exit converging

[sunyaer]

This is a quiz question about Ontario Building Code:
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This is OBC:

3.4.1.2. Separation of Exits

(1) Except as permitted by Sentence (2), if more than one exit is required from a floor area, each exit shall be separate from every other exit leading from that floor area.

(2) If more than two exits are provided from a floor area, exits are permitted to converge in conformance with Sentence3.4.3.1.(2), provided the cumulative capacity of the converging exits does not contribute more than 50% of the total required exit width for the floor area.
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My question:

1. Which exit is the converging exit? Is it the one that is designed to accommodate 400 persons?

2. Could you please draw a sketch to show the converging diagram indicating the 400 persons and 200 persons in the question?

 

[tmurray]

Think about a river with some smaller tributary streams. Each stream contributes to the river, but based on how small they are, there is no visible change to the river.

In our case, the river is the 400 person occupancy exiting. The 200 persons "stream" added to it should not have a severe impact to the exiting of the building.

 

[sunyaer]

..,

In our case, the river is the 400 person occupancy exiting. The 200 persons "stream" added to it should not have a severe impact to the exiting of the building.

3.4.1.2.(2) addresses situations where there are more than two exits, so let’s assume the quiz question is dealing with 3 exits. Lets label the 400 person capacity exit as No. 2, the 200 persons “stream” exit as No. 3.

I think what 3.4.1.2.(2) is saying is that the river (the converged exit, which is 400 person exit No. 2 ) should not contribute more than 50% of the total required exit width for the floor area, the 50% is not limiting the contribution of the “stream”, so the answer could be 300 persons.

The code is really making the test question writer confused, am I confused or am I correct?

 

[tmurray]

3.4.1.2.(2) addresses situations where there are more than two exits, so let’s assume the quiz question is dealing with 3 exits. Lets label the 400 person capacity exit as No. 2, the 200 persons “stream” exit as No. 3.

I think what 3.4.1.2.(2) is saying is that the river (the converged exit, which is 400 person exit No. 2 ) should not contribute more than 50% of the total required exit width for the floor area, the 50% is not limiting the contribution of the “stream”, so the answer could be 300 persons.

The code is really making the test question writer confused, am I confused or am I correct?

Sorry, I was thinking about two different occupancies entering the same exit enclosure, but on re-reading your original post, it looks like it is exits from the same occupancy converging (re-converging?).

The intent of the code is to ensure that no one exit contributes ore than 50% of total exiting capacity (some exceptions to this for single exit allowances). This is to ensure that compromising one exit will not cause significant delays in egress. This clause just clarifies that you can't circumvent this requirement by separating and then re-converging exits.

I was going to say the question is poor, but we just cannot answer it. Insufficient information is given in the question. We also need to know what the required exit width of the floor area is before we can answer the question.

 

[sunyaer]

…
I was going to say the question is poor, but we just cannot answer it. Insufficient information is given in the question. We also need to know what the required exit width of the floor area is before we can answer the question.

Correct.
We need to know:
1. The required exit width of the floor area;
2. The calculated contribution before the converging point (location) of the 400 person capacity exit; (400 person exiting capacity is designed, not the allowed contribution)

Example:
1. The required exit width of the floor area is 800 persons;
2. The calculated contribution before the converging point of the 400 person capacity exit is 300 persons;

so the converging exit (“stream”) can not have more than 100 persons.

Am I correct in the above?


Note: in this example, there may by 4 exits.

 

[tmurray]

Correct.
We need to know:
1. The required exit width of the floor area;
2. The calculated contribution before the converging point (location) of the 400 person capacity exit; (400 person exiting capacity is designed, not the allowed contribution)

Example:
1. The required exit width of the floor area is 800 persons;
2. The calculated contribution before the converging point of the 400 person capacity exit is 300 persons;

so the converging exit (“stream”) can not have more than 100 persons.

Am I correct in the above?


Note: in this example, there may by 4 exits.

If your required exiting width is for 800 persons, the most you could have at any point along the exit would be 400 persons (max. of 50% of the 800 person occupancy).

 

[sunyaer]

If your required exiting width is for 800 persons, the most you could have at any point along the exit would be 400 persons (max. of 50% of the 800 person occupancy).

The designed exiting capacity of the exit could be more than 50% of the 800 person occupancy, but the contribution width can not be more than 400 persons, making required cumulative width of all other exits not less than 50% of the 800 persons.

Am I correct or wrong?

 

[tmurray]

The designed exiting capacity of the exit could be more than 50% of the 800 person occupancy, but the contribution width can not be more than 400 persons, making required cumulative width of all other exits not less than 50% of the 800 persons.

Am I correct or wrong?

I'm not sure I am following you.

The exiting capacity of each individual exit cannot exceed 50% of the required capacity (3.4.3.2.(6)).

3.4.1.2.(2) is just reinforcing this base concept.

In our imaginary scenario,
If there is a 800 person occupancy, at no point can the individual capacity of an exit exceed 400 persons.

We can have two 400 person exits
We can have one 400 person and two 200 person exits...

We cannot have an exit that serves 401 or more persons.

 

[sunyaer]

..,

In our imaginary scenario,
If there is a 800 person occupancy, at no point can the individual capacity of an exit exceed 400 persons.

Maybe I am wrong, as I am using “demand” and “capacity” concept to understand this.

Can exit capacity be over designed, like almost all structural elements in buildings?
In this 800 person occupancy scenario, can we provide two exits, each of which has exiting capacity of 401 person instead of 400 person?

400 person capacity is the “demand” for each exit, while we over design one or both of them with 401 person “capacity”.

It looks logical to me: each of the two exits we provide has not less than 400 person capacity. What the code is trying to prevent is: one of the two exits has 401 person capacity, another one has 399 person capacity.

 

[sunyaer]

You can over design one of the two exits for 500 person capacity, but this would not cause another exit to be designed for 300 person capacity, 400 person exiting demand is still required for any exit.

 

[tmurray]

You can't really overdesign the exiting too much based on the way that this is worded (it's weird I know).

You could probably have a conversation with the official if you were to overdesign all the exits though.

 

[sunyaer]

You can't really overdesign the exiting too much based on the way that this is worded (it's weird I know).

You could probably have a conversation with the official if you were to overdesign all the exits though.

In real life design, I don’t think architects would often over design exits since space is always limited. However, the code doesn’t forbid you to over design ( like most of structural elements), but you are not allowed to over design one exit in order to under design another. There are wordings like “required exit width” or “exit width required” in the code, which implies there are cases where the actual exit width may be larger than the required width.

I can imagine that there may be scenarios where an exit is clearly seen by evacuating occupancy and the exit is over designed too much, people in panic while evacuating may all rush to this big exit, which is not the intent of the code.

I am just splitting hairs here for learning purpose only.

 

[tmurray]

you are not allowed to over design one exit in order to under design another.

I like the way you put this. Yes, that would be the intent.

The intent of the code is to provide some redundancy in case one of the exits becomes compromised.