16.3 Sizing Structural, Mechanical, Electrical & Plumbing Components
Key Takeaways
- Preliminary structural depth is estimated as a fraction of span, and for steel a useful shortcut is a depth in inches of roughly half the span in feet.
- Duct cross-sectional area equals airflow divided by velocity, which converts an engineering quantity directly into a plenum dimension.
- Typical office planning figures are roughly one ton of cooling per 300 to 400 square feet and roughly 400 cfm of supply air per ton.
- NEC Article 110.26 requires working clearance at electrical equipment of not less than 36 inches deep, 30 inches wide, and 6 feet 6 inches high.
- Plumbing fixture counts follow from occupancy classification to occupant load to the code fixture table, so an occupant load error corrupts every downstream count.
PDD objectives 1.2 and 1.3 ask you to determine the size of mechanical, electrical, plumbing, and structural systems and components. You are not performing engineering; you are reserving the right amount of space and confirming that the engineer's sizes fit the architecture. If a formula is required on the exam, NCARB supplies it in the item — what you must bring is the method and the planning rules of thumb.
Structural Depth Rules of Thumb
Preliminary member depth is expressed as a fraction of span. These are planning approximations for reserving space, not design values.
| Member | Approximate depth |
|---|---|
| Steel beam or girder | span / 20 to span / 15 |
| Open-web steel joist | span / 20 to span / 24 |
| Steel truss | span / 10 to span / 12 |
| Wood joist or rafter | span / 20 |
| Glulam beam | span / 16 to span / 20 |
| Concrete one-way slab | span / 24 to span / 28 |
| Concrete beam | span / 12 to span / 15 |
| Concrete flat plate | span / 30 to span / 33 |
| Post-tensioned slab | span / 40 to span / 45 |
A useful shortcut for steel: beam depth in inches is roughly half the span in feet. A 40-foot span suggests a member on the order of 20 inches deep. That single approximation is enough to test whether a floor sandwich fits.
Depth is not the only sizing question. Preliminary column size grows with tributary area and the number of floors above; preliminary footing size is the supported load divided by the allowable soil bearing pressure from the geotechnical report.
Mechanical Sizing
| Quantity | Planning rule of thumb |
|---|---|
| Cooling load | Roughly 1 ton (12,000 Btu/h) per 300–400 sq ft for typical office; far denser for data, kitchen, and laboratory spaces |
| Supply airflow | Roughly 400 cfm per ton of cooling |
| Office supply air | Roughly 1 cfm per square foot |
| Duct main velocity | Roughly 1,500–2,500 feet per minute for medium-velocity commercial mains |
| Duct cross-sectional area | Airflow divided by velocity: A (sq ft) = cfm ÷ fpm |
| Mechanical room area | Roughly 3% to 6% of gross floor area for an all-air system |
| Ceiling plenum depth | Roughly 18 to 36 inches for an all-air system with ducted mains |
The duct area relationship is the one to internalize, because it converts an engineering quantity into an architectural dimension. A 10,000 cfm main at 2,000 fpm requires 5 square feet of cross section — a 30-inch by 24-inch duct before insulation and hangers. That duct will not fit in a 16-inch plenum, and discovering it at design development is the failure this objective is written to prevent.
Outdoor air quantities come from ASHRAE Standard 62.1, which combines a per-person rate with a per-area rate, so occupancy density drives ventilation independently of floor area.
Electrical Sizing
| Quantity | Planning rule of thumb |
|---|---|
| Total connected load, office | Roughly 5 to 10 watts per square foot |
| Lighting power density | Roughly 0.6 to 0.9 W/sq ft under current energy codes, by space type |
| Electrical room | One main service room plus a distribution closet per floor, stacked |
| Working clearance at equipment | Not less than 36 inches deep, 30 inches wide, and 6 feet 6 inches high under NEC Article 110.26 |
| Transformer | Requires ventilation, clearance, and structural support; often located at grade or in a vault |
Architecturally, the sizing question is not the ampacity — it is the volume and clearance the equipment demands, and the riser path connecting the service entrance to every floor.
Plumbing Sizing
Two calculations recur.
Fixture counts come from the plumbing code table of minimum required fixtures, applied to the occupant load derived from the building code. The sequence is fixed: occupancy classification → occupant load → fixture table → required water closets, lavatories, drinking fountains, and service sinks, typically divided evenly between the sexes unless the occupancy justifies otherwise. Get the occupant load wrong and every downstream fixture count is wrong.
Supply and drainage sizing is driven by fixture units: each fixture type carries a water supply fixture unit value and a drainage fixture unit value, the values are totaled, and the pipe size is read from the code table. Hot water heater capacity is sized from peak demand, not from total fixture count.
Static pressure changes by roughly 0.433 psi per foot of elevation, which is why a tall building needs pressure zones and why the architect must reserve space for pressure-reducing stations or intermediate mechanical floors.
Special Systems
Preliminary elevator quantity is driven by population, handling capacity, and interval; preliminary sprinkler design is driven by the hazard classification, which sets the design density and remote-area size under NFPA 13; and preliminary generator sizing is driven by the connected emergency and standby loads under NEC Articles 700 through 702.
Exam Tip: These items are almost always testing whether the engineered quantity fits the architecture. Convert every sizing answer into a dimension — duct depth, shaft footprint, equipment room area, structural depth — and then check it against the plenum, the shaft, the room, and the floor-to-floor height that the design actually provides.
An architect must reserve plenum depth for a 10,000 cfm supply duct main designed at a velocity of 2,000 feet per minute. What approximate duct cross-sectional area is required, and what does that mean for a 16-inch plenum?
For preliminary space planning, an architect needs an approximate structural depth for a 40-foot steel beam span. Which estimate is appropriate, and what is it used for?