Notes
The £5 a kilo rule
Steel is sold by weight. Once you know that, you can read your own steel schedule and ask better questions.
Drawing made for this demo. True proportion, not a design.
Key to the drawings
- Existing masonry A flat tone: everything already there, cut or seen.
- Brickwork, new Single diagonal hatch. The outer leaf, and new brickwork in elevation.
- Concrete Aggregate triangles and fine dots, scattered at random: foundations, slabs, padstones, lean-mix cavity fill, encasement.
- Undisturbed ground Basket weave, as a band that follows the ground line and fades out.
- Hardcore Large open zigzag, the depth of its own layer.
- Timber, cut A box with a cross. Seen along its length, an outline only.
- Steel, cut Solid, at the section's true proportion.
- DPC Heavy dashed line, as Approved Document C draws it.
- Load path Where the load goes, from the roof to the ground; dashed where it runs beyond the cut.
- Load marks Arrows at the symbol weight: loads down onto a member, bearing up under a foundation.
- Hidden, beyond or removed Out of the cut, beyond it, or taken out.
- Named dimension A quantity named, never numbered.
- Cloud Confirm on site. Never a revision mark.
The designation is the whole story
Somewhere in your engineer's pack there is a schedule of steelwork, and on it a line that looks like this: 203x133 UB25. It reads like a part number. It is closer to a recipe.
UB means universal beam, the standard I-shaped section used for almost every domestic opening. The first two numbers are the serial size in millimetres, which is roughly the depth and the width of the section. The last number is the one that costs you money: the mass of that beam in kilograms, for every metre of its length.
So a 4 metre length of 203x133 UB25 weighs about 100 kg. Length in metres, times the last number. That is the only sum you need to do yourself.
Diagram drawn for this demo. Universal beam designations follow the standard serial size and mass per metre convention. Section in true proportion, not a design.
Reading your own schedule
For each beam on the schedule, multiply the length by the last number in the designation, and add the answers up. That is the weight of steel in your house, the number your fabricator is quoting against.
Two caveats. Schedule weights are usually rounded, and weight is not the whole invoice: connections, padstones, delivery, and the labour of carrying steel through a house sit on top. Still, lighter steel is cheaper steel, and easier to get through your front door.
Why does the span drive the weight?
A beam is sized by how hard it bends and how far it sags. Try , then , and watch the load travel to the supports.
Standard results for a simply supported beam under an even load.
Illustrative. Not a calculation and not a design.
M equals w L squared over 8.
Delta equals 5 w L to the fourth, over 384 E I.
- w
- the even load along the beam
- L
- the span
- M
- the largest bending moment
- δ
- the sag at midspan
- E
- the stiffness of the material
- I
- the stiffness of the section’s shape
Double the span and the bending doubles twice over; the sag doubles four times over.
Now try . The sag shallows sharply; the bending does not change. Depth buys stiffness far faster than weight does, which is why a flush beam, kept inside the floor, buys weight instead.
What lean design saved on one project
Re-Structured, a practice that writes openly about this, reviewed the design of a rear extension and found steel the building did not need. Build It published the case: on a £50,000 rear extension, the review identified overdesign that saved 700 kg of steel, and at £5 per kg that is a £3,500 saving on the build cost.
Those are a third party's figures about a third party's project. We quote them because they are unusually specific, not because your house will produce the same number. What travels is the mechanism. 700 kg is not an exotic saving. It is a handful of members, each one size larger than it needed to be.
Illustrative arithmetic at the published rate. Not a price for your project, and not a saving we promise. Steel prices move.
Source: Build It, case by Re-Structured.
Source: Build It, “Structural Costs & How to Save Money on Structural Engineering”, case by Re-Structured. A third party’s figures about a third party’s project. The arithmetic here is that published rate times a weight, nothing more.
Source: Build It, “Structural Costs & How to Save Money on Structural Engineering”, case by Re-Structured. A third party’s figures.
The same practice is candid about why overdesign happens, and none of the reasons are sinister: not enough time; working outside their specialism; risk aversion late in a career; inexperience early in one; fear of an insurance claim; working to obsolete standards; poor software; and no review systems. Their own line is the one worth remembering.
when the excess is say only 20%, it can easily be missed
Re-Structured, on why over-specification is hard to spot
A lean design meets the Building Regulations exactly as a heavy one does. Lean is not a shortcut. It is what happens when someone has the time to look twice.
Questions worth asking
None of these are aggressive, and a good engineer will enjoy them.
- What is the total weight of steel in this design?
- Which member is the heaviest, and what is driving its size: the span, the load above it, or the depth we have to fit into the ceiling?
- Is any beam sized by how far it is allowed to bend rather than by what it has to carry? If so, what would have to change to relax that?
- Would moving a post, or trimming an opening by a few hundred millimetres, take a size out of anything?
- Has the scheme been looked at as a whole, rather than beam by beam?
- Are the padstones and the supports under each beam designed too, and is there anything unusual underneath them?
- If the steel has not been ordered yet, is there still time to look at it again?
The earlier you ask, the more of the structure is still worth asking about.