Module 2 · Lesson 2.1
Combinations, and the case where load helps you
Most of this is shared with every other material. The part that is not is knowing when a load is on your side.
Why this matters
A steel roof can weigh a twentieth of a concrete one. That single fact turns a routine check into the one that governs: wind suction a concrete building would never notice can lift a steel building off its foundations. And the check is easy to get wrong in the unsafe direction, because the instinct to multiply permanent load by 1.35 is exactly backwards when that load is what is holding the building down.
By the end of this lesson you should be able to
- Say what a partial factor is for, and why there is more than one
- Recognise a favourable permanent action
- Carry out an uplift check and interpret the result
- Explain why the governing combination is rarely obvious
What you should already know
- Actions and combinations (Reinforced Concrete Design, Module 2) — the rules are shared
- Equilibrium, and the idea of overturning and uplift
The rules are not steel's rules
EN 1990 sets the basis of design for every material. A partial factor on a permanent action does not change because the structure is steel rather than concrete, and this course does not re-teach the machinery — it is the same machinery, and the Reinforced Concrete course develops it in full.
What changes is which combination governs, and the reason is weight.
A reinforced-concrete floor might be 7.5 kN/m². A composite steel floor might be 4. A steel roof with light cladding might be 0.35 kN/m². Wind suction on that roof might be 0.9 kN/m².
So for a steel roof the permanent load is a fraction of the wind load, and the structure can end up being pulled upwards. That is not an exotic case. It is the routine governing case for a great many single-storey steel buildings, and it changes what the holding-down bolts, the bases and the foundations are for.
Worked example
Uplift on a light industrial roof
Given
- Steel roof: purlins, sheeting and services, 0.35 kN/m²
- Characteristic wind suction on the roof, 0.90 kN/m²
- Frame spacing 6.0 m, rafter length 12.0 m on plan
- γG,inf = 1.0 favourable; γG,sup = 1.35 unfavourable; γQ = 1.5
Find
Whether the roof lifts, and what happens if the wrong factor is used.
Assumptions
- Wind suction taken as uniform for this check; real distributions vary across the roof
- Partial factors in general UK use, not verified against the National Annex here
Try it
Does this roof lift?
Two answers at once: the one the correct favourable factor gives, and the one you get by inflating the self-weight out of habit. Find the band where they disagree about whether there is any uplift at all.
- Permanent, favourable (γG = 1.0)
- 0.35 kN/m² down
- Wind suction, unfavourable
- 1.35 kN/m² up
- Net action, correct factor
- -1.00 kN/m²
- Net action, using 1.35 by habit
- -0.88 kN/m²
- Difference the wrong factor makes
- 0.12 kN/m²
- Uplift on one frame
- 90 kN
- Per base
- 45 kN
- Wrong factor hides the uplift?
- no
Net -1.00 kN/m² — the roof lifts, and it lifts on either factor. The holding-down system and the foundations must carry it.
Things worth trying
- Start at the defaults — a light industrial roof. There is a net uplift of about 1 kN/m², which over one frame is a serious holding-down requirement rather than a nominal one.
- Now find the band. Set the suction to 0.80 and raise the roof weight slowly from 1.0. Somewhere near 1.05 the correct factor still shows uplift while the habitual 1.35 shows a comfortable hold-down. Inside that band the mistake does not shave a margin — it deletes the entire holding-down design.
- Raise the roof weight to 2.5 kN/m², roughly a concrete roof. The uplift disappears entirely, which is why this check is a steel problem and not a general one.
- Increase the frame spacing and the span. The net PRESSURE does not change at all — only the force each frame collects. Uplift is a pressure problem at the sheeting and a force problem at the base.
- Note that the difference between the two answers is always 0.35 × gk. It grows with the roof weight, so the heavier the roof, the more the wrong factor flatters you — which is the opposite of reassuring.
Predict first
A steel building has a heavy concrete floor added at first-floor level. What happens to the uplift check on the roof?
Practice
A steel roof weighs 0.40 kN/m². The characteristic wind suction is 0.85 kN/m². What is the net design action, in kN/m²? Take a downward result as positive.
Practice
That net uplift of 0.875 kN/m² acts on a frame at 6.0 m centres spanning 15.0 m. What total uplift force must the two bases resist, in kN?
Practice
A roof weighs 1.05 kN/m² and the suction is 0.80 kN/m². Using the CORRECT favourable factor, what is the net action in kN/m²?
Check yourself
What decides whether a permanent action is favourable or unfavourable?
Summary
- The combination rules are shared with every material — steel does not have its own
- What is steel-specific is which case governs, and lightness is the reason
- Favourable or unfavourable is a property of the ACTION AND THE CHECK together
- A permanent action resisting uplift takes γG = 1.0, never 1.35
- There is a band of roof weights where the wrong factor deletes the uplift case entirely
- Uplift is checked at every level of the load path, not once for the building
This is educational material. It uses simplified examples to teach principles, and must not be relied on for real design or safety-critical decisions. Module overview and checkpoint