in those areas. Qualifying batch plants for large comkpanies like Granite Rock or Lone Star would have a sufficient number of materials tests conducted in the past to show a history of materials strength for standard batches. So we just generally aim for those standard mix designs, a) because they have a long history for that mix and the materials in it (lower liability due to variability in mix), and b) it is cheaper for construction and allows multiple bids from multiple sources for competition. there ar eplants that provide for landscaping purposes, but those are usually way too small to work on large projects alone, and usually can not hisstorically meet the requirements to receive certifications needed to qualify.

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#3

Re: Rules of Thumb - Building Construction

07/02/2010 1:03 AM

What you would like to achieve is worse than risky and would likely be considered fool hardy by those of us who have been contractors as well as Project Managers. There does not exist any form of general estimating formula or rule of thumb "guesstimation" procedure that can circumvent foundation engineering. The very first thing that you do that doesn't follow the plans and specifications makes you the Architect or Engineer for your changes and the company or corporation that employs you will be liable for any damages that results from changes to the work that you are responsible for.

Where pilings are placed beneath piers and sections of poured concrete footings, I have found that the screw type piling is usually less costly purchase and to install. However this still would require computation and approval by the design engineer.

TMF

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#4

Re: Rules of Thumb - Building Construction

07/02/2010 1:13 AM

deekay -- I agree with passingtongreen on his answer. Even I, as a mechanical engineer with limited construction experience, can see the importance in soil and other site conditions in determining the design of a structure. Until you have some basic understanding of the numbers associated with these factors you will likely struggle with plan and proposal evaluation.

In the Western World the "rules of thumb" tend to be embodied in the building codes that consider the conditions of the local environment and the likelihood of extreme conditions being encountered in the life of a structure. Given the rapid modern development in your country I would expect the building codes are becoming as sophisticated as those in the West at least for development of large buildings in urban centers. You can judge from your local experience whether code enforcement is sufficient to motivate builders to build to code for any particular class of structure.

So there appear to be really two major parts of your evaluations of new projects. The first being the technical question of whether the new facility design is up to "code". The second is whether the quoted project cost is within a range that is acceptable to you.

The first technical issue can only be dealt with by an engineering opinion from a second qualified source. Either you or someone on your staff plays this role or you contract it out to another disinterested engineering firm that you can trust.

The second issue requires a capability to estimate construction costs. In the world of civil engineering and construction there have been many reference books published on estimating construction costs and likely a lot of software offered to use as an estimating tool. Generally the actual unit costs of materials and labor given in such references become dated quite rapidly and it becomes necessary for the estimator to stay current on the actual costs.

In your position as a customer you usually do not need to be as rigorous in estimating the projects as the contractor who must be careful to produce a winning bid that will also result in profitable work. For fairly conventional construction such as common industrial and commercial buildings without a lot of fancy architectural work in the design and new construction techniques a more simple estimating model may well serve your needs. So, for example your model may be based on square meters of one level concrete slab rather than say cubic yards of concrete, tons of rebar, types and amounts of labor and other minutia such as form construction materials. Such an estimating tool can be developed in-house by you in spreadsheet form.

If you have a history of actual contract costs versus specs, size, etc. for previously completed projects you can test your spreadsheet model against them to see what kinds of errors are produced. That will give you confidence in it's use as a management tool and improve it's credibility in the eyes of your superiors.

The other advantage of having technical capabilities for evaluation both technical performance and cost performance of a project is that you now have an independent audit function reportable to you (or under your own fingertips or hat) to give early warning when a project is performing in a troublesome manner likely to adversely affect quality, costs or schedule.

Ed Weldon

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#5
In reply to #4

Re: Rules of Thumb - Building Construction

07/02/2010 2:06 AM

Hello Ed,

From many years of experience in estimating and bidding the work, I have found that money can be made where the plans and specifications differ, and by simply using common sense to determine just where changes will be made to the work. I have bid many contracts for less than the engineer's estimate and made 100% of my profit through the change order process. On a grade road construction project for a central Florida park, that was to be 3 1/2 miles long, and constructed along an abandoned railroad line, and the pre bid site examination the entire site was under 18/24 " of water.

I knew that the order to begin the work would not be let until early May of that year, and historically, That area was bone dry in may, so the water would not be an issue. I submitted a bid that was 3/4 of a million dollars below the next bidder. By law the state had to offer me to abandon my offer with out penalty because of the significant difference between the two lowest proposals. I enforced my right to bid the work and with the extras we turned a nice 33% overal profit. The original estimated profit margin was 15%.

TMF

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Ed Weldon (1); passingtongreen (1); RCE (3); Toomuchfun (5)

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