Why do we need a steel goal post or a box frame?

The use of steel is essential in all structural modifications. Its use allows Structural Engineers and Builders achieve Clients requirements of spanning across openings. Sometimes adding a steel beam is not sufficient and steel goal posts or box frame is required. This page tries to answer frequently asked questions regarding the use of goal posts and box frames.

When the whole or a substantial part of an external wall is being removed there is more to consider than just the vertical dead and imposed loads. For example, many homeowners want to knock down as much of the rear wall as possible to create an open-plan kitchen-diner when building a rear extension. As structural failures are not only caused by vertical forces, it is important to consider resistance to lateral loads such as wind. Traditional masonry houses have cross walls or shear walls. The party walls are “buttressed” by the front/rear walls and by a spine wall in the middle. The masonry can this way transmit wind and lateral loads in all directions safely to the ground. Homeowners carrying out structural modifications to their properties may inadvertently strip their property (and neighbouring properties) of this “lateral stiffness”.

To ensure masonry buildings maintain the required lateral stiffness, the Building Regulations Approved Document A set some conditions on openings in structural walls. Approved Document A limits the amount of external or buttressing wall that can be removed and insists that the corners or returns of these walls are substantial. The guidance states that no more than 66% of an external or buttressing wall can be removed and that a min of a 550mm (665 external measured from the centreline of the party wall or external wall) return is maintained. Please refer to attached Diagram 12. If this guidance can't be achieved a portal or a box frame is required.

Beta Structural Engineering can design steel box frames (also referred to as picture frames) or steel portal frames (also referred to as goal posts) to allow homeowners to achieve an architectural layout that suits the aspirations of modern lifestyle of maximum openness and ease of movement. Below is a description of the main three structural options available to any homeowner undertaking a rear extension or converting a rear reception into a kitchen diner.


Option 1: Use of a steel beam supported on masonry returns

Description: the opening is made in the rear wall leaving a return of 550mm either side of the opening. A steel beam is usually required to support the loads from above. This steel beam is supported on pad-stones that are supported on the masonry returns. The beam width is chosen to ensure it matches with the width of the supported wall. 

Advantages: minimum demolition, short construction time, less expensive than other options. The return of the wall is conveniently similar to a kitchen counter width.

Disadvantages: the opening achieved does not give the maximum openness that some homeowners may want.

Option 2: Use of portal frame (goal posts)

Description: the opening is made as wide as possible. The whole of the rear wall between the two flank walls (party walls) is removed (obviously after the builder secures the first floor and the wall above using properly chosen temporary supports). A steel portal frame is then inserted, made up of two columns and a beam supported on them – using what is referred to as moment/rigid connections. The two columns are supported on new foundations cast at the edges next to the flank wall. The columns would be at the edge of these new foundations. The frame is attached to the masonry walls to ensure the frame provides the structure with the required lateral stiffness.

Advantage: maximum opening can be achieved. Steel columns can be H section 152x152mm to 254x254mm depending on the opening span and loads from storeys above (as well as sway forces from wind).

Disadvantage: the main disadvantage of this option is the need for new foundations. As the columns are at the edge, the foundations size needs to be increased to counteract the eccentric loading (to spread the load without over-stressing the soil). Compared with option 1, this option is requires a higher budget and a longer construction time.


Option 3: Use of box frame (picture frame)

Description: the opening is made as wide as possible. The whole of the rear wall between the two flank walls (party walls) is removed (obviously after the builder secures the first floor and the wall above using properly chosen temporary supports). A steel box frame is inserted, composed of a top steel beam supported on two steel columns, supported in turn on a bottom steel beam. The bottom beam is encased in concrete and supported on the old corbelled brickwork foundation that lies underneath the removed wall. The connections between the columns and the beams are referred to as moment/rigid connections. The connections can either be designed and detailed by the Structural Engineer or by the fabricator if they have an in-house Structural Engineer to undertake a design based on the forces provided by the Structural Engineer.

Advantage: maximum opening can be achieved. Steel columns can be H section 152x152mm to 254x254mm depending on the opening span and loads from storeys above (as well as sway forces from wind). Ground works are minimized by re-using the corbelled foundation of removed wall for supporting the box frame.

Disadvantage: the main disadvantage is the increased cost of steelwork. However, the savings in ground works cost and time outweigh the slight increase in steel cost.



Beta Structural Engineering recommends option 1 for all cases except where the client mind is set on achieving a maximum opening. To achieve the maximum openness, we highly recommend option 3: use of a steel box frame. We have developed a standard method of designing these box frames and provide detailed fabrication drawings that would allow the box frame to be produced accurately. Many steel yards do not have the benefit of an in-house engineer and even if they do, they would charge clients for detailing connections. Our standardized fabrication drawings allow our clients to use any steel yard to procure steel ensuring competitive rates on the steel cost and saving time and cost.

 

Examples:

Opening in rear wall, Seward Road, Hanwell W7                                     Ealing Council Building Control Structural Calculations and Construction Drawings.

Opening in spine wall, Avondale Road, Mortlake SW14                           Richmond Building Control Structural Calculations and Construction Drawings

Opening in spine wall, Maldon Road, Acton W3                                       Ealing Building Control Structural Calculations and Construction Drawings

Opening in spine wall, Cypress Avenue, Twickenham TW2                      Richmond Building Control Structural Calculations and Construction Drawings

Opening in spine wall, Willow Road, W5 South Ealing                             Ealing Building Control Structural Calculations and Construction Drawings

Opening in rear wall, Hatfield Road, West Ealing W13                             Ealing Council Building Control Structural Calculations and Construction Drawings.

 

Goal posts for opening in rear wall, Shrewsbury Wall, Isleworth TW7        Hounslow Council Building Control Goal Post Structural Calculations and Construction Drawings.

Goal posts for opening in spine wall, Mattock Lane, Ealing W13               Ealing Council Building Control Structural Calculations and Construction Drawings

 

Box frame for opening in rear wall, Creffield Road, W5 Ealing                  Ealing Council Building Control Box Frame Structural Calculations and Construction Drawings.

Box frame for opening in spine wall, Shrewsbury Wall, TW7 Isleworth       Hounslow Council Building Control Box Frame Structural Calculations and Construction Drawings.

Box frame for opening in rear wall, Sydney Road, W13 West Ealing         Hounslow Council Building Control Box Frame Structural Calculations and Construction Drawings.

Box frame for new two storey extension, Boston Road, Hanwell, Ealing    Ealing Council Building Control Box Frame Structural Calculations and Construction Drawings.

Box frame for opening in rear wall, Rusthall Avenue, Chiswick, W4          Council Building Control Box Frame Structural Calculations and Construction Drawings.

Box frame for opening in rear wall, Gordon Road, W5 Ealing                   Ealing Council Building Control Box Frame Structural Calculations and Construction Drawings.

Box frame for opening in rear wall, Dean Road, Hanwell W7                    Ealing Building Control Structural Calculations and Construction Drawings.

Box frame for opening in rear wall, Churchfield Road, W13                            Ealing Building Control Structural Calculations and Construction Drawings.

Box frame for opening in rear wall, Loveday Road, W13 Ealing                      Ealing Council Building Control Box Frame Structural Calculations and Construction Drawings.