Structural strengthening: what the code requires, what the procedure costs

You have been handed a report saying the building needs structural strengthening. It is a phrase that frightens people and that almost nobody knows how to turn into decisions.

This is what it means in terms of the code, which signatures are needed, and what can be expected of the procedure.

Which code governs

The code in force is the NSR-10, the Colombian earthquake-resistant construction code, adopted by Decreto 926 de 2010 under Ley 400 de 1997. It has not stood still: it has been amended, in this order, by Decretos 2525 de 2010, 092 de 2011, 340 de 2012, 945 de 2017, 2113 de 2019, 1711 de 2021, 1401 de 2023 and 1580 de 2023. Nine decrees in all, and what governs is the text that results from every one of them, not the 2010 PDF that comes up first in a search.

And on existing buildings the law is explicit about what it orders the code to contain:

In other words: assessing and strengthening what already exists is not a regulatory void. It has a procedure of its own inside Title A of the code, distinct from the one applied to new build, and it has a name and a number: chapter A.10, «Evaluación e Intervención de Edificaciones construidas antes de la vigencia de la presente versión del Reglamento» (the assessment and intervention of buildings put up before the present version of the code came into force). It is worth asking for it that way, by its number: whoever hands you a vulnerability report should be working from that chapter, and should say so on the first page.

When assessment is compulsory

It is not the owner’s free choice in every case. A.10.1.3 sets out the chapter’s own scope, and the cases in which assessment becomes enforceable come from there:

• —The use of the building changes (A.10.1.3.2). A house that becomes a school, an office or a warehouse takes loads other than those it was designed for. And the code counts as a change of use both the planning kind (residential to commercial) and a move from one of the use groups to a higher one.
• —The structure is extended or modified (A.10.1.3.4). The chapter names «ampliaciones adosadas» and «ampliaciones en altura» in so many words: side extensions and extensions upwards. Adding a storey is not an interpretation; it is written down.
• —There has been damage from an earthquake or another event (A.10.1.3.6), where the damage is moderate to severe (in the structure, in the non-structural elements or in both) and the building has not been declared for compulsory demolition.
• —The competent authority requires it. If the municipal disaster-risk office issues a technical opinion finding serious damage to the load-bearing structure, or a judicial or administrative order intervenes, applying for the strengthening permit stops being optional. In propiedad horizontal (commonhold) the holder of that permit is the owners’ general meeting.

How far the strengthening has to go

This is the part almost nobody explains and the one that really moves the budget. The code does not ask the same of every existing building: the target of the strengthening depends on the use and on the period in which it was built. And if the strengthening changes the structural system, the Systems ruled out by seismic zone tool shows which are excluded in your zone.

Who signs what

There is no margin here. The law reserves the signatures, and whoever signs answers for it:

And in buildings that reach or exceed 2,000 m², the review of the structural designs has to be carried out by a professional other than the designer and independent of them in employment terms, at the cost of whoever applies for the permit. Mind the threshold: if the building falls short of that figure but the extension you are about to apply for takes it there once added to what is already built, the independent review is required too.

And there is one more signature that rarely appears in a budget and that the code requires in every case, with no area threshold: the construction of the structural intervention has to be placed under independent technical supervision (A.10.1.6). It is not optional, and the designer does not do it.

The permit: which one, and how long it lasts

Structural strengthening is a modality of building permit in its own right, not a minor work that can be done without permission. And it has three particularities worth knowing before you budget.

• —It lasts 24 months, extendable once by 12. The 36 months quoted everywhere belong to new build, urbanisation and parcelación, its rural counterpart. This modality is not among them.
• —The charges are assessed on a reduced base. Where there is a curaduría (the private office licensed by the state to issue permits), structural strengthening is assessed on 30% of the area to be intervened, not on the whole. It is a real relief within the procedure.
• —A building that never had a permit can still be strengthened. The modality is granted without prejudice to the building being formally recognised afterwards, provided the works were completed at least five years before the strengthening is applied for. Taken on its own, it approves no uses and authorises no works other than the strengthening.

The right order, and the one that wrecks the budget

The most expensive mistake is contracting the works before you have the diagnosis. The sequence that works:

1. 1A survey of what is there. Real measurements, not the ones in the title deed.
2. 2The vulnerability assessment, signed by a structural engineer. This is where you learn whether strengthening is needed, how much and where.
3. 3The geotechnical survey, if the diagnosis calls for it. Sometimes the problem is not upstairs.
4. 4The design of the strengthening, which is a project in itself.
5. 5The permit, in the structural strengthening modality.
6. 6The works, followed by whoever designed them and under the independent technical supervision the code requires in every case.

Skipping step 2 is the reason strengthening quotes turn up varying tenfold between one another: they are pricing different solutions to a problem nobody has defined.

What to expect on cost

I am not going to give figures, and for one reason: no published Colombian source sets out a methodology for the cost of a strengthening scheme. It depends on the structural system, on the damage, on access, and on whether the building can be emptied.

What can be said about the shape of the cost is this:

• —The procedure is the cheap part, and with the reduced 30% base it is cheaper than people fear.
• —The assessment and the design are proportionally expensive against a new build, because what already exists has to be uncovered.
• —The works are where the money is, and their cost is settled by the diagnosis, not the other way round.
• —There is one line that catches people out: in a structural intervention, complying with Títulos J and K (fire protection and the supplementary requirements) is left to the owner’s discretion, except where there is a change of use or an extension, in which case it is compulsory (A.10.1.3.7). That is to say, in the two commonest cases.
• —And in propiedad horizontal you have to add the time: the contingency fund comes first, and the extraordinary levy is only approved if the fund turns out to be insufficient.

Any figure you are given before the assessment is an estimate against an unknown problem. It may serve to tell you whether going on is worthwhile; not to sign a contract on.