Steel Beam size vs Wood

[Spicycheesecake]

Hello,

I am looking at installing a much larger secondary beam to support the 20' 2x6 floor joist span at its mid point. Currently we only have a wood 4x4, and after some calculations by a structural engineer friend of mine, he says it should be replaced with a beam of either 3 2x8s, or 5x 2x6s, ideally of douglas fir. Note in the image attached there is a similar beam on the other side of the house but it isnt shown.

The beam would be 20' long, with support on both ends and a telepost in the middle.

Since my basement is only about 6' high I am wondering if there is an option for either replacing the 4x4 with a 4" tall steel beam, or instead drilling and through bolting a C channel beam onto each side of it. I'm also open to other solutions that could retain the existing headroom.

Is anyone able to help with the need steel beam size or spec that would be as strong or stronger than the wood beam options?

 

[Eng16080]

Yup, really it's not so simple as finding a steel beam with equivalent strength to a wood beam. A good engineer would need to understand the big picture of what you're trying to accomplish and then provide their professional guidance in terms of a reasonable solution. Is this a huge amount of effort for an engineer who regularly does this type of work? No, not really. But it's not simple either, and somebody without this experience will likely overlook a lot of the finer details.

For example, how do you connect the wood framing to the steel beam? You can't just nail to it as you can with a wood beam.

I originally didnt think it was that complicated and I'd find it on a table somewhere but it seems to be a pretty tricky question. Maybe I should try going to a place that sells steel beams?

I'm certainly biased, but I would go to a structural engineer first. You can go "to a place that sells steel beams" and I'm sure they'll sell you a steel beam! They're not going to provide you a stamped design though, and the beam may well not be what you really need.

 

[BAretired]

All I need is to know what size 4" steel beam can hold as much as a wood beam. Specifically it needs to meet or exceed 3x 2x8s or 5x 2x6s.

The strength of 3-2x8 D. Fir is 7.64 kN-m and 18.3 kN for Mr and Vr respectively
The strength of 5-2x6 D. Fir is 8.60 kN-m and 27.0 kN for Mr and Vr respectively
The strength of one C100x7 (C4x4.5) channel is 8.10 kN-m and 58.2 kN for Mr and Vr respectively
Mr and Vr means factored moment and shear.

I originally didn't think it was that complicated and I'd find it on a table somewhere but it seems to be a pretty tricky question. Maybe I should try going to a place that sells steel beams?

It isn't complicated. I found it on a table somewhere, in Canadian handbooks, which is why the units are metric. I have not examined or commented on your particular situation, but this is the information which you sought. Now you and your friend can decide what to do.

 

[Eng16080]

The strength of 3-2x8 D. Fir is 7.64 kN-m and 18.3 kN for Mr and Vr respectively
The strength of 5-2x6 D. Fir is 8.60 kN-m and 27.0 kN for Mr and Vr respectively
The strength of one C100x7 (C4x4.5) channel is 8.10 kN-m and 58.2 kN for Mr and Vr respectively
Mr and Vr means factored moment and shear.

Perhaps this is useful information, but for most residential projects I'm involved in, it's usually beam deflection that controls, not bending or shear.

 

[BAretired]

Perhaps this is useful information, but for most residential projects I'm involved in, it's usually beam deflection that controls, not bending or shear.

For 3-2x8, 5-2x6 (No. 1 or 2 grade) and steel C4x5.4 the EI values are 651e9, 478e9 and 354e9 Nmm^2 respectively. That would be 708e9 Nmm^2 for channel pair, one bolted each side of the 4x4.

 

[EngDM]

On another note, using anything more than a 3ply 2x6 as a beam may require additional fastening considerations especially in side load applications to make sure all ply's are engaged. ForteWeb doesn't even give results for 4ply 2x6 due to this; the amount of nails required can't be done with your typical spacing limits. Not saying you can't do it, but it's just another reason why you should get a professional engineer involved.

 

[BAretired]

Two C4x4.5 steel channels are stiffer and stronger than either wood beam recommended by the OP's engineer friend.

While it may be desirable to get another professional engineer involved, it is unlikely to happen unless ordered by the authority having jurisdiction, which is also unlikely. I suggest that the OP and his engineer friend proceed in accordance with their best judgement. Beefing up the 4x4 wood beam does not address other issues such as live load and joist spans.

 

[Canuck65]

"... he says it should be replaced with a beam of either 3 2x8s, or 5x 2x6s, ideally of douglas fir"

not sure why your friend the engineer is telling you that ideally your beam should be douglas fir. For lumber of the dimensions stated above, in Canada, SPF has more bending capacity, i.e. is stronger.

I agree with many of the above comments. This project requires the services of a structural engineer and there are lots around who would take on a residential project like this.

 

[ChorasDen]

not sure why your friend the engineer is telling you that ideally your beam should be douglas fir. For lumber of the dimensions stated above, in Canada, SPF has more bending capacity, i.e. is stronger.

I'm not familiar enough with CSA086 to know the design values offhand, but within the NDS supplement, which is what is used for U.S. codes, this is incorrect. Also, bending is not the only consideration here, Douglas Fir size-for-size is nearly 15% stiffer, so that may be a consideration in the original recommendation.

 

[BridgeSmith]

I'm not familiar enough with CSA086 to know the design values offhand, but within the NDS supplement, which is what is used for U.S. codes, this is incorrect. Also, bending is not the only consideration here, Douglas Fir size-for-size is nearly 15% stiffer, so that may be a consideration in the original recommendation.

Last time I checked the NDS, the only other common species I saw with allowable stresses as high as Douglas Fir is SYP (Southern Yellow Pine)

 

[ChorasDen]

@Canuck65, looking into it more using some design software, it appears you are correct: for Ontario building codes, SPF has a higher bending capacity than Douglas Fir, that's really interesting. I also see that NBCC lumps together visual grades 1 & 2 for both Douglas Fir and Spruce Pine Fir. Are you able to share what grading agency determines the visual grade criteria, and what the design values are?