S
schornstein21 Apr 2010 14:52Hello everyone,
I hope someone can answer my question.
In the building and performance specification for the roof structure, it was stated that the structural calculation of the purlin roof would be based on a snow load of 0.75 kN/m² (15.7 psf) and wind zone II.
Now we have received the calculation, which considers a snow load of 0.65 kN/m² (13.6 psf) and wind zone I.
We are located in the Karlsruhe area.
How will this difference affect our roof?
I hope someone can answer my question.
In the building and performance specification for the roof structure, it was stated that the structural calculation of the purlin roof would be based on a snow load of 0.75 kN/m² (15.7 psf) and wind zone II.
Now we have received the calculation, which considers a snow load of 0.65 kN/m² (13.6 psf) and wind zone I.
We are located 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 base value for the snow load of sk = 0.65 kN/m² (14 psf) is correctly applied for the Karlsruhe area correct, so I wouldn’t worry about that.
Whether a difference of 0.75 - 0.65 = 0.1 kN/m² (2 psf) has an impact depends primarily on the calculated rafter cross-section b/h (width/height) and the chosen wood grading.
The design of timber components is regulated in DIN 1052. There is basically a distinction between two types of verification:
1. Load-bearing capacity verification:
This verifies the failure of the component, meaning the numerical check whether the timber cross-section b/h can withstand effects such as self-weight, wind, snow, and other so-called live loads without damage.
2. Serviceability verification:
This verifies the deformation (deflection) of a component against a permissible maximum limit. (This is more about appearance; even if this verification is not met, component failure is not expected.)
Now, regarding the actual question of whether this can affect the roof:
This depends on the utilization factor of the selected rafter cross-section!
The utilization factor is the ratio of the acting loads divided by the resisting capacity.
Example of load-bearing capacity verification:
Utilization = actual stress (from loads) / allowable stress (based on wood grading)
Example interpretation of the verification result:
Utilization = 1.3 means the cross-section cannot carry the load, i.e., it is undersized (cross-section damage like breakage is possible)!!
Utilization = 1 means the cross-section has reached its full load-bearing capacity
Utilization = 0.5 means the cross-section is only half utilized, i.e., it is oversized!
If the structural engineer assumes 0.65 kN/m² (14 psf) and selects a rafter cross-section b/h below the limit utilization factor of 1.0, e.g., 0.9, then assuming sk = 0.75 kN/m² (16 psf) will not lead to a significantly different result. The utilization factor would then be somewhat higher (perhaps 0.95). But this also depends on other boundary conditions of the structural system, such as support spans and rafter spacing.
So, as already mentioned, it all depends on which cross-section b/h with what wood grading (grade, species) is used and how conservative the structural engineer was in the design according to DIN 1052 or whether some reserves were allowed.
I hope this helps, best regards BIBO
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 base value for the snow load of sk = 0.65 kN/m² (14 psf) is correctly applied for the Karlsruhe area correct, so I wouldn’t worry about that.
Whether a difference of 0.75 - 0.65 = 0.1 kN/m² (2 psf) has an impact depends primarily on the calculated rafter cross-section b/h (width/height) and the chosen wood grading.
The design of timber components is regulated in DIN 1052. There is basically a distinction between two types of verification:
1. Load-bearing capacity verification:
This verifies the failure of the component, meaning the numerical check whether the timber cross-section b/h can withstand effects such as self-weight, wind, snow, and other so-called live loads without damage.
2. Serviceability verification:
This verifies the deformation (deflection) of a component against a permissible maximum limit. (This is more about appearance; even if this verification is not met, component failure is not expected.)
Now, regarding the actual question of whether this can affect the roof:
This depends on the utilization factor of the selected rafter cross-section!
The utilization factor is the ratio of the acting loads divided by the resisting capacity.
Example of load-bearing capacity verification:
Utilization = actual stress (from loads) / allowable stress (based on wood grading)
Example interpretation of the verification result:
Utilization = 1.3 means the cross-section cannot carry the load, i.e., it is undersized (cross-section damage like breakage is possible)!!
Utilization = 1 means the cross-section has reached its full load-bearing capacity
Utilization = 0.5 means the cross-section is only half utilized, i.e., it is oversized!
If the structural engineer assumes 0.65 kN/m² (14 psf) and selects a rafter cross-section b/h below the limit utilization factor of 1.0, e.g., 0.9, then assuming sk = 0.75 kN/m² (16 psf) will not lead to a significantly different result. The utilization factor would then be somewhat higher (perhaps 0.95). But this also depends on other boundary conditions of the structural system, such as support spans and rafter spacing.
So, as already mentioned, it all depends on which cross-section b/h with what wood grading (grade, species) is used and how conservative the structural engineer was in the design according to DIN 1052 or whether some reserves were allowed.
I hope this helps, best regards BIBO
S
schornstein3 May 2010 22:45Thank you very much, BIBO.
You were very helpful.
I need to work through all the new information first. Our rafters are 8 cm (3 inches) wide and 24 cm (9.5 inches) high. The timber grade is C24 Nk1. Does that mean the average is 0.34? I couldn’t find the utilization ratio; the report only shows abbreviations. I assume that P = 1.00 kN (225 lbf) is this utilization ratio? Or am I mistaken?
In any case, I am relieved.
Best regards, Schornstein
You were very helpful.
I need to work through all the new information first. Our rafters are 8 cm (3 inches) wide and 24 cm (9.5 inches) high. The timber grade is C24 Nk1. Does that mean the average is 0.34? I couldn’t find the utilization ratio; the report only shows abbreviations. I assume that P = 1.00 kN (225 lbf) is this utilization ratio? Or am I mistaken?
In any case, I am relieved.
Best regards, Schornstein
Hello Schornstein,
you are welcome to send me the structural calculations via private message, and I will try to interpret the numbers.
The rafters with dimensions 8cm x 24cm (3 inches x 9.5 inches) are quite solid, especially since the insulation can still be well accommodated between the rafters at a height of 24cm (9.5 inches).
It is also important to know the rafter spacing, is it the usual 80cm (31.5 inches)?
How far apart are the supports (where the rafter rests), and what is the roof pitch in degrees?
- About the limit utilization ratio:
Here is a simple example:
1. Stress [kN/cm²] = Force [kN] / Area [cm²] (maybe you remember from school)
Calculate the existing stress (assuming a contact area for compression of 8cm x 12cm (3 inches x 4.7 inches)):
2. Existing stress = 12 kN / (8cm * 12cm) = 0.125 kN/cm²
Permissible stress according to DIN 1052 for softwood NH C24 (compression perpendicular to grain):
3. Permissible stress = 0.154 kN/cm²
Proof of compliance (both verification methods (a/b) are possible):
4a. Existing stress < Permissible stress
0.125 kN/cm² < 0.154 kN/cm² (proof is fulfilled)
4b. Existing stress / Permissible stress <= 1.0
0.125 kN/cm² / 0.154 kN/cm² = 0.812 <= 1.0 [-] (proof is fulfilled)
From check 4a it is clear that the stress verification is met, but how much is the cross-section utilized?
This is much clearer from check 4b, as the utilization factor can be read directly:
Utilization = 0.812 [-] * 100% = 81.2%
The cross-section is utilized at 81.2%, so there is still a reserve of 18.8%.
There are other verifications that must be performed, but the most unfavorable verification (highest utilization) determines the limit utilization ratio.
- P = 1.00 kN could be the repair load (manload about 100 kg), but it is not the utilization ratio!
- I did not understand how you calculated the “average 0.34”.
Best regards, Bibo
you are welcome to send me the structural calculations via private message, and I will try to interpret the numbers.
The rafters with dimensions 8cm x 24cm (3 inches x 9.5 inches) are quite solid, especially since the insulation can still be well accommodated between the rafters at a height of 24cm (9.5 inches).
It is also important to know the rafter spacing, is it the usual 80cm (31.5 inches)?
How far apart are the supports (where the rafter rests), and what is the roof pitch in degrees?
- About the limit utilization ratio:
Here is a simple example:
1. Stress [kN/cm²] = Force [kN] / Area [cm²] (maybe you remember from school)
Calculate the existing stress (assuming a contact area for compression of 8cm x 12cm (3 inches x 4.7 inches)):
2. Existing stress = 12 kN / (8cm * 12cm) = 0.125 kN/cm²
Permissible stress according to DIN 1052 for softwood NH C24 (compression perpendicular to grain):
3. Permissible stress = 0.154 kN/cm²
Proof of compliance (both verification methods (a/b) are possible):
4a. Existing stress < Permissible stress
0.125 kN/cm² < 0.154 kN/cm² (proof is fulfilled)
4b. Existing stress / Permissible stress <= 1.0
0.125 kN/cm² / 0.154 kN/cm² = 0.812 <= 1.0 [-] (proof is fulfilled)
From check 4a it is clear that the stress verification is met, but how much is the cross-section utilized?
This is much clearer from check 4b, as the utilization factor can be read directly:
Utilization = 0.812 [-] * 100% = 81.2%
The cross-section is utilized at 81.2%, so there is still a reserve of 18.8%.
There are other verifications that must be performed, but the most unfavorable verification (highest utilization) determines the limit utilization ratio.
- P = 1.00 kN could be the repair load (manload about 100 kg), but it is not the utilization ratio!
- I did not understand how you calculated the “average 0.34”.
Best regards, Bibo
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