Building a Bridge - Engineering Part 3

 Altogether, the posts on building a bridge will be numerous posts. I'm doing this to keep from a large volume, and hopefully, keep those interested from being bored. 

The next phase in engineering the building a bridge involves the drawing of plans, final funding, and the specifications for construction. How much goes on in the background is beyond my experience, but I have a feeling many projects are shot down before construction due to politics, and lack of funding. 

The plans are drawn by draftsmen using CAD programs. In the past, there drafting was done with pencils, and the drafting tools used for the craft. In my experience, AutoCad is the leading software package. It's expensive, especially when many of the plug-ins are purchased. Those proficient in the use of the software have my admiration. I base my opinion on what little experience I've had with simple two dimensional CAD programs. AutoCad is much more robust, more complicated, and can be used for some amazing, scaled three dimensional drawings. Considering the complexities, many of the drawings I've seen for construction projects are probably simple drawings for the more experienced draftsman.

With most bridge projects plans I've dealt with, the centerline proposed for the construction is the main control. Regardless of service roads, or surrounding structures, the centerline is the best baseline, since it eliminates any crossover stationing. I ,once, bid on parts of a project designed by a consulting firm, which had a very confusing conglomeration of stationing. Since the project involved many sections of highways with existing stationing, including the stationing was probably their solution to the complex problem of converting all stationing to one baseline. Their job was probably easier, but to the contractor, examining the plans was tedious. 

With a baseline, there is a starting point, which on a new project will be station 0+00. With tie-ins, or renovations of an existing road, the stationing for the project may start at some station as 256+58.5 which would be part, or a continuation, of a project point that had 256 one hundred feet stations, plus 58 feet 6  inches, which is 6/12 or .50 feet. I have worked with metric, when TxDot dabbled in metrics, and found the English units too entrenched to stay at metrics. Metrics are easier to use for calculations, but many manufacturers have English dimensioning, and these dimensions don't quite match the metric dimensions. Soft metrics can be used, but in reality, manufacturers aren't going to spend millions to convert, and in engineering, the calculations don't quite match. Since materials strengths, and dimensions require accurate calculations, errors in converting data can lead to problems. 

In stationing, and the centerline is used for the baseline, offsets are used to indicate future, or existing sections. With a 60 feet wide road, the edges would be defined by left and right offsets. At station 100+00, the far right edge would be station 100+00 - 30 feet right (100 feet from the starting point, and 30 feet perpendicular from that point). The left edge 100+00 - 30 feet left. It's an easy method to determine locations, and when it comes down to existing utilities, it's important to the owners to understand where, and how far to move any obstructing utilities. Utilities are usually moved before projects are let, since the operation can take a long time. Contractors charge for downtime, and justifiably so. Equipment, labor, and time add costs, which a contractor must learn how to handle. Failing to fall procedures outlined in the specifications, which are part of the contract, can lead to monetary losses by the contractor, and a bad working relationship between the engineers and contractor.

The plans are not just drawings of the future bridge. With TxDot, and most Texas municipal drawings that usually follow TxDot standards, included are a multitude of pages with standard drawings, paving designs, drainage structures, elevations views, existing utilities, typical design sheets, phasing, traffic control, environmental protection requirements, quantities, typical sections, and any special design features. There are more, but to really understand the requirements, you would have to examine the TxDot website that details the necessary requirements. Last, but not least, are the specifications for the project, which reference what is commonly called the "Bible". Txdot occasionally updates their standard specification book, which has all the standard construction, and material requirements. An important reference is always available for approved material suppliers. These suppliers provide materials that were tested by TxDot, and in some situations, a contractor can have a problem by bidding using a material not supplied by an approved supplier. The cost difference may be substantial, and lead to financial woes for the contractor.

Usually, there's a pre-bid meeting. The bidders may just meet at the office of the owner, an onsite meeting may be all that's required, or both may occur at the pre-bid. It's at this point questions are asked, answers are produced, or the engineer will answer the questions in writing before the bid date. If an addendum is required, it will be issued before the bid date. Acknowledging the receipt of addendums is usually part of the bid documents, and failing to acknowledge the addendum will lead to a bid being thrown out.

When all is compiled, checked, and approved, the project is released for bids. This leads to the contractor to examine the plans, and specifications, figure out the costs associated to perform the work, add a profit, fill out all the blanks correctly, notify the bonding company, and cross their fingers.

Bidding is once a month for TxDot projects, and at council meetings for cities. The bids are checked for following the rules of bidding, and any numerical irregularities. Whether the bid is local, or let in a multiple bid letting, some allow electronic bidding. It's at this point the contractor finds out if they won the bid, and can now start worrying about what they left out. Some are so far out of balance, the contractor is consulted, and allowed to remove their bid. This can come at a price, if the project requires a bid bond. The contractor forfeits the bond, and the bonding company frowns upon such things. 

The engineers might find they substantially underpriced the project, and wonder why. That, and if their track record with the contractor usually ends up with change orders for all their mistakes, they may wonder what their exposure will be, and how much it will cost. It's now their job to go through all the paperwork of the bid, and the contractor  will eventually be given a contract to sign. After signing, the contractor has to start submitting material sources, subcontractors, schedules, and have all prepared before the pre-construction meeting. When the start construction date is set, time is started on the project. Whether its a few months, or a few days, the contractor submitted schedule is used as a club, when the project falls behind.

Now it's time to start the project, and a new post.