Welcome to the second part of our three part series of posts on navigating the planning and zoning process for this multi-family container based development project.
When we started this project, I didn't realize how much professional engineering goes into a project like this. In this case we have utilized:
Civil Engineering
Drainage
Sanitary Sewer
Alley and Arkansas St. Public Improvements
Utility routing outside building structures
Structural Engineering
Foundations
Connection Systems
Cantilevers
Mechanical Systems Engineering
HVAC
Water and other types of heat
Plumbing
Internal water systems
Internal waste water routing
Electrical
Utility connection
Internal site plan
Fire Sprinkler
Sprinkler design
Riser and distribution design
This post will focus on a few areas of engineering where we spent a lot of time and money that might be able to help others reduce the time and money they spend doing something similar.
Civil Engineering
The largest engineering work product is for Civil Engineering. The two sections below highlight the components we had the most difficulty addressing.
Drainage: flat sites make ground water difficult. In the end, we are creating an internal drainage system, smallish drainage swales, a dry well and utilizing mostly pervious surfaces (Organi-Lock for walkways and Tru-Grid pagers for parking). The diagram below shows the final grading, area drain system and dry well. Looking closely at the patterns, you will notice that the majority of the site walkways are not concrete.
The BV UDC section 16.04.4.1.3 governs the drainage requirements for in-town projects. The basic idea is that you need to manage the precipitation falling from the sky on your property up to the peak runoff during a 100 year storm.
Shallow Sewer: the sewer main in Arkansas street is quite shallow (3' 7"; from what I understand, normal is 6' - 8'.). The standard BVSD connection guidance indicates you must keep your sewer line under 5' and in special circumstances can go to a 3' minimum (top of pipe) depth below grade. Note that there is reference to 1' bury in their code but they would not permit a solution with less than 3' of cover regardless of protection from crushing and freezing. Our run distance for the internal site sewer main is about 140 ft from the connection point in the street. to the last clean out We have sufficient volume that we can increase the main sewer pipe diameter from 4" to 6". This allowed us to decrease the slope from 2 degrees to 1 degree (2' rise per 100' vs 1' rise per 100'). The diagrams below show the final approved configuration. Note that due to the bury depth 3' vs an area frost depth of 30" being deemed too freeze prone, we are required to very expensive insulated pipe with heat trace called Tricon.
Our overall Civil Engineering project expense for design has been about $35,000. It could have been fifteen to twenty thousand less if we knew what the end sanitary sewer solution would look like.
Structural Engineering
The structural elements of this project have been reasonably straight forward. Deepened edge footers surrounding the building sections and standard structural reinforcement of the containers. The trickiest bit was the cantilever design of the office overhanging the community and storage pods. The original design had these all in-line with no overhang. The Buena Vista UDC prevented this simplified design from progressing due to the need for changes in wall plane, etc discussed in the previous post. Overall, the structural engineering design was about $10,000.
Mechanical Systems Engineering
From a mechanical systems perspective, this is a simple project. We have mini-splits for heating and cooling with electric radiant cove heat for backup in strategic residence locations and areas requiring conditioned space for utilities. The overall cost for this portion of design work was about $3,500.
Plumbing Engineering
Plumbing is a little more complicated. We designed the overall solution to leverage
wet venting thereby avoiding most roof penetrations and permitting our wet wall top surfaces to functions as roof drainage instead of extra gutter systems. The diagram above shows the in-ground and sections directly attached to the in-ground plumbing. The overall plumbing engineering design was about $5,000.
Electrical Engineering
The electrical engineering for this project has been a larger learning experience than expected. From the beginning our vision has been to add renewable to this project in the second phase. While this has been communicated with everyone relevant to the conversation, the exact implications of this and the subsequent impacts on the power requirements for the site have taken quite a while to unravel.
A future blog post[s] will go through all of the details of the multi-family commercial power system required to support adding renewable energy. Here we will just scratch the surface to get you pointed in the right direction.
Single Phase or 3 Phase? We started out with a design requiring two legs of single phase power with each one on it's own transformer (167kVA). We are estimating a peak draw of about 70 kW and an average monthly power consumption of 15,300 kWh. Residential solar systems, batteries, inverters, etc are designed for single phase power. Commercial solar systems, inverters, etc are optimized for three phase power. From a basic load perspective, either single or three phase power are perfectly adequate for this site. The driving difference between three phase and single phase is that the flexibility for integrating other future loads, commercial storage and commercial solar is excellent with three phase and quite difficult with single phase. We designed for single phase and are currently in the process of updating that design for 3 phase.
Commercial Power or Residential Power? When we started, I didn't realize this was a question. With our power coop in Buena Vista (SDCEA), we have a number of options: residential individually metered power -- rate schedule 2 or commercial bulk metered power -- rate schedule 3 or 8. Our original plan was the first option -- individually metered power. Unfortunately, the electric utility will force you into the commercial larger power rate as soon as you site aggregate exceeds 50 kW average draw during any 15 minute period during any month. Once your site is bumped up to the large power rate structures, you stay there. The issue with this is that the demand charges are projected to at least double the cost of electricity from this residential load. If you bounce the projections in the Single Phase vs. 3 Phase section above against the residential power cutoff, you quickly see that one of the large power rates is the only path forward. If we pick rate schedule 8, then we just need to find a way to manage demand during the peak billing period and peak residential use period (5pm - 10pm).
The engineering side of this is a little complicated and will be discussed in detail when our plan finalizes. For reference our preliminary and now abandoned single phase one line diagram is below. The design cost for the single phase solution (most will be reused) was about $10,000.
Fire Sprinkler Engineering
Just about any time you are working with a commercial structure or a residential structure housing more than two separate residences you will need to install a fire sprinkler system. Our system design is reasonably straight forward. It consists of a 4" fire line from the water main in the street to a manifold distribution system where we have the ability to isolate apartment groups. From the manifold a 2" line runs to each riser for an apartment or unit group and then sprinkler heads inside the units. The diagram below depicts the first floor of this layout. The overall cost of the sprinkler system design was about $9,000.
Hopefully this post provides you with a good idea of the engineering elements required for a multi-family commercial project. If you have questions or would like additional details, please drop me a note through the contact page.
In the final post of this series we will discuss landscaping, site and public improvements.
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