Extending a Factory or Warehouse in Melbourne: What the Structural Engineer Checks First

Growing businesses in Melbourne's industrial areas, from Dandenong South and Braeside to Laverton North and Campbellfield, often reach the point where the existing shed or warehouse is too small. Extending the building is usually quicker and cheaper than moving, and it keeps the business on a site it knows. It is also more complex than building new, because the new structure has to work with a frame, a slab and a roof that were designed for a smaller building.

This article sets out what a structural engineer checks first when a factory or warehouse is extended, and why each check matters. For new industrial buildings, see Principal Built Engineering's warehouse structural design page.

Why is an extension harder than a new building?

A new warehouse is designed as one system. The portal frames, bracing, purlins, slab and footings are sized together for one set of loads. An extension breaks into that system. Loads move to places the original design did not expect, and the new and old parts have to be joined so they behave well together.

Most single storey industrial buildings in Melbourne are steel portal frames. The article on what a portal frame is in warehouse construction explains how they work. The table below compares the common ways a portal frame building is extended.

What happens to the existing end frame?

This is the most common trap in warehouse extensions. In many portal frame buildings, the frame at each end wall is lighter than the internal frames. It carries only half a bay of roof, and the end wall often has mullions or wind columns that help it. Once new bays are added past it, that end frame becomes an internal frame carrying a full bay of roof, and the end wall columns may be removed to open the space.

The load on that frame can roughly double. If it was designed as a light end frame, it may need strengthening, new members alongside it, or replacement. The engineer checks the original drawings for the frame sizes and connections, or confirms them on site where drawings are not available, before the extension layout is settled.

How do wind loads change?

In Melbourne, wind is usually the load that governs the design of a steel portal frame building. Wind loads are set by AS/NZS 1170.2, and several things can change when a building is extended.

• Building size and shape. A longer or wider building changes the pressure coefficients on the roof and walls, and the end bays of the new building see different pressures from the old ones.
• New openings. Large roller doors are dominant openings. If they can be open in a storm, the internal pressure in the building rises, which adds uplift on the roof and outward pressure on the walls. A new roller door in an extension can change the loads on the existing frames too.
• Bracing paths. Roof and wall bracing carries wind along the building to the ground. A longer building needs bracing that suits its full length, and the bracing in the existing bays may need to be extended or added to.
• Site changes since the original design. New buildings around the site, cleared land or changes in the standard since the original design can all change the wind loads used for the check.

Because the existing frames share the building's internal pressure, the engineer checks them under the wind loads of the extended building, not just the new frames. Where the old frames fall short, options include adding bracing, adding wind columns, strengthening knee and apex connections, or limiting door openings with a wind locked door rating.

What about the slab and footings?

The new floor slab has to meet the old one, and the joint between them is where trouble shows up. The old slab has finished most of its shrinkage and settlement, while the new slab is just starting. If the two are tied rigidly, the new slab can crack as it shrinks. If they are not tied at all, forklifts can chip the joint and the slabs can move apart.

The engineer usually designs a dowelled joint that lets the slabs move along their length while keeping their surfaces level, and sets the new slab thickness, reinforcement and joint layout for the actual loads: forklift wheels, racking legs and machinery. The article on how industrial slabs and hardstands are designed covers this in more detail, and the article on how warehouse racking loads are designed explains why racking leg loads often govern.

Footings bring their own questions:

• New footings next to existing ones must not undermine them during excavation.
• The existing footings under the old end frame may need to grow if that frame carries more load.
• The ground under the extension may differ from the ground under the original building, especially on filled sites, so a current geotechnical report is usually needed.
• Different movement between old and new footings is controlled by the joint details in the frame, wall cladding and slab.

What about the roof, gutters and stormwater?

An extension adds roof area, and that water has to go somewhere. A box gutter at the junction of an existing building and a lean to collects water from both roofs. If it overflows, the water usually comes inside. The engineer and the drainage designer check gutter and downpipe sizes for the combined roof, and the stormwater system for the extra flow and any council detention requirements. Principal Built Engineering provides stormwater drainage design alongside the structural design where the project needs it.

The existing roof structure is checked as well. New sheeting is usually lapped onto the old, and purlins in the old end bay may change from end spans to internal spans. Roof mounted plant, solar panels or skylights added at the same time change the loads on the purlins and frames.

What does the process look like?

What documents are needed for the permit?

The building surveyor will normally ask for structural drawings and calculations covering both the extension and any work to the existing building, a geotechnical report for the footing design, and a certificate of compliance for the structural design. Construction stage inspections are available from Principal Built Engineering if required, although the building surveyor normally carries them out. The article on what structural drawings are needed for a warehouse lists the usual set.

Principal Built Engineering designs industrial extensions, new warehouses, mezzanines and fit outs across Melbourne and Victoria, with design and certification available in all Australian states. More detail on industrial work is on the warehouse structural engineer Melbourne page, and broader services are listed under Melbourne structural engineering services.

Frequently asked questions

Can I extend my warehouse without changing the existing frame?

Sometimes. A separate building with its own frame and a structural gap leaves the existing frame almost untouched. Adding bays to the end of a building usually changes the load on the old end frame, and a lean to adds load to the existing columns, so some work to the existing frame is common. The engineer confirms this early from the drawings and a site inspection.

Do I need a soil report to extend a factory?

In most cases, yes. The footings and slab for the extension are designed from a geotechnical report for the ground under the new part. An old report for the original building may not cover the extension area or meet current requirements, especially on filled or variable ground.

Does a new roller door affect the structure?

Yes. A large roller door is a dominant opening for wind design. When it is open in a storm, it raises the internal pressure in the building, which increases roof uplift and wall pressures on both new and existing frames. The engineer checks the frames for this case, and the door itself needs a suitable wind rating.

Planning to extend a factory or warehouse? Get in touch with Principal Built Engineering with the site address, the existing drawings if you have them and a sketch of the extension, and the team will set out the checks and documents the project needs.