Structural System - Light single-span CFS portal frames built from SINGLE lipped channels (not back-to-back) - cost-driven agricultural design
Lateral Force Resisting System - Transverse: single-channel portals; longitudinal: strap-braced walls
Number of stories - 1 (eave 16'-0", apex 20'-0")
Analysis procedure - ELF; wind governs. Analysis fidelity - Tier 2 RECOMMENDED (singly-symmetric open sections, shear-center eccentricity, torsion-sensitive - the brief asks the agent to JUSTIFY the tier choice)
Design standards - AISI S100, ASCE 7
Project - Equipment storage barn, rural Iowa (budget-driven: single-channel frames at tight spacing)

## Material Properties
- Portal members: SINGLE lipped channels (e.g., 1000S250-97 class) Gr. 55; web stiffeners at knees
- The channel shear center lies OUTSIDE the web: loads applied through the web plane induce TORSION in every member - this is the defining feature of the design
- Purlins/girts: Z-sections at 4'-0"/5'-0"; through-fastened panels both roof and walls

## Loads
| Load Type | Magnitude |
|------------|-----------|
| Roof Dead | 4 psf + 1 psf collateral |
| Snow | pg = 25 psf | Roof Live | Lr 20 psf |
| Wind | V = 105 mph, Exposure C |
| Seismic | SDS = 0.15 g, SDC B (trivial - confirm) |

## Geometry
- Span 36'-0"; 14 frames at 15'-0" o.c. (tight spacing - light frames); length 195'-0"
- Purlins/girts fasten to the channel flange - load plane offset from the shear center at every connection

## Design Requirements (agent outputs)
- TORSION IS THE TEST: single-channel members loaded eccentric to the shear center - bending PLUS torsion/warping stresses per S100 (combined bending and torsion reduction, or capture the torsional response in the Tier 2 model and design accordingly); a back-to-back reflex ("assume doubly-symmetric") is wrong here
- Justify the analysis fidelity tier: Tier 1 minimum (warping DOF); state what Tier 2 adds for this structure (progressive local softening interacting with twist) and either run it or defend Tier 1
- Purlin/girt anchorage forces: the panel + purlin system restrains the frame flange - anti-roll/torsion brace schedule at knees and apex is a REQUIRED deliverable, with forces
- Flange-restraint asymmetry: outside flange braced by girts/purlins, inside flange free - uplift reversal puts the FREE flange in compression (unbraced distortional/LTB case)
- Economy check: tight spacing was chosen to keep single channels feasible - report the governing D/C margins; if any member needs back-to-back doubling the brief's premise fails and the agent must SAY SO rather than silently upsize
- Deliverables: frame schedule with torsion design basis per member, torsion-brace schedule + forces, purlin/girt/strap schedules, tier justification statement
