Live residential design: Taos, NM custom home (Pt. 2)

Designing a wood column for gravity loads

Connor worked through the first example using a post from the deck of the Taos, New Mexico mountain home, the same project used in earlier sessions of the series. The given information included a column height of 10 ft, continuous bracing on all axes, simple supports at top and bottom, and an axial eccentricity of one-sixth the member depth. Gravity loads were dead at 20 psf, snow at 40 psf, and live at 60 psf, applied over a tributary area of 12 ft by 5 ft - giving axial loads of 1,200 lb dead, 2,400 lb snow, and 3,600 lb live.

Within Calcs.com, Connor used the exposed outdoor isolated column preset (ASD), which automatically sets wet-service conditions. He pointed out that the axial eccentricity input accepts expressions like D/6, so as the cross-section changes during member selection the eccentricity updates automatically. After filtering the member selector to Douglas Fir Large No. 1 posts and timber, the 6x6 passed at 31% utilization with combined compression and bending as the controlling check.

Adding wind loads to the column design

The second example kept the same geometry and gravity loads but added lateral wind loads. Connor first ran the components and cladding wind load calculator in Calcs.com, using a wind speed of 115 mph, exposure category C, a flat roof height of 10 ft, and plan dimensions of 10 ft by 24 ft per the drawings. He set the effective member area to 1 sq ft throughout, noting this is the conservative choice that avoids the need to revisit if the area assumption changes. The governing output was at zone 5: 27.6 psf positive pressure and -34.4 psf negative pressure.

He then duplicated the gravity-only column sheet rather than starting from scratch, and added two distributed lateral load inputs - one on the strong axis with a tributary width of 12 ft and one on the weak axis with a tributary width of 5 ft - each carrying the 27.6 and -34.4 psf values. With biaxial lateral loads added, the 6x6 reached 100% utilization. Upsizing to an 8x8 Douglas Fir Large No. 1 brought the result to 40%.

During the Q&A, an attendee asked whether the components and cladding outputs were ultimate-level pressures being fed into an ASD calculation. Connor acknowledged this was a good question, said he believed the calculator handles the wind factor correctly within the governing load combination (noting the detailed mode showed the 0.6W factor applied), but committed to confirming with the engineering team and following up by email.

Seismic loads and the portal frame calculator

The third example was more involved: Connor set up a freestanding seismic-resisting frame to demonstrate how the seismic analysis and portal frame calculators can feed a member design. He used spectral acceleration values taken from the Taos project drawings, a building risk category of 2, a story height of 10 ft, and an assumed effective seismic weight of 50 kips. The seismic force resisting system was set to timber frame, which he flagged as a conservative and somewhat artificial choice for a deck - in most real projects this column would be a component of the building's overall lateral system rather than the primary resisting element by itself.

The resulting base shear was applied as a lateral point load at the top of the portal frame analysis wizard, with a frame width of 24 ft and column height of 10 ft. Connor then created a wood member design calculator, selected "link to analysis module," and connected it to the left leg of the portal frame. He walked through how the member selector filters by species, grade, and size, and ended up at an 18x18 Douglas Fir Large No. 1 at 83% utilization. He also noted that after changing the member size in the design calculator, the analysis calculator's member definition needed to be updated to match, otherwise Calcs.com flags a mismatch error.

Connor was explicit that connections at the frame joints - which would be a separate and significant design task for a wood frame - are not currently handled in Calcs.com, and he mentioned that specifically so attendees would not expect it.

Getting site design parameters and Q&A

For engineers who do not have wind, snow, or seismic data on the drawings, Connor demonstrated the ATC Hazards by Location tool: enter an address, select risk category and site class, and the tool returns the site-specific design values. He used the White House as a live example to show the format of the output. Attendee Matthew noted during the Q&A that NOAA has released its own updated hazards tool and that the ATC tool may no longer receive updates, so Connor recommended checking that newer resource.

Other questions from the session covered how load combinations are displayed (switching from standard to detailed mode shows the governing combination for each check), how to modify bracing assumptions mid-design, how to filter the member selector by species and grade, and how to build up posts from multiple plies. Connor closed by inviting attendees to submit past design drawings to be used in future webinar examples, and previewed the next session on heavy timber framing.