Snow load, wind zone, snow load zones

Hello, everyone. I hope someone can answer my question. In the construction and performance description of the roof structure, it was stated that the static calculation of the purlin roof was made considering a snow load of 0.75 kN/m² and wind zone II. Now we have received the calculation in which a snow load of 0.65 kN/m² and wind zone I were considered. We are in the Karlsruhe area. How will this difference affect our roof?

Hello Schornstein,

All of Germany is divided into so-called snow load zones. This is regulated in DIN 1055-5 Actions on Structures – Part 5: Snow and Ice Loads.
The basic value for the snow load of sk = 0.65 kN/m² is set correctly for the Karlsruhe area richtig,
I wouldn’t worry about that.
Whether a difference of 0.75–0.65 = 0.1 kN/m² has an effect depends significantly
on the calculated rafter cross-section b/h (width/height) and the selected wood quality.

The design of timber structures is regulated in DIN 1052. Here, two verification types are fundamentally distinguished:
1. Load-bearing capacity verifications:
here, the failure of the component is proven, i.e. the calculative verification
whether the timber cross-section b/h is capable of carrying the actions
such as self-weight, wind, and snow and other so-called imposed loads without damage.
2. Serviceability verifications:
here, the deformation (deflection) of a component is verified against an allowable
maximum limit (this is more about aesthetics; even if the verification is not met, failure of the component is not to be expected).

Now to the actual question of whether this can affect the roof.
This depends on the utilization rate of the chosen rafter cross-section!
The utilization rate is the quotient between the acting forces divided by
the resisting forces.

Example load-bearing capacity verification:
Utilization = existing stress (from action) / allowable stress (from wood quality)

Example interpretation of the result from the verification:
Utilization = 1.3 means the cross-section cannot carry the action, meaning the cross-section is undersized (cross-section damage, breakage, etc. are possible)!!
Utilization = 1 means the cross-section has reached its full load-bearing capacity
Utilization = 0.5 means the cross-section is only used to half its capacity,
meaning the cross-section is oversized!

If the structural engineer assumes 0.65 kN/m² and chooses a rafter cross-section b/h below
the limit utilization rate of 1.0, e.g. 0.9, then assuming sk = 0.75 kN/m² will not result in anything different; utilization will just be somewhat
higher (maybe 0.95). However, this still depends on other boundary conditions of the static
system, such as the support spans (bearings) and the spacing between rafters.

So, as already mentioned, it all depends on which cross-section b/h with which wood quality
(grade, type of wood) is to be installed, and how strict the structural engineer was in the design according to DIN 1052, or whether certain reserves were allowed.

I hope I could help, regards BIBO

Thank you very much, BIBO.
You have been very helpful.
I first have to work through so much new information. Our rafters are 8 cm wide and 24 cm high. Wood class C 24 Nk1. Does this mean that the average is 0.34? I haven’t found the utilization factor; there are only abbreviations in the report. I suppose P=1.00 kN is this utilization factor? Or am I mistaken?
In any case, I am reassured.
Best regards, Schornstein

Hello Schornstein,

you can gladly send me the static calculation via PN, then I would try to decipher the numbers.
The rafter b/h = 8cm x 24cm is already quite decent, since the insulation can still be well accommodated between the rafters at a height of 24cm.
It is also important to know, what is the rafter spacing between each other, usually 80cm?!
How far apart are the bearings (where the rafter rests) from each other, and what roof pitch in degrees is present?

- About the limit utilization rate:

Here is a very simple example:

1. Stress [kN/cm²] = Force [kN] / Area [cm²] (maybe still known from school)

calculate existing stress (assumption contact area for pressure absorption 8cm x 12 cm):
2. existing stress = 12 kN / (8cm * 12 cm) = 0.125 kN/cm²

permissible stress according to DIN 1052 for softwood NH C24 (compression perpendicular):
3. permissible stress = 0.154 kN/cm²

carry out the verification (both verification methods (a/b) are possible):
4a . existing stress < permissible stress
0.125 kN/cm² < 0.154 kN/cm² (verification is fulfilled)
4b . existing stress / permissible stress <= 1.0
0.125 kN/cm² / 0.154 kN/cm² = 0.812 <= 1.0 [-] (verification is fulfilled)

From verification 4a it is evident that the stress verification
is met, but how is the cross-section utilized?
This is much clearer in verification 4b, because the utilization rate can be read directly:
utilization rate = 0.812 [-] *100[%] = 81.2 %
The cross-section is utilized here at 81.2 %, so there is still a reserve of 18.8 % in the cross-section!

There are other verifications that must be carried out, the most unfavorable verification (highest utilization rate) indicates the limit utilization rate

- P = 1.00 kN could be the repair load (man load about 100kg), this is not the utilization rate!
- “Average 0.34 is?” I did not understand how you arrive at this value.

Best regards, BIBO