Engineering & Design
Collateral and Mezzanine Loads in Metal Buildings
Collateral load is the weight of permanent materials other than the metal building system itself, such as sprinklers, lighting, ductwork, ceilings and solar panels. Mezzanine loads are the dead and live loads from an intermediate floor supported within the building, including its deck, finishes, occupants, storage and equipment.
Key facts
- Collateral load is a permanent load and is combined with dead load in design.
- Concentrated hanging loads should be specified separately from a uniform collateral allowance.
- Mezzanine live loads depend on the intended occupancy defined by the governing code.
- Mezzanine columns bring new point loads to the slab and foundations.
- Adding services or solar panels later without checking the structure is a common cause of overload.
What counts as a collateral load
Collateral load is a term used widely in metal building practice for permanent weight that the owner adds to the structure. Typical items include fire sprinkler mains and branch lines, lighting, cable trays, HVAC ducts, suspended ceilings, acoustic linings and rooftop or roof-mounted solar arrays.
Codes classify these items as permanent or dead loads, but the building supplier cannot know them unless the owner states them. A uniform allowance is usually applied over the whole roof, and heavier local items are listed separately with their weight and location.
Why concentrated loads need their own definition
A uniform collateral allowance assumes the weight is spread evenly. Real services are often concentrated: a large air handling unit hung from two purlins, a heavy sprinkler main running along one frame line, or a hoist hung from a rafter. These loads can overstress a single purlin or connection even when the average roof load is low.
The owner or services engineer should provide a list of hanging items with weights, support points and any dynamic effects. The building engineer then provides suitable supports, such as purlin strengthening, sub-framing or loads taken directly to the frames.
How mezzanine floors are loaded and supported
A mezzanine adds a floor deck, joists or beams, columns and often stairs and handrails. Its live load is set by the governing code for the intended use, which may range from light office use to heavy storage, and the owner should state the actual use rather than a generic figure.
Mezzanines can be framed independently on their own columns or connected to the main building frames. Connecting them to the frames can save columns but adds load and lateral restraint questions that the engineer must resolve. Vibration and floor stiffness also need attention where people walk or machines operate.
How the four code families treat these loads
In the United States, ASCE 7 defines dead and live loads and the IBC sets occupancy live loads, while metal building practice uses the term collateral load for the owner-supplied permanent items. Canada treats such items as dead load under the NBCC, with use and occupancy loads given in Part 4.
In Australia and New Zealand, AS/NZS 1170.1 sets permanent and imposed actions, including floor loads for different occupancies. In Europe, EN 1991-1-1 covers self-weight and imposed loads for buildings, with categories of use and values that may be adjusted by the National Annex. Each approach reaches a similar outcome by different terms.
What to specify for collateral and mezzanine loads
- A uniform collateral allowance agreed with the services designers.
- A list of concentrated hanging items with weights and locations.
- Any roof-mounted solar arrays, including racking weight and wind effects.
- Mezzanine use, storage type, and any forklift or pallet truck traffic.
- Mezzanine floor finish, partitions and ceiling under the floor.
- Whether future services or a future mezzanine should be allowed for now.
Frequently Asked Questions
Is collateral load the same as roof live load?
No. Collateral load is permanent weight that is always present, while roof live load represents temporary maintenance traffic and materials. Codes combine them differently, so they must be specified separately.
Can solar panels be added to an existing metal building roof?
Often yes, but only after an engineer checks the roof sheets, purlins, frames and connections for the added weight and wind effects. Arrays can also change snow accumulation and drainage, which should be included in the check.
Should a mezzanine be supported by the main building frames?
It can be, but independent framing often keeps the main frames lighter and simplifies future changes. The choice depends on the mezzanine size, loads and layout, and is confirmed by the project engineer to the governing code.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
