Structural Steel Design / Fabrication, erection and durability
Durability and erection: the two limit states nobody analyses
One is reached slowly over decades, the other in an afternoon halfway through the programme. Neither appears in the frame analysis, and both can govern.
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Read the lesson register and outstanding review notesWhy this matters
A structure has to survive two conditions that no static analysis describes.
The first is time. Steel in the open corrodes, and the design life is a number somebody wrote in a brief. Whether the structure reaches it depends on the environment, the coating, and — decisively — on details that no calculation sees.
The second is the afternoon halfway through the programme when half the steel is standing and most of the bracing is not. That configuration is a structure. It carries load. It is frequently the least stable arrangement the steel ever finds itself in, and it does not appear anywhere in the design.
By the end of this lesson you should be able to
- Treat corrosion as a design-life problem rather than a paint specification
- Say why access decides whether a short coating life is acceptable
- Name the details that defeat a coating whatever is specified
- Quantify how much less stable a partly erected frame is
What you should already know
- Frame stability and αcr (Modules 4 and 16)
- Bracing systems and their relative efficiency (Module 16)
Corrosion is arithmetic before it is chemistry
Three numbers decide it.
The environment sets a rate. Corrosivity categories run from C1 — a heated interior, where unprotected steel loses under a micrometre a year — to C5, marine or aggressive industrial, at well over a hundred. That is a range of more than two hundred to one, and it is set by where the building is, not by anything the designer controls.
The coating buys years. A system's life to first maintenance rises with its thickness and falls with the corrosivity.
The brief states a design life. Fifty or sixty years, typically, for a building.
The design question is whether the second covers the third in the environment set by the first — and if not, what happens instead.
Rule classification: engineering recommendation. The rates and coating lives here are indicative figures used to teach the comparison. A real specification is written against a manufacturer's system data and the relevant protective-coating standard, not against these.
Worked example
A 50-year building in three different places
Given
- The same structure, with the same 200 µm coating system
- A 50-year design life in every case
- Only the site changes: C2 rural, C3 urban, C5 marine
Find
Whether the coating lasts the design life at each site, and what follows if it does not
The details that defeat any specification
A coating fails first where the drawing let it down. Five recurring cases:
Water traps. An upward-facing horizontal surface holds water and dirt against the coating indefinitely. The local corrosivity at that spot is effectively a category worse than the rest of the structure, and it is where the failure starts.
Unsealed crevices. A gap between plates draws water in by capillary action and will not release it. The coating never reached inside, and the corrosion proceeds unseen.
Bimetallic contact. Two different metals with an electrolyte between them form a cell, and the less noble one corrodes preferentially. Stainless fixings into carbon steel is the common case; electrical isolation must be detailed across the complete connection, which may require sleeves, gaskets and washers rather than a washer alone.
Sharp edges. Coating pulls back from a sharp edge by surface tension as it cures, so the film there is a fraction of its nominal thickness. Rounding edges is a fabrication instruction with a durability purpose.
Unsealed hollow sections. An open hollow section breathes moist air, condenses it, and corrodes from the inside where nobody will ever look.
Every one of these is a drawing correction, not a specification correction. Writing a heavier coating system on a detail that traps water does not help.
Try it
Coating durability: an illustrative comparison
Explore an assumed relationship between exposure, thickness and maintenance. The rates and fitted coating-life expression are unverified teaching inputs; they cannot select a coating system or predict maintenance dates.
Corrosivity category
Reachable for maintenance
Upward-facing surfaces that hold water
Hollow sections sealed
- Assumed unprotected rate
- 25 um/year
- Model loss at constant rate
- 1.25 mm per face
- Coating thickness
- 200 um
- Illustrative model duration
- 25 years
- Required design life
- 50 years
- Model duration comparison
- below the design life
- Detailing issues
- 0
Expert review required. Use exposure assessment and manufacturer system data to specify protection and maintenance. Check that inspection and maintenance access remain practicable. The model does not establish adequacy or a maintenance date.
Things worth trying
- At the defaults, the fitted expression returns about 25 years. This is an example output, not a verified maintenance interval.
- Double the thickness while holding the exposure category fixed. The model duration doubles because thickness enters linearly; actual coating performance does not follow this expression in general.
- Change the exposure category to see how the assumed rate changes the result. The category descriptions are broad examples, not an exposure assessment.
- Maintenance access affects whether a proposed protection strategy is practicable. It does not change the numerical curve here.
- The bare-steel loss calculation assumes a constant rate over the entire period. Do not use it to size a corrosion allowance.
- Use the detail controls to identify issues for review. They do not quantify local deterioration or establish that an unflagged detail is satisfactory.
- A durability specification needs the actual environment, surface preparation, manufacturer system data, inspection requirements and a maintenance plan.
The structure that nobody analysed
Here is a frame that reaches its design intent: four braced bays, αcr of 12, comfortably non-sway by the criterion of Module 4. Everything checks.
Now consider the afternoon when 80% of the steel is standing and one of the four braced bays has been completed.
The lateral stiffness is a quarter of the finished value. The vertical load is 80% of it — so the critical load factor falls on both counts at once.
What it calculates: The critical load factor of a partly erected frame, as a first estimate
- critical load factor at this stage of erection
- critical load factor of the completed frame
- braced bays installed so far, including any temporary bracing
- braced bays in the completed frame
- f
- fraction of the frame's steel already standing
This assumes
- Lateral stiffness is roughly proportional to the number of braced bays
- The bracing installed is distributed similarly to the completed system
- Vertical load scales with the steel erected
In plain terms: For 1 of 4 bays with 80% of the steel standing this gives about 3.75, against 12 for the structure everyone checked — the stage is 3.2 times less stable than the finished frame, and it is nowhere in the design.
Rule classification: derived. This is proportionality, not a code rule: αcr scales with lateral stiffness and inversely with the vertical load that stiffness has to stabilise. It is a first estimate for reasoning about sequence, not a substitute for analysing the stage properly once it has been identified as critical.
Try it
Bracing through an erection sequence
This simplified model makes lateral stiffness proportional to the number of braced bays and vertical load proportional to the steel erected. Compare the trend; each real erection stage needs its own structural assessment.
- Completed frame
- 12.0
- This stage
- 3.75
- Without temporary bracing
- 3.75
- Fraction of the completed value
- 31 %
- Worse than the finished structure
- yes
- Comparison with reference 10
- below
This illustrative stiffness/load model gives alpha-cr = 3.75. The reference value of 10 is not an adequacy criterion for the erection stage. Verify each stage using its actual load paths, restraints, actions and relevant instability modes.
Things worth trying
- At the defaults, the model gives alpha-cr = 12 × (1/4) / 0.8 = 3.75. This follows from the assumed stiffness and load fractions.
- Set two of four braced bays and 100% steel erected. The model gives 6 because its stiffness is halved while its load is unchanged.
- Add two equivalent temporary bays to the default case. The result becomes 11.25. Passing the reference value of 10 does not establish stability or adequate strength.
- Move the steel-erected control with the bracing fixed. The curve falls as the assumed load increases. Real erection loads include effects not represented by this slider.
- Restore four braced bays and 100% steel erected to recover the assumed completed-frame value of 12.
- A real stage assessment needs the installed members, restraint and connection stiffness, temporary works, construction actions, imperfections and relevant instability modes.
Practice
A frame will finish with 4 braced bays and αcr = 12. At one stage, 1 bay is braced and 80% of the steel is standing, and the erection engineer proposes temporary bracing equivalent to 2 permanent bays. What is αcr at that stage with the temporary bracing in place?
Check yourself
The same 200 µm coating system gives 66 years in C2 and under 10 years in C5. What follows for a coastal building with a 50-year design life?
Predict first
A designer specifies a heavy coating system, and the detailer draws a bolted splice with an unsealed gap between the cover plates. Where does the structure corrode first?
Summary
- Corrosion is three numbers: an environmental rate, a coating life, and a required design life
- The teaching model gives illustrative lifetimes of 66, 25 and 9.8 years; these are not verified coating-system data
- A short coating life is acceptable only if the steel can be reached — which is a building decision
- Water traps, crevices, bimetallic contact, sharp edges and open hollow sections defeat any specification
- Those are drawing corrections, not specification corrections
- A partly erected frame with 1 of 4 braced bays and 80% of the steel up sits at αcr ≈ 3.75 against 12
- Every erection stage is a structure, and its temporary bracing is a load path somebody must design