The most common and most likely answer to this crossword clue is the 10 letter word CROSSBEAMS
I've seen this crossword clue in The Guardian
C
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O
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More clues leading to the result CROSSBEAMS
Buddy explains!

The straight part
We know that "girders" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case girders = crossbeams.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does CROSSBEAMS mean?
CROSSBEAMS noun- A horizontal structural beam that runs perpendicular to the primary support beams; a girder.
- A beam laid across the bitts, to which the cable is fastened when riding at anchor.