Convince father-in-law about sizing

[olssono]

Hello,

We are building an attefallshus and plan to have a sliding door section 34x21 in one of the facades. I used the Swedish Wood's dimensioning tool and got the following result for the support beam:

Glued laminated timber 115x225 (GL30c), deformation 11 mm (L/324), utilization rate 55%, support reaction 13.133 kN

The house is 4x7.5m, metal roof 27 degrees, snow zone 2.

My father-in-law is helping with the construction and thinks it's completely absurd to use such a large glued laminated beam. He has indeed worked as a carpenter for 35 years and believes a standing top plate of 145 mm is sufficient.

Although he provides many great suggestions and his help is invaluable, I still feel that he's a bit too far from what I’ve learned for me to fully trust him. I would like to avoid a situation where in a few years I encounter problems with the sliding doors because the deflection has become too large.

Is there anyone kind-hearted who can help me with arguments that would resonate with a 70-year-old carpenter with half a lifetime in the industry? Is there perhaps any validity to what he says, could we reduce the dimensions? Maybe screw-glue two 45x220 pieces?

 

[justusandersson]

Assuming that only the outer walls carry the roof (no central wall, in other words) and a rough calculation seems to confirm your assessment. I do think that in this scenario, one would want to further reduce the deflection. The simplest way is to increase the width to 140x225 or 165x225. Increasing the beam height has more effect, but it might be a limiting factor? With a 45x145 beam on edge and a 3.4 m span, there isn't much left of that sliding door section after a winter... Two screw-laminated 45x220 beams are in the right range but also result in unacceptable deflection. Tell your father-in-law that when spans increase, calculations are necessary; intuition isn't enough. However, age and experience are always a plus!

 

[Nyfniken]

Father-in-law probably just needs to understand that the house is yours and not his. It doesn't really matter who is more right.

 

[witten]

There is a difference between holding and holding. Calculation according to building codes (not optional, by the way, but mandatory) takes into account 50-year snow load, a beam with quality in the lower percentile, and a safety factor on that. About 50% on dead weight and a little less on snow load. The end result should be a beam with a probability of failure that is on the order of one in a million during the building's lifetime. Deformations, however, are not calculated with these margins; instead, loads are reduced.

This does not yield the same result as when you design by feel and it holds here and now. I must also say that I have seen quite a few examples of under-dimensioned roofs and floors designed by carpenters, with both deformations and deflection. But many do not reflect that it is under-dimensioning that's at issue.

It may be difficult to get emotional confirmation of this safety thinking in this example, but the same formulas apply to a beam with a 40 m span over a sports hall. Then it preferably shouldn't make your stomach flutter a little.

 

[olssono]

Assuming that only the outer walls carry the roof (meaning no heart wall) and roughly calculated, your calculation seems correct. I think that in this situation, one would want to further reduce the deflection. The simplest way is to increase the width to 140x225 or 165x225. Increasing the beam height has more effect, but it might be a limiting factor? With a 45x145 beam on edge and a 3.4 m span, there won't be much left of that sliding door section after one winter... Two screw-laminated 45x220 are in the right ballpark but also result in unacceptable deflection. Tell your father-in-law that when spans increase, calculations are necessary; gut feelings aren't enough. However, age and experience are always a plus!

Thanks for the quick response! Indeed, no heart wall, just a long ridge beam 115x450 and rafters (though I didn't need to argue the dimensions of that beam with him ).

So it will at least be a 115x225. I could probably fit 270 in height, though. Would that mean I could go down in width to 90x270 maybe? That would give me some extra insulation on a wall that otherwise won't be more than 120+45.

 

[prototypen]

If you reduce the width, you get closer to the point where the beam wants to topple over.
I can't say how big the risk is, but others know.
You can block with the rafters, but it involves some tinkering.

Protte

 

[olssono]

Thank you for all the answers.

It ended up being a 90x270. Didn't tell the svärfar and just bought it and nailed the wall together. Svärfar shook his head a little but said it looked good.

Nailed it into the hammarbandet lying directly on top and secured with 90x90 brackets on the pillars (2 pieces glued together 45x120), thinking it should protect against tipping.

 

[Daniel 109]

Thanks for the quick reply! Indeed, no heart wall, just a long ridge beam 115x450 and rafters

The ridge beam serves the same function.