Does unprotected stair need to be enclosed?

[TickityBoo]

Imagine a two storey office block - ground and first - with a protected stair at one end and an unenclosed stair at the other. Distance between stairs approx 35m (floor area approx 400m2).

Strictly speaking, this unprotected stair is not an accommodation stair as it is not surplus to moe requirements.  Its needed, given that two storey exits are required (the single direction TD to the protected stair is over 18m, if the unprotected stair is not counted as a moe). 

I know that in a new build, both stairs would be enclosed.  However, this is an existing building where travel distance from any point on the first floor is within 45m to either the protected stair at one end or the final exit on the ground floor via the unprotected stair which is at the other end. Is there a problem? Does the unenclosed stair really need to be enclosed in what is a low risk existing building.  You can always turn your back on a fire and reach a place of safety and the nearest place of safety is always within 45m.

NB It is accepted that the need for a refuge for disabled evac would strengthen the argument for enclosing the unprotected stair e.g. if the protected stair was taken out by the fire.  However, let's assume the first floor is unaccessible to wheelchair users / others requiring a refuge.

Interested to hear your thoughts.

[kurnal]

  However, this is an existing building where travel distance from any point on the first floor is within 45m to either the protected stair at one end or the final exit on the ground floor via the unprotected stair which is at the other end.

 You can always turn your back on a fire and reach a place of safety and the nearest place of safety is always within 45m.

I have justified such an arrangement through risk assessment in many workplaces.

One asumes there is an exit close to the base of the open staircase and that a fire cannot possibly affect both exit routes? Alarm System or fully staffed? And that no wedges are ever ever used on the protected route? Very many existing shops are like this.

[TickityBoo]

There is an exit close to the base of the unprotected stair and there is a Cat M system.  Nearly all offices are occupied. Fire would not affect both exit routes as one of the routes is via a protected stair and there is also cross corridor doors for smoke control on each floor.

There is no protected route other than the protected stair. Doors off the protected stair are nominal 30 mins FR, self closing with smoke seals etc

[Midland Retty]

From what you describe I would suggest that you have satisfied current benchmark guidance which recommends you should hit a place of comparative or ultimate safety within 45 metres

As you point out a fire should be quickly detected by staff and so long as people can turn their back and seek alteratives should their chosen route be compromised i dont see an issue.