masonry diaphragm aspect ratio

[broncosfan]

We are adding a 90' long x 25' wide connector between two warehouses. There will be two 90' long, 12" thick reinforced masonry walls. The roof will consist of 25' long joists spaced at 6' on center with 18ga type B metal roof deck. The 18ga metal roof deck will be tied into the precast concrete shear walls of the existing warehouses.

I have a 90'x25' metal deck diaphragm which works for strength and deflection (36/7 puddle welds with #10 sidelaps at 12"). This diaphragm length-to-width ratio exceeds IBC table 2109.2.1.2 which states the maximum length to width ratio is 2:1 between masonry shear walls. BUT, my connector building is 90' between PRECAST CONCRETE SHEAR walls (we are not adding 25' long masonry shear walls at each end). So, am I ok here? Am I missing something? Thanks for the input.

 

[sundale]

Your situation is certainly open to interpretation with shear walls of two different materials like that. I would suspect that the intent of the IBC maximum allowable diaphragm aspect ratios is to limit out of plane wall forces and effects due to large diaphragm deflections and perhaps chord forces. Personally, I would satisfy the letter of the code requirements with an intermediate 25' shear wall or provide pc or tilt up wall (non-cmu) wall system on your 90' sides.

Are you going to allow thermal movement between the infill structure and the existing buildings? Given the potential for an expansion joint and having to analysis existing structures for new lateral loads and detailing requirements, you may wish to consider providing a completely separate lateral system for your infill building.

 

[JAE]

With metal deck (per IBC Table 2109.2.1.3) I would suspect that you still must maintain the 2:1 ratio regardless of what sort of orthogonal shearwalls you have.

Since you violate the 2:1 with a 90:25 roof, I would think you simply have to leave 2109 (Empirical Design) and use 2107 or 2108.

 

[broncosfan]

Thanks. I hear what you are saying about out of plane forces due to large diaphragm deflections. But I calculated a maximum diaphragm deflection of 0.164" at the midpoint (45' from end). This seems very small to me. The owner wants room to operate forklifts thru the connector, so adding a shear wall at midpoint is not going to work. I thought of adding a rigid steel frame at the midpoint, but feel that it might not do anything, since it would be so flexible compared to the masonry walls. Masonry walls are being pursued because they would be much cheaper than precast or tilt wall in this situation. Not sure if I can help the thermal expansion issues in this case. I guess I'd rather tie everything together for stability. The building is 90' x 25' x 35' tall.

 

[bjb]

by tieing in to the existing lateral structure, are the increased forces below the limits of 1614.1.1 of the IBC?

If the above is a problem, could you provide isolation joints at each end of your connector, and provide a series of steel portal frames along the length of the connector?

 

[haynewp]

Why are you looking at emperical design?

 

[JAE]

Agree with haynewp - that's my point above - get out of 2109 and simply design your fairly straightforward walls with 2107 or 2108.