WHY USE A BOX FRAME WHEN ONE BEAM IS OK

When designing large openings or knocking through the main house wall (which was the external wall into the extension area) we have builders question the engineering drawing design constantly "do we really need to install a BOX FRAME when we can get away with one beam?". It can get quite frustrating sometimes as it's nearly a routine question on 35% of all our projects we carry out for our clients. We tend to find that the builders who do not question the design and crack on are the more experienced builders out there as they have been used to installing BOX FRAMES for the past 10+ years on all projects. This article is to give a small brief on why BOX FRAMES are needed for large openings or knock throughs to enter into the newly extended area which is becoming very popular in extension designs these days.

WHY THE BOX FRAME WHEN WE CAN USE ONE BEAM TO SUPPORT THE OPENING?

BOX FRAME DESIGNS CANNOT BE REPLACED WITH A ONE SINGLE BEAM OPTION

ENGINEERS RESPONCE

John, Thank you for raising the same question you have asked me on the previous project you sent over to me last week. I've noticed that you keep raising the same question and it's been on approx. 40% of the projects I do for you.

With regards to BOX FRAME designs - please find below a few main reasons why it is needed.

When a solid masonry wall (such as an external solid wall) is removed - the existing building is losing part of the lateral stability system which needs to be enhanced by an additional solid wall (or long brick piers) or alternatively by a steel box frame. It is essential to properly recognize the existing stability system and the implications of the removal of one of its key elements (i.e. solid wall). The steel Box frame is designed not only to resist vertical loads but also designed to resist lateral wind loads. Simple Beam and pier solution may not be sufficient to resist lateral wind pressure or vertical loading pressure.

The load from above such as first-floor wall, floor and ceiling joists, roof and in some instances a loft conversion cannot just pinpoint the load on an existing brick pier which is usually designed 1.5 brick width. The foundations more likely will not be sufficient in these circumstances too. A BOX FRAME is designed to withstand the loading in all directions to hold the additional loads from above.

How can an unregulated, unqualified builder even consider mentioning using a single beam? This is alarming that any builder advises this decision to the homeowner or Architect and even action in some cases by using a single beam ignoring the design carried by the qualified engineer on someone else’s house.

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Good working practise using a Box Frame method - Waltham Cross

This is not just negligence, it’s deliberate cowboynisem. The builder knows what the correct design is and needs to follow it. Comments like the design is overkill are very common to get out of going for this building method.

THREE REASONS TO AVOID BOX FRAMES COMING FROM A BUILDER

Some Builders are very good at making alternative suggestions up to the design given to them to make his life easier for him/her by avoiding the BOX FRAME option altogether for only three reasons we can think of.

1. The Builder has quoted the client (homeowner) before seeing the final engineering design/drawings. After engineering review/input/design is shown to the builder, a surprise in the amount of steelwork needed to complete the project HAS NOT been accounted for. The builder is now in a bad position to go back to the homeowner and ask for a further £4000+vat for materials and £2000+vat for installation and labour costs. The only way out of this embarrassment is to go back to the engineer and fight with him/her to design the new opening to a ONE BEAM alternative. Going back the homeowner and asking for more money would be crazy idea as the homeowner has chosen this builder with a fixed price given and written contracts in place. Asking for a further £6000+vat does not look very professional at all. This shows that the builder is not being straight and looks like further surprises can happen at any time. Homeowners are usually on a budget and do not have £6000 laying around to give to builders on demand by click of fingers, the quote was (say for example) £40,000 for the build not £46,000.   

2. The Builder has no experience or has not got the required people power for the installation of such large complex building methods. The installation of such beams and columns (some spanning 8 meters in length and weighing up to 800kg) can be challenging. The temporally propping methods are to be carried out by also a skilled experienced builder that knows what they are doing.

3. Just pure laziness. Installing a BOX FRAME can take all day as opposed to one hour for a single beam installation.

To cut the long debate a BOX FRAME is the only option and needs to be followed. The design has been carried out by a qualified engineer that provides not just the steel fabrication detailing showing the builder how all is fitted together but to provide the supporting structural calculations as workings to back the engineer's design up to be issued to building control for approval. The engineer is putting their responsibility on the design to work not fail. The Builder has no say in this area as is unqualified and cannot produce any form of engineering design or calculations.

ALTERNATIVE OPTION TO CONSIDER BY USING ONLY ONE BEAM

For the builder to get their way and be allowed to use ONE BEAM then this can be achieved by the following design method below: To achieve similar behaviour of a steel box frame design, we would need to introduce a minimum of 1000mm long Engineering Brick Pier (215mm wide) with mass concrete footing under - minimum 650mm wide x 1000mm deep. This, however, would need to be installed next to the neighbour's foundation hence it's likely that a Party Wall award would be needed for acceptance. (if the property has any neighbours) The client (homeowner) in most cases would not be happy with two large brick walls sticking out by 1000mm from their proposed living room at either end. The new wall is highlighted in red.

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DETAILED BOX FRAME DETAIL

STEEL BEAM EXTENSIONS
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I hope you understood this article and find it useful for your proposals we just want you to get the right results and not to make a mistake in doing the build the wrong way around as this could affect the sale to the next purchaser. Regards John. D.

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