Updated Design Narrative_RRCabin_L T_09012020.pdf

Tongass National Forest

Raven’s Roost Cabin Replacement

DESIGN NARRATIVE

July 2018 Synopsis for Design Contract

Amended July 2020 to include loft and outhouse construction

US DEPARTMENT OF AGRICULTURE FOREST SERVICE | ALASKA REGION

TONGASS NATIONAL FOREST

USFS Raven’s Roost Cabin: Design Narrative Synopsis - 1

PROJECT SUMMARY

This document provides supplemental information for use in cabin design work described in the Special Project Specifications, Raven’s Roost Cabin Construction contract.

The new cabin will be a pre‐engineered, timber‐framed structure with glulam timber foundation beams. Design will be based on the USFS‐provided drawings for a two‐story (with loft) milled timber 16’x20’ kit cabin. The cabin’s lower level will be wheelchair accessible and have a large covered deck. Match deck width to cabin width.

CABIN SITE DESCRIPTION

Subsurface conditions at the cabin site: approx. 2.5 feet of organic sub‐alpine muskeg over bedrock.

The foundation will bear directly on bedrock.

OVERVIEW OF CABIN FEATURES

Base the design of this cabin on the USFS‐provided two‐story, 16’x20’ cedar kit (see Drawings). Incorporate additional elements as outlined in this document, including but not limited to:

Insulated cabin floor and roof.

Propane heat.

Doors centered at front and rear.

Gable roof of clear polycarbonate panels over rear cabin door.

PLAN VIEW OF CABIN AND COVERED

12’-long Deck

(Fibergrat

16’x20’Cab

USFS Raven’s Roost Cabin: Design Narrative Synopsis - 3

Front deck with heavy timber framing and Fibergrate decking.

Front deck roof will be clear polycarbonate roof panels.

A guardrail is required around the deck.

The foundation will bear on and be anchored to bedrock.

Glulam foundation beams.

Propane Heating The primary heat source will be a direct‐vent propane heater, Empire Heating System model DV25 with a thermostat, a timer with automatic shut‐off valve, and fuelled by a 500 gallon tank.

Accessibility The cabin, deck, outhouse and the route between the cabin and outhouse will be fully accessible. Design all cabin and outhouse access points to be in conformance with the Forest Service Outdoor Recreation Accessibility Guidelines (FSORAG) and the ABA Accessibility Standards (ABAAS).

STRUCTURAL DESIGN VALUES & MATERIALS SPECIFICATIONS

Design Values The cabin will be constructed in accordance with the following Code and Load criteria:

Code: 2012 International Building Code as amended by the State of Alaska Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7, current edition)

Structural Risk Category II Loads Snow

Estimated Ground Snow Load: 375 psf

Exposure Coefficient: Ce = 0.9, Exposure C, Fully Exposed Thermal Coefficient: Ct = 1.2, Unheated Building

Slope Coefficient: Cs = 0.75, 10:12 slope with un‐obstructed partially slippery surface (metal roof)

Load on sloped roof: 213 psf) Floor Live Load: 40 psf

Wind

Load: Exposure C Wind Speed per ASCE 7‐2010: 137 mph, Structural

Risk Category II Velocity Pressure: as required by ASCE‐7

Seismic Design ‐ as required by ASCE‐7

Site Class C – Rock Importance, Ie = 1.0

R = 4.0; Light framed walls with shear panels of other materials (not sheathed with plywood)

USFS Raven’s Roost Cabin: Design Narrative Synopsis - 4

Material Specifications Cabin Kit Style and wood species

T&G D‐Log Timbers

Alaska Yellow Cedar or Western Red Cedar

Floor Material

Cabin flooring: Simpson SkidGuard High Density Overlay (HDO) Plywood

Cabin loft decking: 3x6 T&G Fir or similar approved light‐colored wood species

Front deck and rear porch deck fiberglass grate surface material: Safe‐T‐Span T2510 or T2515 (depending on span) pultruded pedestrian grating as manufactured by Grating Pacific or approved equal. Surface must be ADA‐ compliant. Install per manufacturer’s recommendations using stainless steel Type R hold down clips. Color: gray.

Submit grating sample for final approval; submit color samples if available in more than one shade of specified color.

Windows and Doors

Window frame type: Vinyl, dark brown; submit color for approval

Door material: Wood, type: Fir; apply stain as for cabin

Door style: Standard exterior door with window in top 1/2

Roof Color: Dark brown; submit color for approval

Insulation

Floor insulation: Yes; R‐value: 20 Ceiling insulation: Yes; R‐value: 20

Foundation Foundations will be round concrete columns with rebar cages, with rebar embedded in the bedrock. The columns will have a galvanized steel connector made of welded plates connecting to the concrete with galvanized anchor bolts.

Front deck supports will be supported with a concrete rock anchor foundation. The design intent is to prevent sliding down the bedrock surface and resist uplift forces. Since the foundation will bear on the bedrock surface the compressive roof and deck loads will be transmitted directly to the bedrock surface.

When installing the concrete foundation, loose/weathered bedrock will be removed to provide a competent foundation surface. If the bedrock surface is steeply dipping, the surface under the footing shall be chipped to provide a level bearing surface and allow secure installation of the rock anchor. A deeper anchor may be required to engage the required mass of bedrock to resist uplift forces. The front porch rock anchor is intended to resist uplift loads, and resist some lateral loads from both seismic and wind forces.

Framing Cabin foundation beams will be treated glulam beams. The foundation beams will support the timber floor joists that will likely be sawn 2x or 3x members which in turn support a timber deck made of 2x or 3x material. The perimeter of the deck will support timber walls which will support the roof. Roof framing will include rafters at periodic intervals supporting a heavy timber deck. The rafters will be supported by the ridge beam at the top. The deck structure will include reinforced concrete columns and glulam beams supporting the fiberglass deck panels.

Roof The cabin roof should be sloped at 4 to 12 (vertical to horizontal) or steeper so that snow will shed off the roof. The cabin eaves should extend a minimum of 2’ beyond the edge of the cabin and the deck so that snow that does shed will not land against the cabin walls or the deck.

USFS Raven’s Roost Cabin: Design Narrative Synopsis - 5

Provide a detail in the drawings to show how the structure will be adequately ventilated.

Load Design The cabin lateral load resisting system will include the roof diaphragm, timber shear wall panels and the floor diaphragm. The shear walls will likely be made with stacked solid timber connected to resist shear forces. Steel connectors will be needed to resist shear and overturning forces due to the high winds and significant earthquake forces when the large snow load mass is included.

Deck To meet accessibility design criteria, the fiberglass decking will meet maximum gap size requirements as specified by the FSORAG and the ABAAS. Wet snow may accumulate on the deck in amounts that approximate the ground snow load despite the presence of a roof. The deck will need to be braced under the grating to resist lateral seismic loads.

The polycarbonate clear roof panels will be Unitrex Ultra‐R (12” R‐Panel) on a 12” on‐center purlin arrangement to resist snow loads. The use of plastic panels requires the roof be braced with cross bracing to resist lateral loads and transfer the loads back to the cabin shear walls while still allowing light to pass through. Design cabin walls and guardrails around deck with sufficient strength to support multiple hammock anchor locations. Identify these locations in submitted drawings.