Rooftop mechanical units are the point loads that arrive by crane, and StructLoads treats them the way the structure experiences them: as concentrated forces at curb rails and dunnage posts, at operating weight, on specific members, with drift piling against their windward faces. The mistake to design out is the plan-note habit, “allow 1.5 kPa for mechanical,” because a real rooftop unit does not spread itself politely across a zone: a packaged unit plants 30 kN on four curb corners, a chiller full of water doubles its catalogue dry weight, and the beam that happens to run under the rail carries the machine while its neighbours carry the allowance. The honest workflow reads the mechanical schedule, weighs each unit wet, and lands each one on the members that actually hold it.
Operating weight, not catalogue weight
Every unit has at least two weights and only one of them is structural. The shipping or dry weight is what the crane lifts; the operating weight adds everything the unit holds in service, and for wet equipment the difference is dramatic. A chiller carries its refrigerant charge and, more significantly, its water-filled evaporator and condenser barrels; cooling towers hold basins of water; boilers and buffer tanks are mostly water by weight. Operating weights running 20 to 60 percent above dry are routine, and a cooling tower’s basin can push the ratio further. Air handlers and packaged DX rooftop units sit at the drier end, but even there, accessories accumulate: economizer hoods, sound attenuators, filters loaded with a year of dust.
The schedule discipline is simple and worth stating: take the operating weight from the mechanical submittal, not the preliminary catalogue, and re-check when the submittal changes, because value engineering swaps units late and the replacement is rarely lighter. The classification is dead load, permanent equipment at operating weight, with the usual caveat inherited from every superimposed dead load: where the weight stabilizes something, count the empty state, because the chiller is drained exactly when the wind blows hardest during commissioning.
Curbs, rails, and dunnage: where the load actually lands
Units meet structure three ways, and each is a different load path. Small packaged units sit on full-perimeter curbs, which spread the weight into a line load around the opening, the friendliest case, though the deck edge at the curb still sees it all. Larger units sit on rails or sleepers, two lines of support that turn the unit’s weight into a pair of line loads whose position relative to the framing decides everything: rails parallel to the joists load two joists heavily and the rest not at all; rails across the joists distribute better but load each crossing as a point. The heaviest equipment sits on dunnage, dedicated steel posts and beams rising from the roof framing, which delivers the entire unit as four to six concentrated forces onto specific beams, chosen, ideally, by the structural engineer rather than by wherever the pipefitter preferred.
Two complications ride along. Vibration isolation, spring mounts and inertia bases under rotating equipment, adds real weight: a concrete inertia base can weigh as much as the fan it steadies, and it is part of the operating load. And every unit needs an opening or a penetration cluster near it, ducts, pipes, conduits, so the framing around a unit is simultaneously carrying more load and missing more deck, the combination that makes the members beside a large RTU the busiest steel on the roof. The reactions from each support point then enter the takedown like any other point load, delivered to beams and traced to columns, and the tracing matters because mechanical loads cluster: plant areas put several units near one column line, and that column’s premium is invisible in any roof-average number.
| Support type | How the load arrives | Design attention |
|---|---|---|
| Perimeter curb | line load around opening | deck edge, opening trim |
| Rails / sleepers | two line loads | position vs framing decides |
| Dunnage steel | 4-6 point loads on beams | chosen landing members |
| Inertia base + springs | adds mass, filters vibration | weight counts in full |
The loads that come with the unit
A rooftop unit changes its neighbourhood. Snow drifts against any projection taller than the snow layer, so each unit grows a windward wedge of drift load computed exactly like a parapet drift in the snow provisions, and a cluster of units can trap a courtyard of deep snow between them; screens and enclosures around plant areas make walls for drift on both sides. Wind pushes on the unit and its screen, and the overturning couple resolves as up and down forces at the mounting points, another reason the anchorage into the uplift-checked roof structure is a designed connection rather than gravity and habit. Maintenance brings people and parts: codes require the roof around serviceable equipment to carry live load for the technician, the toolbox, and the replacement compressor staged on the deck.
Seismic design in active regions adds the anchorage case, the unit’s mass times its component acceleration, restrained at the same mounting points, and it is worth remembering that spring-isolated equipment needs snubbers precisely because the isolation that protects bearings in service lets the unit surge in an earthquake. None of these loads is exotic; all of them attach to the unit’s location, which is why the mechanical plan and the roof framing plan have to be read together rather than in separate offices.
The schedule is a structural document
The mechanical schedule is where these loads live, and reading it structurally means extracting four things per unit: operating weight, support type, footprint and rail orientation, and anchor pattern. The HVAC design process iterates, so the extraction has to happen twice, once at design for member sizing and again at submittal for the truth, and the second pass is the one that catches the value-engineered substitute whose weight moved and whose rails rotated.
Two lifecycle loads finish the picture. Installation first: the crane pick and the temporary setdown mean the unit may rest, briefly, somewhere other than its final rails, on a deck that never expected it, so staging positions belong in the rigging plan with the same seriousness as ballast staging on a green roof. Replacement second: rooftop equipment lives 15 to 20 years while the structure lives 50 or more, so every roof will see at least two generations of units, each generation historically heavier as efficiency standards add heat-exchange surface. The kindest thing a structural engineer can leave the building is documented capacity at the plant zones, a stated allowance per landing beam in the takedown record, so that the engineer replacing the chiller in 2045 checks a number instead of excavating assumptions. StructLoads makes that record nearly free: the unit loads are already discrete objects in the model, and the margin between demand and capacity at each landing member is a report away rather than a reconstruction.
A worked example: one AHU on rails
Take a 4 m by 2 m air handler, illustrative round numbers throughout: 22 kN operating weight, on two 4 m rails across a roof framed with joists at 1.2 m centres spanning 7.5 m. Each rail carries 11 kN as a line load of 2.75 kN/m and crosses four joists, so each crossing delivers roughly 2.75 x 1.2 = 3.3 kN as a point load, landing wherever along the joist’s span the rail happens to sit. At worst, mid-span, that point load produces the bending of about 6.2 kNm, equivalent to adding roughly 0.9 kPa of distributed load to that joist’s whole tributary, on top of what it already carries.
Now the neighbourhood effects. The unit stands 1.5 m tall, so the drift check adds a wedge of snow against its windward face, say a peak surcharge of 1.1 kPa tapering over a few metres, loading the same joists again. Wind on the unit’s 4 m by 1.5 m face at a design pressure of 1.0 kPa applies 6 kN horizontally at 0.75 m height, an overturning couple of 4.5 kNm that the four anchor clusters resolve as roughly plus and minus 1.1 kN of vertical force, small, but tension where the joist connection may never have been asked for any. The two joists under the rails, their connections, and the column line collecting them are the members that care; the roof average, about 0.15 kPa if the unit were smeared over its bay, would have flagged nothing. Drawing the rails as line loads in StructLoads takes a minute and produces the real answer, and shifting the rails half a joist spacing in the model, so each rail lands nearer a support instead of mid-span, often halves the worst joist moment before any steel is touched.
Key takeaways: rooftop mechanical unit loads
Weigh units wet, at operating weight from the submittal, water-filled chillers and inertia bases included, and land each one where it actually sits: curb lines, rail line loads, or dunnage point loads on chosen beams. Add what the unit brings with it, drift wedges against its faces, wind overturning at its anchors, maintenance live load around it, seismic restraint in active regions, and trace the clustered reactions down the columns. The plan-note allowance describes no member’s real load; the schedule, the map, and the takedown do.
Quick answers
How do you account for rooftop mechanical unit loads?
Take each unit’s operating weight from the mechanical submittal, wet weights for chillers, towers, and boilers, isolation bases included, and apply it where its support system actually lands: perimeter curbs as line loads, rails as positioned line loads, dunnage as point loads on chosen beams. Add drift against the unit, wind overturning at anchors, and maintenance live load around it, then trace the reactions through the takedown, in StructLoads or by hand, to the columns that collect the cluster.
What is the difference between dry weight and operating weight?
Dry (shipping) weight is the empty machine the crane lifts; operating weight adds everything held in service: refrigerant, water-filled barrels and basins, fuel, and accessories. For wet equipment the gap is 20 to 60 percent and can exceed it for cooling towers. Structure designs for operating weight as dead load, but uses the empty state wherever the weight would helpfully stabilize, since drained equipment coincides with commissioning winds.
Why is a mechanical allowance like 1.5 kPa not enough?
Because units do not spread: a 22 kN air handler on rails loads two specific joists at several times their share of any allowance while the rest of the zone carries nothing, and clusters of units concentrate on one column line. The allowance is a placeholder for early sizing; the design check needs each real unit at its real location, which is exactly the bookkeeping a takedown model exists to do.
Do rooftop units change snow and wind loads?
Yes: every unit taller than the snow layer grows a windward drift wedge computed like a parapet drift, clusters and screens trap deeper snow between them, and wind on the unit’s face resolves as an overturning couple, vertical plus-and-minus forces, at the mounting points. The roof near equipment therefore carries more than the field in both winter and storm cases, on exactly the members already taking the unit’s weight.
When should you not rely on the original roof design for new equipment?
Whenever a unit is added, relocated, or replaced by a heavier one, which is the normal life of a roof: the original design carried an allowance or the old unit’s map, not the new reality. Replacement units trend heavier, submittals change late, and a rail rotated ninety degrees moves the whole load to different joists. Rerun the affected members with the new schedule before the crane is booked, not after.