Hello
I hope to get some help with sizing calculations for a glulam beam for an extension of a balcony. I have attempted to calculate a beam section in the balcony between two supports.
Some information and a small image attached below.
https://delafil.se/385ee9736efc07eb/dimensionering_limträbalk_balkong_pa1.pdf
I have some questions that I hope you can help answer.
I hope to get some help with sizing calculations for a glulam beam for an extension of a balcony. I have attempted to calculate a beam section in the balcony between two supports.
Some information and a small image attached below.
- Estimated glulam beam approximately 115x333.
- Max span =5.8m (due to parking + garage doors under the balcony)
- cc_beam =1.5m
- Safety class 2
- Climate class 3
- Design value for the load became 5.5kN/m^2 (gk=0.7kN/m^2 qk=3.5kN/m^2)
- Point load is not determining.
https://delafil.se/385ee9736efc07eb/dimensionering_limträbalk_balkong_pa1.pdf
I have some questions that I hope you can help answer.
- I have assumed climate class 3 for the calculation. The balcony is more than half weather-protected by the house. So is this a required choice? This imposes tough requirements on the sizing. Can I go to climate class 2?
- Have I calculated reasonably? It feels like the size of the glulam beam becomes substantial.
- Is there anything else I need to consider in the sizing?
Your beams are too weak (at least the middle beam). Since it supports a load over a width of 1.5 m, the design load will be 6.3 kN/m. To meet climate class 3 (a reasonable choice), the dimension should be 115x405 mm.
Thank you for your response justusandersson. I have tried to follow the standard for the calculation and would like to understand how you arrive at 6.3kN/m. According to my design, I need to design for 8.25kN/m (5.5kN/m^2*cc_balk = 5.5*1.5m= 8.25kN/m)
Your figure of 6.3kN/m is less than my figure of 8.25kN/m, yet your beam is larger? What I have designed for is the maximum bending moment M. I arrive at my dimension according to the calculation below:
Maximum bending moment M = load*span*span/8 = 8.25*5.8*5.8/8 = 34.96kNm (maximum bending moment for a simply supported beam between two supports)
Design bending strength from GL30c beam f = kmod*f/gamma_m = 0.65*30/1.25 = 15.6 MPa (partial coefficients for design bending strength)
kmod = 0.65 because it is climate class 3 and duration class M for the load case
gamma_m = 1.25 partial coefficient for glulam
Required bending resistance W = M/f = 2.2*10^-3 m^3
The bending resistance for a rectangular beam is W=b*h^2/6, solving for h and inserting W and b=115mm above gives h = 341 mm
Is it possible for you to describe where your numbers come from so I can understand where I might be going wrong in my calculations?
Your figure of 6.3kN/m is less than my figure of 8.25kN/m, yet your beam is larger? What I have designed for is the maximum bending moment M. I arrive at my dimension according to the calculation below:
Maximum bending moment M = load*span*span/8 = 8.25*5.8*5.8/8 = 34.96kNm (maximum bending moment for a simply supported beam between two supports)
Design bending strength from GL30c beam f = kmod*f/gamma_m = 0.65*30/1.25 = 15.6 MPa (partial coefficients for design bending strength)
kmod = 0.65 because it is climate class 3 and duration class M for the load case
gamma_m = 1.25 partial coefficient for glulam
Required bending resistance W = M/f = 2.2*10^-3 m^3
The bending resistance for a rectangular beam is W=b*h^2/6, solving for h and inserting W and b=115mm above gives h = 341 mm
Is it possible for you to describe where your numbers come from so I can understand where I might be going wrong in my calculations?
Best answer
You are designing with respect to the ultimate limit state while I am calculating the serviceability limit state, which is common for building structures such as floors, etc. This also imposes significantly higher demands. One calculates the maximum deflection and ensures it is below 1/300 of the span. For a simply supported beam with a uniformly distributed load, the deflection is 5qL^4/384EI. q is the distributed load per meter, L is the span, E is the modulus of elasticity, and I is the moment of inertia. You can use any units you like as long as it's consistent. E must be chosen with respect to climate class and load duration. Usually, deflection is also checked. A point load consisting of 1 kN placed at the beam's center should not cause a greater immediate deflection than 1.5 mm regardless of the span.
As for the load, you arrived at 5.5 kN/sqm (live load 3.5 and self-weight 0.7). To determine the distributed load on the central beam, you need to multiply by 1.5 to find the correct width of the load area. Here I see I've made a calculation error. 5.5*1.5=8.25 not 6.3. I blame the computer's calculation program. If you don't press AC, old numbers can be included. Embarrassing. 115x405 gives the deflection 1/257 which I think is acceptable in the context. Otherwise, you have to increase to 115x450.
As for the load, you arrived at 5.5 kN/sqm (live load 3.5 and self-weight 0.7). To determine the distributed load on the central beam, you need to multiply by 1.5 to find the correct width of the load area. Here I see I've made a calculation error. 5.5*1.5=8.25 not 6.3. I blame the computer's calculation program. If you don't press AC, old numbers can be included. Embarrassing. 115x405 gives the deflection 1/257 which I think is acceptable in the context. Otherwise, you have to increase to 115x450.
How good!
Thanks for the clarification, then my calculations are quite accurate. I have also calculated the deflection in the service state and can conclude that it probably won't be a major problem even with a slightly smaller beam. I will also make some adjustments regarding the maximum span for the middle beam, and possibly add another post.
I will probably go with a 360 height on the glulam, as I don't want to hit my head when entering through the front door.
Thanks for all the help and support, really appreciated!
Thanks for the clarification, then my calculations are quite accurate. I have also calculated the deflection in the service state and can conclude that it probably won't be a major problem even with a slightly smaller beam. I will also make some adjustments regarding the maximum span for the middle beam, and possibly add another post.
I will probably go with a 360 height on the glulam, as I don't want to hit my head when entering through the front door.
Thanks for all the help and support, really appreciated!