Write about Earthquake resistance steel structureIn the below figure you may find the plan view of a building which consists of two identical special moment frames (SMF) in x direction and two identical special concentrically braced frames (SCBF) in y direction for resistance against lateral loads. The whole beam-to-column connections expect for the ones of SMF are simple, so the inner columns are gravity columns. Actually, you will not deal with the overall building, you will focus on the design of SMFs during your studies. Briefly, you will assign section properties to these two identical perimeter SMFs, only. You will use the plan area just for the determination of seismic effective weight during the equivalent lateral load procedure. SMF will resist against the earthquake load affecting in x direction. You may neglect the torsional effects (including accidental torsion) and carry on analyses with 2D simplified systems. The building is 8-story. Initially, you shall determine the appropriate locations for column splices. (Assume that the standard profile length is 30-40 ft.)
Total dead load value assigned to the normal stories is 3.83 kN/m2 (80 psf) including the structural steel elements. Live load applied to the normal stories is 3.11 kN/m2 (65 psf) including the weight of partition walls. Dead and live load values assigned to the roof used for promenade purposes are 3.11 kN/m2 (65 psf) and 2.87 kN/m2 (60 psf), respectively. This structure will be an office building so occupancy category is II and importance factor is 1.0. Please consider that the site class is D. The mapped maximum considered earthquake spectral response acceleration values at short and 1 second periods are 1.5g and 0.6g, respectively. Wind load shall be neglected and the earthquake load is assumed to be the dominant lateral load affecting the building. During the whole structural analyses for design, rigid-end offsets due to the fully restraint beam-to-column connections shall be taken into consideration. Furthermore, the presence of composite slab yields the rigid diaphragm behavior which shall not be overlooked during structural modeling. The whole structural elements including the braces of SCBF shall be assigned as wide flange sections with the structural steel type of A992 (Fy = 34.5 kN/cm2, 50 ksi). The assigned sections for columns shall be from W14 series.
For pre-design: For SMF, you may start with W14x342 for the bottom parts of the columns (from ground to the first column splice), and you may gradually decrease the column section for the upper parts (ie. W14x283 for the middle part and W14x257 for the top). Similarly, you may decrease the height of the cross section of beams gradually starting from the bottom and starting with W21x101 or W24x84, etc. for the first story. But do not forget 1-) The cross section elements shall be seismically compact, 2-) The height of the beams and columns shall be consistent with the qualified connections. (Please revisit the limitations for beam and column sections for prequalified beam-to-column connections and pick up one type, be consistent with it.)
The final cross section properties can be very different from the above initial trial cross sections !
Do not forget that for SMF drift limitations will most probably dominate the design!
a (ft) b(ft) c (ft) H (ft)
30 32 22 12