Seismic and Wind Design Loads on Rooftop Modular Unit

[P1ENG]

I have been asked to provide stamped calculations for a 10x40 (dimensions simplified for example) modular building that will be sat on top a 10-story building in LA county, CA. I don't know anything about the supporting building other than it is 100' to the roof line. Now, I am questioning my method for vertical distribution of seismic forces and calculation of wind pressures. The design is based on ASCE 7-05.

SEISMIC
Since I don't know the mass, period, or natural frequency of the supporting building I am assuming that the unit is located at grade. This is the same as thinking the supporting building has infinite mass and/or rigidity (as in: the earth) compared to my 10x40 building. I came up with this assumption after considering both methods in the attachment "Seismic Comparison". The highlighted information shows the changes between the two assumptions. (Ignore the overturning moments).

WIND
I am using Figure 6-6 for MWF and Figure 6-17 for C&C. C&C loads are straight forward. However, the MWF calculations brought up some questions. For the walls, I am conservatively using qz at the modular unit's roof elevation (110') and multiplying by G and Cp. Since MWF won't control over C&C, I am ignoring internal pressures since I am only concerned with the net lateral pressure on the walls. Now for my uncertainty. For the roof, I am assuming the value "h" is relative to grade and therefore is 110'. Also, "L" is a dimension of the supporting structure. Since "L" is unknown, I have to consider h/L < 0.5 and h/L > 1.0 (the roof of the modular has low roof pitch). Also, since I don't know "B" and "L" of the supporting roof, I must assume that the modular unit is within h/2 from the windward edge of the supporting roof. Therefore, I must use the worst case Cp assuming h/L equal to 0.5 and 1.0 with applicable reduction due to modular unit roof area (400 ft^2).

I also initially thought the modular unit could be considered rooftop equipment, but I have since changed my mind. Equipment is considered rigid and used to obtain base shears for anchorage. The modular unit is structural and can't be assumed to be rigid.

Finally, the modular unit is only 1-story but will be placed on a 10-story building. Any opinions as to whether I will need an SE to stamp this or if a Civil PE will suffice.

Any thoughts on this?

Juston Fluckey, E.I.
Engineering Consultant

 

[frv]

For seismic, I would think that your situation is more similar to a Penthouse than anything else.

See section 13-3, and Table 13.5-1.

 

[P1ENG]

I will check that in the morning, but isn't chapter 13 for Non-structural items? This is a 10x40 manufactured building with occupancy, so it needs to be structural.

Juston Fluckey, E.I.
Engineering Consultant

 

[wannabeSE]

If the modular structure is occupied, it may not qualify as a penthouse per chapter 15 of the building code. You might want to contact the building department to see if it may be analyzed as a penthouse per chapter 13. The alternative is to count it as an additional story.

Your seismic calculations do not match your assumptions. An infinitely rigid building would have a higher C_s since the period, T, would be almost 0 and the R could be 1. Also, 20 kips is extremely light for a 10 story infinitely heavy building. If you want to check your theory, try performing the Equivalent Lateral Force (ELF) with 11 stories and estimate the mass (if you have no idea on the mass, try 100 psf for fun and guess at the building dimensions). Use the R for the main structure's lateral system and estimate the period per section 12.2.8.1 ( if you don't know, try R = 3 or 4 and T=1 for fun).

I doubt that this project qualifies for a two stage analysis per ASCE 7-05 section 12.2.3.1. It seems unlikely that the period of the entire 10 story structure will be less than 1.1 times the period of the modular structure. Also, I have never seen anyone count the walls/cladding as a separate level in seismic analysis. Typically, the mass of walls is distributed to the floor above and below.

A Civil PE may be OK if the project is not DSA or OSHPD. But, you need to check the contract documents and the ask the building department.(If it is the City of Los Angeles, I would not be surprised if they have a building height limit for Civil). I have seen project specifications written different ways. Sometimes they will require a structural engineer and sometimes they require an engineer licensed to practice structural engineering.

 

[P1ENG]

Wannabe: I think you misunderstood my attached calculations. When I assume the supporting building has infinite mass and rigidity, I then do a standard calculation on the modular unit as if it were on grade. That is the second page and that is the worst-case. The 20 kips you mentioned would be the mass of the modular unit. The first page is a standard vertical force distribution assuming the supporting structure has a mass of zero. I figured I would be somewhere between the two values (which are close anyway).

My walls are a separate level of mass, but ultimately half goes to the roof and half to the floor. Notice the 0.5 factor in the calculation of the diaphragm force.

I say it has occupancy, but really it will have an occupancy category and rarely be occupied. It is an equipment shelter.

Juston Fluckey, E.I.
Engineering Consultant

 

[structSU10]

I would run a check with the approximate period response from ASCE for the building, with an estimated weight of the building per floor, just to see what effect it may have. I would think if the mass of the new structure is large enough relative to the existing, it could have some effect. With mathCAD it should be easy to mess around with this to see what effect it has.