When I want to talk about creating a structural model of a building or bridge, should I say modelling the structure or modeling the structure?
Both are correct and acceptable. Modeling is commonly used in the U.S. while modelling is used in the UK and many other places.
The last consonant often gets doubled in present participle and past participle when it's a part of a stressed syllable.
For example, the t in 'submit' is a part of a stressed syllable so it's doubled in present participle (submitting) and past participle (submitted).
The last t in 'target' is a part of unstressed syllable so it doesn't get doubled in present participle (targeting) and past participle (targeted).
However, the final -l- in unstressed syllables gets doubled in present participle and past participle in British English and many other dialects.
- travel → travelling, travelled
- label → labelling, labe**ll*ed
- cancel → cancelling, cancelled
- model → modelling, modelled etc.
- travel → traveling, traveled
- label → labeling, labeled
- cancel → canceling, canceled
- model → modeling, modeled etc.
(You might also want to read the Wikipedia article on doubling of consonants.)
It depends on which English dialect you are referring to:
In American English, the verb model becomes modeled and modeling. Outside North America, the preferred participles are modelled and modelling, with two l‘s. Canadians prefer the double-l forms, though the single-l forms appear about a third of the time. (In contrast, the double-l forms are almost nonexistent in 21st-century American writing, and the single-l forms are similarly rare outside North America.) These spelling preferences extend to modeler (U.S.) and modeller (outside the U.S.).