Load Transfer-Built Up Beam Loaded One Side 1

[4thorns]

Hello All. Hypothetical situation. I have a (4)-2x12 built up wood girder. It's part of a floor system and loaded from one side. All members are (theoretically) identical in size and grade and if properly connected, more than capable of carrying the side load. Joists have adequate connection to outer member (Hangers with appropriate nails) to transfer floor loads into first member of the girder.

Now, if the 4 members are not adequately connected there is going to be a failure. That's a given. Well, it failed.

My first instinct was that the first member is acting in single shear with the second. The second with the third and so on. After further thought this seemed to make sense with nails only penetrating 2 plys at a time. If this is the case, then if the first two member connection fails then the rest of the members seem to be irrelevant.

My questions are;

Is my instinct correct in the case of nails?

How would this situation differ if through bolts penetrating all plys were used instead?

Thank you for your time.
Doug

 

[BAretired]

More nails are needed between the first and second ply because you are transferring 3/4 of the load in single shear. Between second and third, you are transferring 1/2 of the load and between third and fourth, only 1/4 of the load.

If through bolts are used, they need to be designed for the worst case in single shear, namely 3/4 of the load. They would be loafing between the other plies.

You could use through bolts for 1/2 the load and add nails between first and second plies for 1/4 load.

BA

 

[4thorns]

Thanks BA. Understanding how the loads are shared makes all the difference. I posted some time ago inquiring about how stacked beams behave. Didn't make the connection concerning load sharing in this situation. You pointed it out quite well. (I did recognize the shear so you have to give me a cookie for that!)

Thanks again,
Doug

 

[BAretired]

One cookie for you.

BA

 

[woodman88]

Be careful of mixed fastener connections.

My 2001 NDS (I do not have the later ones here at home)section 10.1.4 states "Methods of analysis and test data for establishing design values for connections made with more than one type of fastener have not been developed. Design values for mixed fastener connections shall be based on tests or other analysis (see 1.1.1.3)"

Otherwise, I agree with BA.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[BAretired]

Thanks Garth, I wasn't aware of that.

BA

 

[XR250]

You also have to worry about the eccentricity of the load on the 6" wide member. I typically put straps under the joists and nailed to all plys of the beam to resist the torsion on 4-ply members.