3 Sided Building Addition 4

[BadgerPE]

Hey all,

First off, I apologize for the simple nature of this question. It is something that has had me puzzled for some time now. For an arbitrary 3-sided building addition where the 4th side is made up of the existing building wall, what is the proper way to design the MWFRS for lateral loads in the transverse direction? Do I design the ceiling diaphragm to transfer transverse lateral loads to the collectors and then on to the new shear wall?

 

[MiketheEngineer]

I usually try to:

1. Tie into existing building OR

2. Install adjoining "4th" wall to old wall - now you have two buildings.

Choice #2 might be easier and cheaper because you wouldn't theoretically need to re-analyze the original building. That assumes it will even still engineer out??!! Remember - if you change forces by more than 5% (I think) you need to do a complete re-engineer and possible retro-fit.

Some problems to note - they might bang into each other during a seismic event.

 

[peconsultant]

The geometry of the two buildings will be important in deciding how to design the add-on. If the addd-on is very small in comparison to the existing structure, the new loads may be negligible.

Also, again depending on the geometry of the add-on, it might be possible to resist the transverse wind loads through a couple in the two walls.

If you end up designing the add-on so that it does not transfer load to the existing structure, make sure you check the deflections to make sure no load transfer occurs.

 

[firai]

Make sure that the existing structure and foundation are adequate and that you wouldn't be obligated to bring it up to current code before you tie into it. You also have the option to build a new wall adjacent to the existing one so that you have a complete box (though have a new footing right next to an existing one is tricky). Another possibility might be to make a 3-sided building, but you'll have to make sure that you design for the torsion. In either of the last cases, you'll have to make sure that the deflections (including torsional rotations) are smaller than the structural separation.

Structural Design Engineer
New York, NY

 

[a2mfk]

Are we talking about your average residential addition or enclosing a porch?

If so, I usually design the new or existing roof as a diaphragm to transfer that shear into the existing diaphragm, using light gage flat metal along the valleys as this transfer mechanism.

Its usually for residential a small amount since most additions are in the 20ft range, meaning only an additional 10' of wind shear is put into the existing back wall. Many times for me this is CMU and a non-factor, but the existing roof trusses may need some blocking/bracing to ensure a proper shear transfer mechanism since this is rarely done correctly in the first place...

For bigger buildings there are other options discussed by the other guys that are right on. 3-sided diaphragm if the rigidity of the walls and diaphragm are compatible, small return walls parallel to the existing wall, using a gable truss over the existing walls as a shear wall, or a braced frame (worse case)...