Discontinuous Loading on Second Floor Beam

[socaljoe13]

I'm calculating discontinuous loads on an 18' wood beam for a story story single family dwelling. The loading includes roof and ceiling loads, exterior wall over the beam and second floor joist loads parallel to the beam. And the beam also is carrying two points loads from the ridge above (D + Lr).

I understand the application of special load combo 1.2D + 0.5D + 2.5E (for flexible diaphragm) for loads imposed on the beam.

My question: Should I include the roof and ceiling loads in addition to the second floor and point load when applying SPLC over the beam?

Thanks

 

[JAE]

Usually the special overstrength load combinations don't apply to the basic design of a beam.

Which code are you working under?

Should that have been 0.5L and not o.5D?

 

[XR250]

FWIW, deflection will likely control this design.

 

[socaljoe13]

Thanks for the response. And yes....it is 0.5L (typo). Sorry about the confusion.

I'm working under the 2013 CBC and ASCE 7-10.

I've already calc'd out the basic design of the beam 1. Now I having to check it as an element supporting a discontinuous wall Section 12.3.3.3 thus having to apply the load combination with overstrength factor per Section 12.4.3.2. I'm applying 1.2D + 0.5L + 2.5Em.

I applied all dead and live loads from the roof and second floor in the basic design of the beam 1. Now when checking for overstrength load combination, should I include the roof/ceiling dead and live loads in addition to the second floor dead and live loads + 2.5 Em (load shear wall end)??

Secondly, I know to design beam 1 and connection at end of shear wall above per the overstrength load Omega x 2.5. Do need to apply the resulting reaction load of beam 1 to beam 2, beam 3...etc all the way down to the foundation??

Your advice is greatly appreciated.

 

[jdgengineer]

You should apply all of the loads on the beam and sort them out with the necessary load combinations. Assuming you are using ASD combinations you have to check both the case of full seismic without live load and reduced seismic with reduced floor and roof live load. Therefore you would need to include the roof dead and live loads.

Couple of things to note...

1) If the ceiling live load is only the 10 PSF live load (no storage) the building code does not require that you assume this is concurrent with other live loads.
2) If you are using ASD you are allowed to increase your allowable stresses by 1.2 when using overstrengh combinations.
3) You should trace the overstrength combination loads all the wall down to complete the load path. Whether you have to design the foundation with over strength is open to interpretation.
4) As Excel Engineering mentioned your beam will likely be deflection governed if you follow the code requirements. Technically, you should be determining the angle of rotation of the beam at the shearwall and adding this to the overall drift of the building (similar to holdown slip). However, I suspect most residential engineers do not do this.

 

[socaljoe13]

JD. Thanks for the clarification. All makes sense and will implement as mentioned.

The only one that seems a little vague to me is the transmission of overstrength loads down to foundation for all members associated with beam 1. Section 12.3.3.3 mentions 'design of the connections of such discontinuous elements (hold down) to the supporting members (beam 1)'. When designing for maximum axial force on the supporting member, I'm designing for positive and negative point load result E x 2.5 (in addition to D+L) thus checking deflection of beam and uplift capacity of hold down.

I'm not arguing code requirement but it seems a bit excessive if I'm having to trace and design (positive and negative forces) for all other members and their respective connections (beams, hangers, column caps, posts, foundation, etc) associated with beam 1. Especially in residential construction. I was interpreting Section 12.3.3.3 as for that particular member and it's connection. The code should be a bit more clear on its requirement.

Thank you.