By Ron Thornton
Specialty engineered underground structures are products such as wet wells, valve vaults, holding tanks, cisterns, large septic tanks, grease interceptors and other products for which the structural design is specified by the engineer-of-record to be provided by the precast producer.
Coming up with the sales price of a specialty engineered underground precast structure can be fraught with peril if there is not enough information to accurately determine the amount of concrete and reinforcing needed for fabrication. The same size structure may have significantly different member thicknesses and reinforcing requirements based on what may appear to be subtle differences in specification requirements.
Material cost is critical, of course, but it is not the only factor in determining the sales price. Pricing also includes what a customer is willing to pay for the product, all things being equal, and may not be determinative of manufacturing cost.
Value is the perceived worth of purchasing the product from you as opposed to your competitor. Value includes both tangible and intangible attributes, such as reputation, experience, plant capacity, responsiveness and on-time delivery.
Quality sometimes means different things to different people, but it comes down to consistently meeting the project plans and specs. “Exceptional quality” is hard to measure and often more for marketing than actual production. Poor quality, on the other hand, is easy to recognize and quickly spoils a producer’s reputation.
HOW TO MEASURE MARKET PRICE
Measuring market price is fairly straight-forward when it comes to standard or commodity products such as septic tanks, catch basins and manholes where materials and labor are well established.
Specialty engineered structures are more of a challenge because of variables such as custom-built forms or mixtures. Guessing at concrete thickness and reinforcing requirements can have drastic consequences.
Overestimating costs and labor quickly drives a company out of the market. Underestimating may result in low margins or even losing money on projects.
In order to prepare a credible cost estimate for a project, start by clearly defining the scope of work:
- How many structures are included in the project?
- What is the bid date?
- What is the construction schedule?
- Is there a list of bidders?
- Are there any site constraints that may affect weight limits or accessibility?
Next, secure the following pertinent information:
- Contract plans showing the location of each structure on the site, profiles for drainage and utility structures, project notes sheets and detail sheets.
- Project specifications, including any prebid meeting notes, RFIs and addendums.
- The geotechnical report may have important information regarding water table and soil properties if not stated in the written specs.
A complete and comprehensive cost estimate should include all labor and materials, including concrete, reinforcing steel, miscellaneous hardware, accessories (bar chairs, tie, wire, etc.), lifters, coatings and joint sealants. It also should take into account any special tooling or formwork required as well as labor for dry finish, yard storage, inspection and site representation, if required.
Shipping is a major cost item that is affected by the number of loads, mileage, permits, chase/escort vehicles, fuel surcharge, contract haulers and tolls. Other costs include engineering (in-house or specialty consultant), QA/QC and overhead.
Overhead includes expenses that support business but do not generate revenue. These include management and administration, utilities, mortgage and lease payments, depreciation, taxes, fees and insurance. There are many ways to allocate overhead, and every business should define a method to incorporate overhead into each estimate.
Concrete and reinforcing steel make up the vast majority of material costs, and many other cost categories are contingent upon them. The challenge is how to determine concrete reinforcing quantities without incurring a lot of upfront design expense, particularly for companies without the engineering expertise in-house.
Ideally, there will be a similar structure design from a previous project as a guide. Be certain that the design parameters are the same. Do not just mimic the size of a vault or structure. Otherwise, obtain a preliminary design from a specialty engineering consultant for a nominal fee or use preliminary design software.
With a preliminary design, there is critical information to consider, such as:
- Specified design codes.
- Live load, defined as a dynamic force from occupancy, vehicles and intended use. Live loads are considered transient and can change over time.
- Additional specified loads.
- Depth-of-bury over the top of the structure.
- Access openings, i.e. castings and hatches.
- Pipe openings.
- Water table depth.
SPECIFIED CODE MATTERS
There are several ASTM standards related to underground precast structures, and the governing design code in each of them is ACI 318 “Building Code Requirements for Structural Concrete.” However, many specifications either do not reference the appropriate ASTM standard or override the standard by calling out a different design code. ACI 350 “Code Requirements for Environmental Engineering Structures” is similar to ACI 318 but has requirements that reduce allowable stresses and increase the amount of minimum reinforcing.
Design live loads can vary greatly, and an estimator must be able to recognize how these variations can affect the final design. For example, terms such as H20, HS20, HS20-44 and HS20+30% impact all mean the same thing. (See Figure 1) A-16 is an ASTM designation that means the same thing as H20. HS20 is simply a truck designation that was defined by AASHTO in 1944.
HL93 is a more recent loading definition from AASHTO. The truck actually is the same as HS20, but the method of analysis (called LRFD) is significantly different.
Pedestrian live load, according to ASTM C857, ”Standard Practice for Minimum Structural Design Loading for Underground Precast Concrete Utility Structures,” and C890, “Standard Practice for Minimum Structural Design
Loading for Monolithic or Sectional Precast Concrete Water and Wastewater Structures,” is a uniform 300 psf. Other codes, such as ASCE-7 “Minimum Design Loads and Associated Criteria for Building and Other Structures” have different values for pedestrian loading.
Other live loads to watch for include railroad and aircraft. Railroad loads typically are identified as Cooper E80 according to the American Railway Engineering Manual (AREMA). Aircraft loads often are confusing and misleading. A common specification might call for a 100,000-pound aircraft load. What the spec omits is what the load applies to. Is it the weight of the aircraft, the landing gear or the wheel? If it is the landing gear, what is the wheel spacing?
Additional specified loads may include pulling iron forces, surcharge from adjacent buildings, sluice gate actuators, thrust loads and seismic. A critical mistake in estimating would be to ignore these requirements or believe that they are not significant.
Dead loads include self-weight and soil over the top of the structure. Traffic-rated aluminum hatches typically require a minimum slab thickness in order to properly transfer load from the hatch to the slab. Pipe openings can affect joint locations in deeper structures.
GROUND WATER LOCATION MATTERS
Minimum wall thickness and reinforcing for underground structures are dependent upon several factors. Lateral soil loads (EPh) increase with depth below the surface. The rate of increase depends upon the type of soil and water table depth. For normal soils in a drained condition, EPh = 40psf/ft is a reasonable assumption. This value can more than double below the design water table depth. (See Figure 2)
ASTM allows for the use of active soil pressure for precast structures. However, specifications often require the use of at-rest pressure, which can increase soil loads by 30% to 40%. This can be the difference between an 8-inch wall and a 10-inch wall. Sometimes, even though at-rest pressure is not in the spec, reviewers may reject designs using active pressure.
With buoyancy provisions, generally, a safety factor of 1.1 is reasonable if the water table is well defined or taken at-grade. Increased safety factors, when specified can result in extraordinary measures such as extra-wide base extensions or thickened members. You can use a soil-wedge analysis to improve safety factors, but even though it is a valid method, written specs often preclude the use of this.
KNOW AND DEFINE YOUR MARKET
There are several things producers can do to avoid costly issues in estimating and engineering underground structures, as follows:
- Know your market and quote prices accordingly.
- Make sure to obtain all relevant plans, written specs and geotech report, if necessary.
- Look for land mines in plans and specs. If there is a provision you do not understand, ask for help or clarification.
- Perform a preliminary design. Obtain assistance from a specialty engineer or use available software.
- Build your library of common codes and standards. Many of these are available on-line for free or at a reasonable cost. PI
Ron Thornton, P.E., is the executive director of The Precast Concrete Association of New York.
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