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4 replies
6k views
4 replies
Dimensioning of beam in 1920s wooden house
I am about to replace a steel beam in my 1920s wooden house.
The beam is used to support a wall that was removed between the living room and the glass veranda on the first floor.
The house is 157m2 and has 2 floors with a mansard roof. All the walls in the entire house are made of construction planks 65*175, and 600CC studs with the dimension 75*210. The current beam (I don't know if it's H or I) is 5.5m long, with the dimensions 100*100*5.
I now want to extend the span that the beam supports to about 6.5m. I have been offered to buy a second-hand H-beam with the dimensions 120*120*5 which is sufficiently long, about 7m.
Do you think this extra length on the beam requires larger dimensions, or will this 120 beam suffice?
Also, can someone explain the difference between H and I beams?
Best regards,
D
The beam is used to support a wall that was removed between the living room and the glass veranda on the first floor.
The house is 157m2 and has 2 floors with a mansard roof. All the walls in the entire house are made of construction planks 65*175, and 600CC studs with the dimension 75*210. The current beam (I don't know if it's H or I) is 5.5m long, with the dimensions 100*100*5.
I now want to extend the span that the beam supports to about 6.5m. I have been offered to buy a second-hand H-beam with the dimensions 120*120*5 which is sufficiently long, about 7m.
Do you think this extra length on the beam requires larger dimensions, or will this 120 beam suffice?
Also, can someone explain the difference between H and I beams?
Best regards,
D
The beam you have is likely an HE100A with a bending resistance (Wx) of 73 cm3. The proposed beam is likely an HE120A with a bending resistance (Wx) of 106 cm3.
The latter beam is therefore approximately 45% stronger against deflection than the former, but that doesn't really say anything.
Furthermore, you probably cannot safely determine the steel quality the proposed beam is made of. If it is an older used beam you bought from the scrapyard, it likely has an SIS number (4 or 6 digits) that can sometimes be read stamped in the web (depending on which end it is cut, the standard length is 12 m). If it is a newly manufactured beam, bought new or used from the scrapyard, it has a number containing both letters and numbers in a long sequence. The quality, in turn, determines how much stress the steel can withstand/cm2.
By multiplying Wx with the allowed stress, you obtain the maximum bending moment (Mmax) that the beam can withstand.
To know which dimension is required, you must know the length between the supports and the weight that will rest on the beam (or the loading area x weight/m2). Then you can calculate how large the maximum bending moment will be using the formula: Mmax=q x L2/8. This Mmax must be less than the former for it to hold.
All other approaches involve taking a gamble.
The latter beam is therefore approximately 45% stronger against deflection than the former, but that doesn't really say anything.
Furthermore, you probably cannot safely determine the steel quality the proposed beam is made of. If it is an older used beam you bought from the scrapyard, it likely has an SIS number (4 or 6 digits) that can sometimes be read stamped in the web (depending on which end it is cut, the standard length is 12 m). If it is a newly manufactured beam, bought new or used from the scrapyard, it has a number containing both letters and numbers in a long sequence. The quality, in turn, determines how much stress the steel can withstand/cm2.
By multiplying Wx with the allowed stress, you obtain the maximum bending moment (Mmax) that the beam can withstand.
To know which dimension is required, you must know the length between the supports and the weight that will rest on the beam (or the loading area x weight/m2). Then you can calculate how large the maximum bending moment will be using the formula: Mmax=q x L2/8. This Mmax must be less than the former for it to hold.
All other approaches involve taking a gamble.
Absolutely correct. If we are talking about the same dimension, that is.anaitis said:
But here they went up to a larger dimension, and then it is not certain that the deflection increases even if the length does.
And as usual, the deflection should not exceed 1/400 of the span in long-term load. Therefore, one typically calculates the strength for the moment first, and then checks how large the deflection will be for that dimension under long-term load. If it becomes larger than 1/400 of the span, one should increase the dimension by one or two steps or choose better material quality as an alternative.
But that's not what the thread creator asked about. My answer merely pointed out the path he must take to ensure it won't collapse in the first place.
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