The number of city workers forced out of their offices because of an improperly installed weight-bearing beam in Calgary's Municipal Building is now up to 500.

Portions of five floors were cleared out in September after crews discovered the problem.

City manager Jeff Fielding said officials recently did another examination of what would happen if that beam failed.

As a result, they have decided to move another 180 employees as a precaution.

Diagrams show the location of a truss that was incorrectly installed in the Municipal Building three decades ago. (Screenshot)

"Do I think the building's going to fall down? I don't. Are we being overly cautious? We are," Fielding said.

"But you know, nevertheless, you never know. There's a risk involved and that risk was higher, much higher than what you would normally, would expect."

Four city councillors are among those forced to leave their offices.

Coun. Shane Keating said it's inconvenient but not a huge problem.

Fielding said the beam will be shored up early next year, at which point some employees should be allowed to return.

A long-term repair plan is still being worked on.

Historic old city hall, located just north of the Municipal Building, is currently undergoing a multi-year, $34-million renovation that had already displaced the offices of the city councillors.