Options for 1.5" deep beam?

[Redacted]

Hi there,

I'm working on a project where a wall will be coming out and a new 6' doorway will be put in. The architect will be bringing up the floor level, which reduces the space to add a beam to 1.5". The beam is lightly loaded and is just holding up the roof. However it also needs to hold down the roof in heavy winds. I've never had to design a beam this shallow before so I would appreciate any advice on how to go about doing this correctly.

As there is only 1.5" of depth allowance for the beam, I'm thinking that the only main elements that I can consider are :

1. Flat plate
2. Steel angle (although would I have issues with torsion with this element).

I tried to sketch my thoughts below.

The timber wall plate is only 4", so I'm thinking that the width of the flat plate would only need to be 4" as well. However, my concern is that this would also need to hold down the roof in high winds and I'm wondering if this element would cause the roof to be bouncy, as it doesn't have much rigidity? The wall that the steel beam would be bearing on would be made of filled CMU block.

Note that the opening is only ~6'.

Is there a better approach that I can and should take for this?

Any help would be greatly appreciated.

 

[phamENG]

I'd go for shoring the roof, cutting out the existing plate and flush cutting the rafters on either side and slipping a flush beam into the gap. Refasten rafters to the beam, tie the beam down. Strap along the beam into the plate on each end if needed for in plane continuity.

 

[SlideRuleEra]

...space to add a beam to 1.5".

...I would appreciate any advice on how to go about doing this correctly.

I'm wondering if this element would cause the roof to be bouncy, as it doesn't have much rigidity?

Note that the opening is only ~6'.

Don't bother trying to design a 1.5" deep simple beam for a six foot span...

Span-to-depth ratio = 72" span / 1.5" deep = 48:1

The beam will be "bouncy"... it's just physics (beam deflection based on its' minimal moment of inertia). "Clever" design of a reasonably shaped beam that shallow will not resolve the bouncy issue.

A composite (total thickness >> 1.5") beam with steel plate (or perhaps a 1.5" thick HSS) plus the timber wall plate may have a chance at working properly.

 

[le99]

I would design a steel plate composite with the timber wall plate to help resist the tensile stress. The trick is how to make the connection to ensure composite action.

 

[LittleInch]

A 4" wide inverted channel?

And bolt through to the wall plate?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BridgeSmith]

1.5" x 4" steel structural tube (HSS)? Wall thickness depends on the load.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[haynewp]

It’s guessing without knowing the actual loads.

 

[Redacted]

Thanks for the responses so far everyone! They are very helpful.

For the comments regarding creating a composite section with the timber wall plate : This may be possible, however the timber wall plate is only 1.75" thick. The max depth of composite section I can get would be 1.75"+1.5"(beam clearance) = 3.25", which may still be a bit too shallow for the deflection criteria. Although I will check this.

@phamEng that is an interesting concept and may be the way to go. The existing rafters are 2x6's; if I can somehow cut/remove the wall plate and cut the rafters flush and place a steel beam where the rafters are and attach the rafters to the side of the beam, that would give me an additional 6" to play with. Total space would be 9.25", which would allow a proper 6" or 8" W beam to be installed. However, how would the connection of the rafters to the steel beam look? The rafters are pitched.

Would it be something like this but with the rafters notched to allow for the pitch? Or is there a typical type of detail for pitched rafters to steel beam connections?

You also mention that the steel beam can be strapped to the existing adjacent wall plate. What would that look like?

@haynewp this may be the case however this is a small residential property; the loads will not be large (it's only supporting a roof), I reckon the deflection criteria will govern but I'll check the loads tomorrow.

 

[phamENG]

What kind of trib width? At 6', a multi-ply sawn lumber beam would probably work. If it doesn't, ripping down an LVL should give plenty of capacity.

If you have to go with steel, the detailing is a little harder. But I'd bolt lumber to the web and use a twist strap between it and the top plate of there's room.