Beam Strength

[Brudaniell]

I have a 16' wooden beam across the carport...that was 4' short of the opening, so the contractor put another support post to extend the beam.

I want to remove post after I sandwich the 4.75" wide beam with two 8' long steel plates. They would be attached by pass through bolts in the center, and wood bolts along the perimeter.  

 

Does this appear to be well engineered?  Do I need 1/4" steel plate or will 3/16" suffice?

 

 

[dimreepr]

If the steel plates are straight/flat I don't think it's strong enough, try using angle iron bolted every 6" and supported at one end.

[studiot]

22 minutes ago, Brudaniell said:

I have a 16' wooden beam across the carport...that was 4' short of the opening, so the contractor put another support post to extend the beam.

 

I want to remove post after I sandwich the 4.75" wide beam with two 8' long steel plates. They would be attached by pass through bolts in the center, and wood bolts along the perimeter.  

 

 

Does this appear to be well engineered?  Do I need 1/4" steel plate or will 3/16" suffice?

 

 

Kinda depends upon the load the beam is carrying !

The basic idea seems good, I assume the 4ft sidespan is also currently timber of the same size so you can have two 4ft connections.

This would introduce continuity to allow removal of the post.

I assume you mean coach screws by "wood bolts" so that a positive fixing into the timber is achieved.
Relying on friction induced by clamping pressure is less reliable than a positive fixing.
There should be a large number of these to distribute the load between the weaker and stronger materials and they should be arranged in a series of vertical lines to transfer the shear.
This is very important.
 

You haven't told us the depth of the beam so I can't say  how much plate you would need since it is the depth of the plate that carries the shear (acts as the web).
The flitch construction would be sound against web buckling of the plate.

[dimreepr]

 

2 hours ago, studiot said:

Kinda depends upon the load the beam is carrying !

1

 

You have to assume snow loading and over a 20' span that would be substantial, so too the sheer, but I agree the OP needs to provide much more detail.

3 hours ago, Brudaniell said:

I have a 16' wooden beam across the carport...that was 4' short of the opening, so the contractor put another support post to extend the beam.

I want to remove post after I sandwich the 4.75" wide beam with two 8' long steel plates. They would be attached by pass through bolts in the center, and wood bolts along the perimeter.  Does this appear to be well engineered?  Do I need 1/4" steel plate or will 3/16" suffice?

2

Whatever you do, to repair, it will never be as strong as a 20' beam.

Edited June 28, 2018 by dimreepr

[studiot]

45 minutes ago, dimreepr said:

 

 

You have to assume snow loading and over a 20' span that would be substantial, so too the sheer, but I agree the OP needs to provide much more detail.

Whatever you do, to repair, it will never be as strong as a 20' beam.

Whilst I expect the shear to be the main issue, going from a 16 ft beam to a 20 ft beam will increase the maximum moment, so the ability of the existing beam to resist this would need checking. Otherwise it could need reinforcing along a greater length than 4 ft.

Edited June 28, 2018 by studiot

[dimreepr]

2 minutes ago, studiot said:

Whilst I expect the shear to be the main issue, going from a 16 ft beam to a 20 ft beam will increase the maximum moment, so the ability of the existing beam to resist this would need checking.

No doubt why the contractor chose to install an additional support. 

[studiot]

41 minutes ago, dimreepr said:

No doubt why the contractor chose to install an additional support. 

Well the new maximum moment will be 56% greater than the old and still occur within the old unstrengthened timber section.

[Bender]

- Shear is negligible for such a beam. Only the moment matters. (Failure modes in wood can be complicated and can be caused by shear stress, but this stress is indirectly caused by the moment and not the shear force)

- The new structure could very well be as strong as one with a single wooden beam, because steel is much stronger than wood. Depends on thicknesses.

- relying on friction is quite common and reliable and has the additional advantage of less stress concentrations. 

[studiot]

19 minutes ago, Bender said:

1)- Shear is negligible for such a beam. Only the moment matters. (Failure modes in wood can be complicated and can be caused by shear stress, but this stress is indirectly caused by the moment and not the shear force)

2)- The new structure could very well be as strong as one with a single wooden beam, because steel is much stronger than wood. Depends on thicknesses.

3)- relying on friction is quite common and reliable and has the additional advantage of less stress concentrations. 

1)

There is currently zero moment at the post to be removed. 
But there is substantial shear being transferred. (otherwise no post would be needed)
Making the beam continuous exchanges this shear for increased moment.
But at the expense of increased shear in the joints.
Stress can only be transferred from the wood to the steel by shear connectors.

So shear considerations are vital.

2)

Yes of course it could be designed so. We are all agreed on that. But certain parts of the existing beam, notably at 10 feet from the end, will be subject to a greater moment than before.

3)

Relying on friction in properly controlled conditions yes. And with proper subsequent inspections to confirm that no relaxation has taken place.

Relying on unknown workmanship and subsequent regimes is questionable.

Edited June 28, 2018 by studiot

[Bender]

1 hour ago, studiot said:

There is currently zero moment at the post to be removed. 

Which is why it is useless to consider that part of the beam in a strength calculation, as it won't fail there.

Moment will be biggest in the middle (since we don't have info on the load, I assume uniformly distributed load), and that is where such a beam fails. 

But the load situation considered here is with the post removed. In that case there is definitely a moment to be transmitted at that location. It is true that this position is not critical, so the connection can be considerably weaker than the beam without failing. I would strive to make it equally strong anyway to be on the safe side.

Shear stresses are required to transmit the moment (and negligible shear force) from one part to the other, but overall in a beam which is more than 5+ times as long as it is high, nobody calculates shear forces.

[studiot]

1 hour ago, Bender said:

Which is why it is useless to consider that part of the beam in a strength calculation, as it won't fail there.

Moment will be biggest in the middle (since we don't have info on the load, I assume uniformly distributed load), and that is where such a beam fails. 

But the load situation considered here is with the post removed. In that case there is definitely a moment to be transmitted at that location. It is true that this position is not critical, so the connection can be considerably weaker than the beam without failing. I would strive to make it equally strong anyway to be on the safe side.

Shear stresses are required to transmit the moment (and negligible shear force) from one part to the other, but overall in a beam which is more than 5+ times as long as it is high, nobody calculates shear forces.

 

I wonder if you have, perhaps, misunderstood the original description.

 

There are two beams not one.

 

These two have a common bearing at the intermediate post and probably some loose connection.

 

There is therefore maximum shear acting at this post, on both these beams which should be considered as simply supported.

 

Do you agree with this?

 

 

8 hours ago, Brudaniell said:

I have a 16' wooden beam across the carport...that was 4' short of the opening, so the contractor put another support post to extend the beam.

I do wonder what this piece of timber is actually supposed to support. Perhaps only itself?

 

 

 

 

Edited June 28, 2018 by studiot

[J.C.MacSwell]

On thing to consider is run out of the reinforcement, It might not be the point of maximum moment but it would still be higher than the maximum in the original case, and the bolts would add stress rises in that area from both the bolt holes and loads transferred from the steel reinforcing through the bolts themselves. It might be better to reinforce a fair bit past the centre point rather than stop short.

Edited June 29, 2018 by J.C.MacSwell

[Bender]

7 hours ago, studiot said:

 

I wonder if you have, perhaps, misunderstood the original description.

 

There are two beams not one.

 

These two have a common bearing at the intermediate post and probably some loose connection.

 

There is therefore maximum shear acting at this post, on both these beams which should be considered as simply supported.

 

Do you agree with this?

 

 

I do wonder what this piece of timber is actually supposed to support. Perhaps only itself?

 

 

 

 

How I understood it, he now has two beams with 16' and 4' span and a post in between. He wants to connect the two and remove the post to get a single beam with 20' span.

Edit: I suppose the beams carry crossbeams which support the roof.

4 hours ago, J.C.MacSwell said:

On thing to consider is run out of the reinforcement, It might not be the point of maximum moment but it would still be higher than the maximum in the original case, and the bolts would add stress rises in that area from both the bolt holes and loads transferred from the steel reinforcing through the bolts themselves. It might be better to reinforce a fair bit past the centre point rather than stop short.

Indeed an important point to consider if the original 16' beam was not dimensioned for a 20' span.

[studiot]

8 hours ago, J.C.MacSwell said:

On thing to consider is run out of the reinforcement, It might not be the point of maximum moment but it would still be higher than the maximum in the original case, and the bolts would add stress rises in that area from both the bolt holes and loads transferred from the steel reinforcing through the bolts themselves. It might be better to reinforce a fair bit past the centre point rather than stop short.

 

Not sure if you are referring to the proposed plates as reinforcing?

 

4 hours ago, Bender said:

How I understood it, he now has two beams with 16' and 4' span and a post in between. He wants to connect the two and remove the post to get a single beam with 20' span.

Edit: I suppose the beams carry crossbeams which support the roof.

Indeed an important point to consider if the original 16' beam was not dimensioned for a 20' span.

 

Pretty much as I see it, with the addition that the timber transfers the shear load (sorry to keep mentioning that) to the supports and will continue to do so.

Thus the cross connections to the plates must transfer that shear, in turn, to the plates.

[J.C.MacSwell]

1 hour ago, studiot said:

 

Not sure if you are referring to the proposed plates as reinforcing?

 

 

Pretty much as I see it, with the addition that the timber transfers the shear load (sorry to keep mentioning that) to the supports and will continue to do so.

Thus the cross connections to the plates must transfer that shear, in turn, to the plates.

Yes,  basing it on the plates as roughly described, and 8 feet long, 4 feet either side of the joint (a different placement is really no better in this case, 8 feet doesn't lend itself well to better placement)

To the bold, I think this is where local shear stresses would be considered. Otherwise the point of maximum shear loading is at the ends and equal to half the weight of the total load if evenly distributed, and reduces toward the centre. It is usually not a concern for constant section beams as by the time you cover the maximum bending it is more than covered....but good to be cautious when looking  at an atypical situation like this...adding a joint without support directly underneath.

As it is we don't  even know the height of the cross section of the beam, so we can't even guess what reasonable loading might be expected if currently well constructed.

[studiot]

8 hours ago, J.C.MacSwell said:

Yes,  basing it on the plates as roughly described, and 8 feet long, 4 feet either side of the joint (a different placement is really no better in this case, 8 feet doesn't lend itself well to better placement)

To the bold, I think this is where local shear stresses would be considered. Otherwise the point of maximum shear loading is at the ends and equal to half the weight of the total load if evenly distributed, and reduces toward the centre. It is usually not a concern for constant section beams as by the time you cover the maximum bending it is more than covered....but good to be cautious when looking  at an atypical situation like this...adding a joint without support directly underneath.

As it is we don't  even know the height of the cross section of the beam, so we can't even guess what reasonable loading might be expected if currently well constructed.

Yes I think we are all agreed on these good points, which have been mentioned at various stages in the discussion.

To facilitate discussion I have sketched the situation show clearly that the existing timber would be subject to a significant increase in stress.

This must occur in the existing timber alone as it is beyond the 'reinforced' part.

One question occurs to me.

Carports and garages are usually longer than they are wide so why was there only one extra support post to remove.

What happens at the back end? and what about the middle?