Transmitter Buildings: Build or Pre-built?

February 7, 2014


Most of the time when we plan to build a new transmitter site we're referring to the addition of a new transmitter, in an existing space, in an existing building. Rarely does one have the chance to build a new transmitter site from the ground up. If you were to be assigned that project, though, where would you start?

transmitter site

First, let's make the assumption that you already have a spot for the new building, and at this spot there is nothing but a patch of ground. It's a blank slate. Step one would be to make a list of requirements for the proposed structure:
■ What will the building contain?
■ What are the contents' electrical requirements?
■ How much heat do the contents generate?
■ What will the physical environment be like?
■ Will the structure be designed for future expansion of contents?

To answer the first three questions you will clearly need to have a plan for your transmitter facility -- probably a block-diagram (at minimum). Using a spreadsheet then you can easily list the space requirements, the electrical requirements, and heat generated by all of the proposed equipment.

What about the physical environment? By this I mean will the site be on a mountaintop in the desert, or in a more temperate climate (like northern California) or maybe even Alaska? The structure may have special requirements based on where it is to be located.

Perhaps the most difficult question to answer is the one on future expansion. You may only need 10' by 10' of floor space to accommodate your needs, but what about the possibility of future tenants, or your own expansion? Think about it: if the site is good enough to develop for one tenant (you) then isn't it quite likely someone else will also want to make use of it later on? In the future, expansion is always going to be more expensive (because things don't get cheaper as time moves on) so the time to invest in extra space is really at the outset of the design and construction.

Stick-built or pre-fab?

Once you have answered these questions, the next is whether or not to go with a stick-built structure (that is, built on-site) or with a pre-fabricated building.

Stick-built is going to appeal to some because there is a greater deal of control over the final result. The design can be custom; and as the customer you can be right there at the offices of the architect to provide all your input on how the building should be designed, and what it will look like. You can drop in to the offices of the MEP (mechanical, electrical, and plumbing) engineer to look over his shoulder to make sure all the electrical design is correct. You can go right to the construction site every day to visit your general contractor, adding your input on how well the process is going. His (or her) stress can become yours! And finally, you can be right there to make out the punch list yourself.

As you can probably tell, I'm not inclined to go with the stick-built route for a new transmitter building. If you still need to be convinced that a modular, prefabricated building is the better option, then let me add a few more points to do my best to change your mind.

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Stick-built Advantages Pre-fab Advantages
■ More control over final result
■ Custom design
■ Oversee construction
■ Personally make punch list
■ No environmental delays
■ No damage from elements or vandals during contruction
■ Same codes/standards, but generally stronger
■ Cost effective
■ Speedy timeline
■ Conserve your energy

Modular (or prefabricated) buildings are structures that are manufactured in a facility, and eventually delivered to the customer site. The manufacturing facilities are enclosed facilities, and that prevents environmental factors from delaying the construction. Additionally, construction materials are delivered to the facility location, where they are safely and securely stored, preventing damage from the elements and potentially even vandalism or theft.

Modular buildings are designed to the same codes and standards as stick-built facilities but are generally stronger than conventional structures because they need to be able to withstand the rigors of transportation and craning on to the foundation upon delivery. The same materials are used: wood, steel, concrete.

But those factors aside, let's look more closely the project elements, comparing stick-built to pre-fabrication.

Design. One of the few advantages to stick-built previously mentioned is that you, as the customer, will have a higher degree of control of the final result if you go with a custom project. On the other hand, you will be paying an architect for a "one-off" design. The cost of the design is spread out over many projects in the case of modular construction, providing an obvious cost advantage. The same goes for the MEP design; there will be some customization for the electrical design, but that will be cheaper than hiring an independent engineer for a one-off.

Building foundation. Your construction time-line can be accelerated somewhat because the foundation can go in while the pre-fab building is being made; no need to wait for the foundation to be finished before construction begins on the building itself. At least one of the modular building manufacturers claims that the schedule can be 30 to 50 percent shorter than that of a stick-built project.

There are a couple of options as far as foundations go for modular buildings. A slab foundation, which is a large concrete pad, is one. The slab layer construction consists of sand or gravel, at the bottom of the pad. Over that is placed a vapor barrier, which is a thin layer of impermeable material (typically polyethylene sheets) used to isolate the concrete from the damp ground. On top of that goes a wire mesh, and then the concrete. The edges of the concrete are then covered with an insulating material.

Another option is the crawlspace foundation; the modular building is placed on piers and therefore permanently raised above ground level. This is certainly a good option if you plan to build in any area that potentially floods.

In either case, in the permitting process you will need to adhere to local building codes and regulations; it's outside the scope of this article but suffice it to say that stick-built buildings will require permitting and adherence to local rules as well.

Mobilization expense. I think it is safe to say that the construction of a given size structure, inside of a factory, is going to use less man-hours than the equivalent structure at a remote transmitter site (especially a mountain top). There are many possibilities for construction delays at a remote transmitter site. In either case a foundation is going to be necessary, but in the case of the modular building, fewer trips to the site will be required after the foundation is ready.

One of the last steps in a project like this will be the delivery of the modular structure to its intended home. In most cases the structure (or structures) will be delivered by way of a truck and flatbed trailer combo. A crane is used onsite to pick the structure from the flatbed and to set it on the foundation.

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Knowing that the delivery might very well be difficult (especially to a mountain top) it's wise to study the necessary route and to keep these items in mind:
■ Inspect the delivery route for obstructions. Look for 15' of height (minimum) under all obstructions along the way. Of special concern are bridges, power lines, and tree branches.
■ Check for steep grades along the way. A gradient in excess of 5 percent (1 foot in height gained along 20 feet horizontally) could add costs to the delivery.
■ The pathway should be at least 20' wide all the way along the route.

Part of the process of deciding between modular and stick-built is knowing that delivery and craning of a modular building is even going to be possible (and not excessively expensive) at chosen site.

Aside from those activities, you need to consider your own as well: While taking care of all the day-to-day things that the typical broadcast engineer takes care of, the last thing you want is to add visits to an architect and/or MEP engineer's office. Perhaps more importantly, you will probably want to avoid extra trips to the remote site to make sure your general contractor is on-track. The modular type of building provides an advantage to you here -- not as much of your time will be required to get the results you expect.

If you simply consider all the telecom sites that are relatively new (especially cell telephone sites) it's clear that the large telecom providers see the benefits of modular construction. Choosing between stick-built and modular would seem to be fairly straight forward, but clearly there are going to be instances where it just won't work. You must do your research carefully, as the results, good or bad, will last for years to come.


 

Pre-fab suppliers

Probably one of the most well known providers of modular type buildings is Thermo Bond. Thermo Bond equipment shelters are constructed to customer's specifications and shipped fully assembled, including lights, outlets, air conditioners, heaters, generators, transfer panels, ventilation systems, cable ladder, and grounding systems. Shelters are available in sizes ranging from 4' width x 6' length to 24' width by 42' length.

Fibrebond provides pre-fabricated structures with 5,000 PSI concrete walls. Its standard design provides 100 PSF roof live-load capacity, 150 PSF floor load (with perimeter foundation) and 500 PSF floor capacity with a slab foundation, in addition to 150 mph wind load (for exposure D, which applies to flat, unobstructed areas and water surfaces outside hurricane-prone regions).

Enviro Buildings interestingly says its shelters can be shipped pre-assembled or in a knock-down, panelized configuration. The modular shelters can be transported by all-terrain vehicle, helicopter or up freight elevators. It also claims that "assembly is so easy a two-man team can easily install an Enviro Building shelter."

Mobile Modular says one of the additional benefits to its concrete construction is that its buildings have a bullet resistance; a 30:06 won't go through the wall at point-blank range. Concrete construction resists all boring insects, such as termites; and lastly, concrete construction resists mold and mildew, which would certainly extend the lifetime of the structure at a mountaintop or a flood plain.


 

Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at doug@dougirwin.net.


 


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