Box frame or goal posts?

Box Frame or Goal Posts?

The use of steel is essential in all structural modifications. Its use allows Structural Engineers and Builders to achieve Clients requirements of spanning across openings. Sometimes adding a steel beam is not sufficient which means the project needs a steel box frame or goal posts. This page tries to answer frequently asked questions regarding the use of a box frame or goal posts during the structural modification of masonry structures and buildings (residential, commercial and industrial).

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A box frame or a goal post?

Clients and contractors often ask this question. When removing a substantial part of an external wall, there is more to consider than just the vertical 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 have “buttressing” walls that provide lateral stability which are the front/rear walls and a cross 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”. It is important to carry out these works in a safe and compliant manner, as otherwise it could lead (in the worst scenario) to the collapse of the party wall and may contribute to the collapse of a whole block of flats or a terrace of houses like a “house of cards”.

Compliance and Building Regulations

To ensure masonry buildings maintain the required lateral stiffness, the Building Regulations Approved Document A sets 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 Diagram 12 in Approved Document A of the Building Regulations. If this guidance can’t be achieved a portal or a box frame is required. We encourage clients interested to refer to Document A of the Building Regulations or to LABC technical guidance.

Why you need a Structural Engineer?

Beta Structural Engineers can design steel box frames (picture frames) or steel portal frames (goal posts). Our designs allow homeowners to comply with Building Regulations and 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. If you have any question regarding the information provided below, please do not hesitate to contact us to seek advice. We are happy to explain to you the technical, regulatory and cost impacts of each option.

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

If the client or the contractor is able to retain a minimum return of 550mm either side of the opening, then this would be the right choice. A steel beam would support the loads from above. This steel beam is rests on pad-stones that are spread the loads to the masonry returns preventing the crushing of the relatively soft masonry. The beam width matches the width of the supported wall and takes into account whether there is any moisture.

The Structural Engineer would produce a design and a drawing for the use of the builder who is pricing and carrying out the works. The design and drawing also form part of an application to Building Control to ensure compliance with the Building Regulations Approved Document A.

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.

 

For an initial consultation, email us on fm@betadc.co.uk attaching all available plans and drawings

Option 2: Use of portal frame (goal posts)

The opening is as wide as possible because the clients intends to remove the whole of the rear wall between the two flank walls (party walls). The builder secures the first floor and the wall above using adequate temporary supports. Following the securing of the property, the contractor inserts a steel portal frame. The steel frame refers to two columns and a top beam. The top column transfers the loads to the two columns using moment/rigid connections. The two columns transfer loads to new foundation cast at the edges next to the flank wall. The columns would be at the edge of these new foundations. The frame columns provide lateral stability to the masonry walls so the contractor needs to use chemical fixings to attach the steel columns to the masonry walls to compensate for the lost lateral stiffness.

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

Disadvantage: the main disadvantage is the need for new foundations. Columns are at the edge of the foundations, causing eccentric loading so foundations size could be quite large to counteract the eccentric loading and to spread the load without over-stressing the soil. Option 2 requires a higher budget and a longer construction time than option 1.

 

 

Option 3: Use of box frame (picture frame)

Description: the opening is as wide as possible. The whole of the rear wall between the two flank walls (party walls) would require removal (obviously after the builder secures the first floor and the wall above using properly chosen temporary supports). The contractor would need to insert a steel box frame (a top steel beam on two steel columns sitting 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 moment/rigid connections and we provide the details and design for them. Alternatively the fabricator may have an in-house Structural Engineer to undertake the design and detailing based on the forces from the Structural Engineer.

Advantage: this options results in a maximum opening. 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). The use of the corbelled masonry foundation of the original wall to support the box frame would minimize ground works.

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.

 

So what is better, Box Frame or Goal Posts?

Beta Structural Engineers recommend option 1 for all cases except where the client has already decided to achieve 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 accurate production and fabrication of the box frame steel. 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:

Use of Steel Beams

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.

Opening in internal walls, Rossdale Drive, Brent, NW9
– Brent Council Building Control Structural Calculations and Construction Drawings.

Goal Posts

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

Goal posts for office conversion, Cleveland Road, Ealing W13
– Ealing Council Building Control Structural Calculations and Building Regulations Drawings

Box Frames

Box frame for opening in rear wall, Kent Avenue, W13 West Ealing
– Ealing Council Building Control Box Frame Structural Calculations and Construction Drawings.

Box frame for opening in rear wall, Bradley Gardens, W13 West Ealing
– Ealing Council Building Control Box Frame Structural Calculations and Construction Drawings.

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

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

Box frame for new two storey extension, Boston Road, W7 Hanwell
– 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.