Steel Box Frame & Goalpost Frame Structural Calculations • RHS/SHS Design • Building Control Ready
Fixed-fee structural calculations for steel box frames and goalpost frames. Used for garage conversions, large residential openings, open-plan remodels and commercial applications. Building Control-ready packs delivered for your builder.
Get Your Fixed-Fee Box Frame Quote
Describe your opening and what is above it — we'll confirm a fixed price within one working day.
What Is a Steel Box Frame and When Do You Need One?
Box frames and goalpost frames use hollow steel sections to carry loads where a single RSJ beam is not appropriate.
🟦 Steel Box Frame
A steel box frame uses rectangular or square hollow sections (RHS or SHS) as both columns and beams. The frame transfers loads through the columns into pad or strip foundations at the base. The hollow profile gives a compact, clean appearance suited to residential and commercial settings where an I-section beam would look out of place.
🏉 Goalpost Frame
A goalpost frame is a two-column, single-beam steel structure — named for its resemblance to rugby goalposts. Used where a single beam needs vertical support at both ends because the existing masonry cannot safely carry the point load reactions. Common for wide garage openings, large glazed openings and open-plan remodels.
RSJ Beam or Box Frame — Which Do You Need?
RSJ / Steel Beam (Single Beam)
- Shorter spans where masonry can carry point loads
- Load above is moderate, spreads across the bearing
- Padstone bearings into existing brickwork are sufficient
- Standard wall removal between rooms or door widening
Steel Box Frame / Goalpost Frame
- Wider spans or heavy loads where masonry would be crushed
- Columns needed to route loads directly to pad foundations
- Large garage openings or bi-fold glazing across wide span
- Aesthetic requires slim hollow section rather than I-beam
Where Steel Box Frames and Goalpost Frames Are Used
The most common projects we design box frames and goalpost frames for across the UK.
Garage Conversions
Converting a garage to habitable space by removing the front wall and replacing it with a steel frame to carry the floor or roof above.
- Goalpost frame replaces garage door lintel and piers
- RHS columns founded on new pad foundations
- New glazing or infill panel designed around frame
Large Residential Openings
Wide bi-fold doors, glazed walls or large openings where a single RSJ bearing on blockwork is not sufficient.
- RHS beam spans opening with column support at each end
- Column bases transfer loads to pads below floor level
- Suitable for spans up to 8 m in typical residential use
Open-Plan Remodels
Creating open-plan kitchen, dining and living areas by removing multiple walls and replacing them with a steel frame structure.
- Multi-column box frame carries combined loads from floors above
- Loads routed to new pad foundations where walls are removed
- Works alongside RSJ beams where some walls are retained
Commercial Shopfronts
Replacing masonry shop fronts with open steel frames to carry floors and facade above a new wide opening.
- Box frame takes facade loads from above ground floor
- Connections designed for gravity and wind load
- Building Control calculations for landlord and council
Structures Between Buildings
Linking adjacent buildings across a gap where a steel box frame creates a covered walkway or structural connection.
- Frame designed for roof loads and any floors above link
- Connection details to existing party or gable walls
- Movement joints considered where buildings may settle differently
Extensions and Ground Floor Openings
Single storey extensions where the junction to the existing building requires a goalpost frame to span without loading weak masonry.
- Frame replaces load-bearing wall at extension junction
- Designed to work with extension foundations and floor slab
- Included in Building Control calculations for the extension
What Our Steel Box Frame Design Pack Includes
Everything Building Control needs to approve your steel box frame or goalpost frame — and everything your builder needs to install it.
RHS / SHS Member Sizing
Beam and column sections chosen for strength, deflection and the available clearance in the building.
- Gravity, wind and notional horizontal load cases
- Bending, shear and axial checks to BS EN 1993
- Deflection limits appropriate to the application
Base Plate and Connection Design
Column base plates, holding down bolts and beam-to-column connections sized for all load combinations.
- Holding down bolt forces including uplift under wind
- Base plate thickness and weld design
- Bolted or welded beam-to-column options
Pad and Strip Foundations
Foundations sized for column point loads with bearing pressure, sliding and overturning checks.
- Pad dimensions and reinforcement details
- Foundation depth for frost and bearing conditions
- Strip options where two column pads are close together
Drawings and Notes for Builder
Clear arrangement and detail drawings your builder can use on site without interpreting calculations.
- Frame arrangement with section sizes marked
- Base plate and holding down bolt layout
- Foundation plan and section with reinforcement
From Enquiry to Building Control Approval
Most box frame designs are completed without a site visit. Share photos and dimensions and we handle the structural design.
Send Your Project Details
Share photos, drawings or sketches showing the opening, the loads above and the floor construction. We confirm what we need for a fixed fee.
Fixed Fee Agreed
You receive a clear scope, fixed price and turnaround time before any design work begins. No hourly rates.
Calculations and Drawings
We produce the full structural calculation pack — member sizing, base plates, foundations and builder drawings — ready for Building Control.
Building Control Support
We respond directly to Building Control queries and provide revisions until the frame is formally approved.
Information We Need to Design Your Steel Box Frame
Clear information at the start avoids rework.
Opening Dimensions and Loads
- Width and height of the opening needed
- What is directly above: floor, roof, masonry or combinations
- Number of storeys and floor construction type
Existing Structure
- Photos of the existing wall, floor and ceiling
- Wall construction: brick, block, timber frame, stone
- Existing foundation type if known
Site Address and Use
- Full address or postcode for wind loading
- Building use: residential, commercial, mixed
- Any specific clearance requirements for column positions
Drawings (if Available)
- Architect or builder drawings in PDF or DWG format
- Planning drawings showing elevations and sections
- Any previous structural reports on the building
Steel Box Frame Design Packages
Fixed prices agreed before any work begins. No hourly rates. No hidden extras.
Goalpost / Box Frame
RHS beam and columns with base plates for a standard residential or commercial opening.
- RHS or SHS member sizing
- Base plate and connection design
- Builder-ready arrangement drawing
- Building Control calculation pack
Garage Conversion Frame
Front elevation steel frame and foundations for a full garage conversion to habitable space.
- Goalpost frame replacing garage opening
- Column pad foundation design
- Connection to existing structure
- Building Control submission ready
Complex Multi-Column Frame
Larger frames, multiple load combinations or frames serving multiple openings across a floor plan.
- Multiple column and beam layout
- Combined load path analysis
- Multiple foundation pads or strip design
- Full Building Control drawing set
With Pad Foundations
Pad or strip foundations designed and detailed as part of the commission where new foundations are required.
- Pad dimensions and reinforcement schedule
- Foundation depth and bearing checks
- Included in Building Control pack
- Combined with any frame type above
Steel Frame Projects We Have Designed
Comments from homeowners, builders and architects on box frame and goalpost frame projects across the UK.
"We needed a goalpost frame to convert our garage and carry the first floor above. SECalcs produced clear calculations and a drawing that showed the builder exactly where the columns had to go and how to fix them. Building Control passed without any questions."
"We wanted a 6 m wide bi-fold opening at the back of our kitchen extension. The engineer explained why a single beam wasn't suitable and designed a compact RHS goalpost frame that fitted within the wall thickness. The result is exactly what we wanted."
"SECalcs confirmed the existing blockwork couldn't carry the point loads and designed a box frame to route the loads to new pads below floor level. The calculation pack gave Building Control and our structural warranty provider everything they needed."
Steel Box Frame and Goalpost Frame Questions Answered
Common questions from homeowners, builders and architects planning a steel box frame or goalpost frame project.
A steel box frame uses hollow rectangular or square steel sections (RHS or SHS) as columns and beams to carry loads across an opening or span. Unlike a portal frame, the joints are typically bolted rather than welded, making them well suited to residential openings, garage conversions and smaller commercial applications where a compact, lighter frame is appropriate.
A goalpost frame is a two-column, single-beam steel structure named for its resemblance to rugby goalposts. It is used where a single beam needs vertical support at both ends because existing masonry cannot safely carry the point load reactions. Common uses include wide garage openings, large glazed openings and open-plan remodels.
You need a box frame or goalpost frame when a single horizontal beam cannot carry the loads safely. This typically happens when the span is wide and loads are heavy; when masonry cannot carry the beam point loads even with padstones; or when columns are needed to transfer loads directly to pad foundations below floor level.
Yes. Any structural steel frame that carries building loads requires Building Control approval. We provide a complete Building Control-ready calculation and drawing pack as part of every commission — everything the inspector needs to approve the structure.
Steel box frame and goalpost frame calculations typically start from £450 for a straightforward residential opening or garage conversion. More complex frames with multiple columns, heavy loads or separate foundation design are priced on enquiry. All fees are fixed and agreed before any work begins.
A portal frame uses I-section members with moment-resisting haunch joints, suited to larger industrial spans of 10–40 m for farm buildings and warehouses. A steel box frame uses RHS or SHS hollow sections with bolted connections, better suited to residential and commercial openings of 2–8 m. For larger portal frame buildings, see our portal frame structural design page.
Ready to Get Your Steel Box Frame Designed?
Send your opening dimensions and a photo of the existing wall. We'll confirm a fixed fee — most enquiries answered within one working day.
- Head Office: Corwell Lane, Uxbridge, England, UB8 3DE.
- 07359 267907
- info@structuralengineercalcs.com
- Company No: 12653163