Emergency Roof Shoring B2610 - Final Report

Mountain Home AFB, Idaho

QYZH 07-0063

Emergency Roof Shoring Facility 2610

Final Report 10 October 2007

Emergency Roof Shoring-Facility 2610 (QYZH 07-0063) Mt. Home Air Force Base, Idaho 10 Oct 2007

Table of Contents Section Page

A. EXECUTIVE SUMMARY

A-1. PURPOSE

A-2. SCOPE

A-3. OBSERVATIONS AND CONCLUSIONS

A-4. RECOMMENDATIONS

B. INTRODUCTION

B-1. PURPOSE OF SHORING SYSTEM

B-2. SCOPE OF INVESTIGATION

B-3. MEETINGS DURING INVESTIGATIVE PHASE

B-4. CRITERIA, METHODS AND TECHNIQUES

B-5. PERFORMANCE OBJECTIVE

C. DESCRIPTION OF STRUCTURE

C-1. GENERAL

C-2. DATES OF CONSTRUCTION, ALTERATIONS, AND REPAIRS

C-3. HISTORICAL SIGNIFICANCE

C-4. COLLECTED DATA

D. SITE VISITS

D-1. OVERVIEW

D-2. FIELD OBSERVATIONS

E. STRUCTURAL LOADS

E-1. GRAVITY LOADS

E-2. SNOW LOADS

E-3. WIND LOADS…………………………………………………………………………………… ... 8

E-4. SEISMIC LOADS

F. SHORING ANALYSIS

F-1. STRUCTURAL ANALYSIS SUMMARY

F-2. GRAVITY ANALYSIS………………………………………………………………………………10

F-3. WIND ANALYSIS

F-4. SEISMIC ANALYSIS………………………………………………………………………………. 10

G. CONCLUSIONS AND RECOMMENDATIONS

G-1. CONCLUSIONS

G-2. RECOMMENDATIONS

Table of Contents (cont'd)

ATTACHMENTS

(Attachment page numbers are preceded by the number of the attachment)

ATTACHMENT 1. STATEMENT OF WORK

1.0 GENERAL REQUIREMENTS

ATTACHMENT 2. SELECTED AS-BUILT DRAWING DATA

2.1 ARCHITECTURAL FLOOR PLAN OF EXISTING FACILITY (PROVIDED BY BASE CE)

2.2 AS-BUILT ROOF FRAMING PLAN (1973 ORIGINAL CONSTRUCTION)

ATTACHMENT 3. SHORING DATA

3.0 WJA EMERGENCY SHORING PLANS

3.1 THYSSENKRUPP SAFWAY SHORING DATA

ATTACHMENT 4. FIELD REPORTS

4.0 WJA FIELD REPORTS

4.1 PHOTOGRAPHS

ATTACHMENT 5. SHORING CALCULATIONS

5.0 WJA SHORING CALCULATIONS

ATTACHMENT 6. SHORING CERTIFICATION

6.0 WJA SHORING CERTIFICATION LETTER

A. EXECUTIVE SUMMARY

A-1. PURPOSE

This report by WJA provides a record of temporary emergency roof shoring which was designed by WJA and installed by RSCI into the Consolidated Service Center Facility 2610 at Mt. Home Air Force Base, Idaho. The shoring was ordered by Mt. Home Air Force Base as emergency action to prevent a structural roof collapse.

A-2. SCOPE

The scope of work is limited to design and construction of temporary emergency roof shoring to prevent structural roof collapse in Facility 2610, visual examination of the roof structure for structural integrity, and issuance of a statement from the shoring Structural Engineer that the shoring has resulted in a structure that is safe from structural collapse.

The intent of the project is to provide emergency structural stabilization to provide a relatively safe environment so that additional more detailed evaluations and permanent repairs can be performed to the building. The scope does not include detailed structural analysis nor detailed material condition assessments of the existing structure.

A-3. OBSERVATIONS AND CONCLUSIONS

• The extent of unsafe roof trusses which require emergency shoring is limited to an area between Grid lines F and G. An emergency shoring system was designed by WJA and successfully installed by RSCI to prevent total roof collapse in this local area.

• Now that an emergency shoring system has been installed, the roof structure appears to be safe from structural collapse provided that the roof is not overloaded beyond 25 psf roof design live load.

• Although the shored structure is safe from structural collapse, the presence of shoring frames, loose ceiling tiles, debris, dust, inadequate heating and ventilation, disconnected electrical wiring, and a compromised fire protection system results in special non-structural life-safety hazards. The presence of these hazards should be considered by the Building Official prior to issuing any level of occupancy.

A-4. RECOMMENDATIONS

• We recommend that the Government install a new temporary roof drain in the failed, depressed roof truss area to provide positive drainage. In addition, standing water should be continuously monitored and removed in the depressed roof area starting immediately until a new temporary drain is installed (see Photos F and G). We recommend that the Government protect the structure to limit roof live loads to 25 psf maximum.

• Fire protection sprinkler piping, mechanical ductwork, and electrical power have been disconnected in areas as necessary to install the emergency shoring system.

Penetrations through fire rated gypsum wall-board corridor ceilings have also resulted from the installation of the temporary shoring in areas. We recommend that the Building Official consider the special non-structural hazards which have been introduced with installation of the shoring system before allowing any level of occupancy.

B. INTRODUCTION

B-1. PURPOSE OF SHORING SYSTEM

A local roof area at the Consolidated Service Center Facility 2610 at Mountain Home Air Force Base nearly experienced a dangerous structural collapse. The problem was first observed by users of the facility who noticed sprinkler heads protruding abnormally approximately 6” to 8” below the ceiling. Upon further investigation by the Base Fire Department and the 366 Civil Engineering Squadron, several wood roof trusses which span 90 feet in a local area near Grid F.5 and 1.5 were observed to have bottom chords that were severely distressed, ruptured, and/or split. After realizing the severity of the safety problem, Mountain Home Air Force Base alertly ordered that all personnel leave the building indefinitely, and issued an emergency contract to RSCI to provide emergency shoring to the structure to prevent total collapse.

The emergency shoring system will prevent the roof from collapsing until permanent repairs can be engineered and constructed within the next year.

B-2. SCOPE OF INVESTIGATION

The scope of work is limited to design and construction of temporary emergency roof shoring to prevent structural roof collapse in Facility 2610, limited visual examination of the remaining roof structure for structural integrity, and issuance of a statement from the shoring Structural Engineer that the new shoring has resulted in a structure that is safe from structural collapse.

Performance of work is based upon Statement of Work QYZH 07-0063 “Emergency Roof Shoring-Facility 2610” dated 17 September 2007 (see Attachment 1). The Contract was awarded to RSCI on Monday, 24 September 2007. RSCI and WJA performed the work during a fifteen day contract performance period, and completed the shoring installation and structural observations on Monday, 8 October 2007.

B-3. MEETINGS DURING INVESTIGATIVE PHASE

The team members for this project include:

• 366th Civil Engineering Squadron, Mt. Home AFB, Idaho

• RSCI Construction, Meridian, Idaho

• WJA Design Collaborative, Seattle, Washington

Structural Engineering Consultant to RSCI

A project kickoff meeting and site visit was conducted with the 366 CONS, 366 CES, RSCI, and WJA on 25 September 2007. A walk-through inside the facility was conducted on this date to familiarize RSCI/WJA with the project conditions. The Base fire department emphasized special safety procedures required for this project. The Base provided access to the roof on this date, so that RSCI and WJA could observe the extent of standing water accumulation in the failed roof area. Attendants at this meeting included Ray Prazak/366 CONS, Karen Rogow/366 CES, Marty Ray/366 CES, Stephanie Workman/366 CONS, John Walker/366 CES Fire Department, John Henderson/RSCI, Tim Scott/RSCI, and Doug Heit PE/WJA.

On 02 October 2007, RSCI, WJA, and representatives from Weyerhauser Trus Joist Commercial met at the project site. The purpose of this meeting was to familiarize Trus Joist with the nature of the truss failures, and to obtain Trus Joist’s opinion regarding the extent of required shoring to stabilize the structure. Attendants at this meeting included John Henderson/RSCI, Tim Scott/RSCI, Ted Osterburger PE/Senior Engineer/Trus Joist Commercial, Alan Eskuri PE/Division Engineer/Trus Joist Commercial, and Doug Heit

PE/WJA.

A third meeting was conducted at the project site on 04 October 2007. Representatives from the 366 Civil Engineering Squadron met with RSCI and WJA to observe the shoring progress, and to discuss informally possible permanent solutions. The Base stated that it intends to order a more comprehensive structural study of the entire facility, and request 3 permanent design options developed to 15% design level for consideration.

Attendants at this meeting included Colonel Laffey, Scott Mayberry/366 CES, Tim Scott/RSCI, and Doug Heit /WJA.

A fourth meeting was conducted at the project site on 04 October 2007. Attendants were John Walker/Mt. Home AFB Fire Department, Marty Ray/366 CES, Tim Scott/RSCI, and Doug Heit/WJA. The purpose of this meeting was to familiarize Mr. Walker with the extent of fire protection, HVAC, electrical, and ceiling penetration damage that resulted from installation of the emergency roof shoring.

B-4. CRITERIA, METHODS AND TECHNIQUES

This structural evaluation and analysis is performed using codes and standards as noted in this report.

• ASCE 7-05 “Minimum Design Loads for Buildings and Other Structures” For determination of snow loads and wind loads

• AISC/ASD “Manual of Steel Construction, Allowable Stress Design 9th Edition” For evaluation of structural shoring beam capacities

B-5. PERFORMANCE OBJECTIVE

The performance objective category for the emergency shoring is Collapse Prevention for approximately one year duration.

C. DESCRIPTION OF STRUCTURE

C-1. GENERAL

The Consolidated Service Center Facility 2610 is located at 480 E-Street, between Gunfighter and Phantom Avenue at Mt. Home Air Force Base (See Photos A-E). The structure is primarily one-story, with overall dimensions approximately equal to 338 feet by 145 feet (49,010 square feet) (see Attachment 2.1). The exterior walls are approximately 24’-9” tall above the main floor level. In addition, the structure has various elevated storage and office mezzanines. The main pedestrian entrance is located at the southeast wall, with the northwest wall used primarily as truck loading access. For purposes of this report, we will define the southeast wall as the “south” wall, and the northwest wall as the “north” wall.

The roof is relatively flat, particularly for use with long-span roof trusses. The roof has a single ridge-line which runs from the east to the west, and is located approximately 90 feet north of the main south wall. The roof has approximately 1” of gravel ballast which was field measured by RSCI/WJA during our investigation to weigh approximately 4.5 psf. According to our interviews, the roofing has been replaced in recent years, and is currently a “Malarkey” builtup roof system, consisting of gravel ballast over 4-ply, hot mopped membranes, over 4” rigid insulation, over ¾” perlite.

The roof structural system is one of two types depending upon location. In areas of the original construction, including the area that partially collapsed, the roof structure appears to be ¾” plywood over Trus-Joist TJH trusses (see Attachment 2.2). The roof structure over more recent facility expansions is 1 ½” Type B steel deck over steel bar-joist trusses.

Roof joists in wood framed areas are supported by interior glulam wood beams. Bar joists in steel framed areas are supported by interior wide-flange steel beams.

The exterior walls are load-bearing, precast concrete, with an exposed aggregate architectural finish.

C-2. DATES OF CONSTRUCTION, ALTERATIONS, AND REPAIRS

Over the 34 year history, there have been numerous structural additions, alterations and repairs made to this structure. WJA has limited information regarding the exact dates and history of the alterations and repairs. We will briefly describe the significant alterations which are documented on the Government provided As-Built drawings or projects that we know of in recent years:

• Original construction of a Commissary Store circa 1973 consisting of approximately 230’ x 145’ wood framed TJH roof truss system.

• 1979 Commissary Addition consisting of approximately 52’ x 145’ of steel framed roof office addition to the east end of the existing structure.

• 1984 Commissary Addition consisting of approximately 56’ x 145’ of steel framed roof storage addition to the west end of the existing structure.

• 1996 United States Postal Service Interior and Exterior alterations

• 1997 Conversion of Facility 2610 to a Consolidated Service Center interior tenant improvements

During our field investigations, we observed that several TJH wood roof trusses in the local vicinity of the area of partial collapse have been previously repaired. More specifically, several roof truss bottom chords have been reinforced with steel plate each side, and bolted through the trusses in various different ways (See Photos J and V).

According to representatives from Trus-Joist, the roof trusses may have been damaged nearly 20 years ago when excessive gravel roof ballast was stockpiled on the roof during a roof replacement project.

C-3. HISTORICAL SIGNIFICANCE

The structure is not classified as a historical structure.

C-4. COLLECTED DATA

In order to visually observe the existing structure, RSCI removed ceilings in areas to gain visual access. Where accessible, a man-lift or ladder was used to extend above the ceiling plane to observe the existing structure.

• Photographs were taken by WJA in areas to document the conditions

• WJA collected As-Built data from the 366 Civil Engineering Squadron, including the original 1973 construction, and the 1979, 1984, 1996, and 1997 alterations

• RSCI and WJA obtained shoring product data from Thyssenkrupp/Safway

• RSCI collected As-Built roof assembly data from Quality Tile and Roofing

• WJA collected original truss design loads from Trus-Joist

D. SITE VISITS

D-1. OVERVIEW

Site visits were conducted to obtain the following information:

• Determine practical areas where emergency shoring could be successfully installed

• Determine the extent of shoring required to stabilize the roof structure

• Determine locations of severe truss chord damage, so that supplemental temporary reinforcing could be engineered and installed

• Determine an approximate level of correlation between the actual as-built conditions and the as-built construction documents provided by the Government

• Observe and inspect the emergency roof shoring system

• Identify significant modifications to the structure which are not documented on the as-built drawings which could influence the shoring design

• Perform a collateral load survey for equipment suspended from the roof

• Observe areas of the roof structure throughout the facility to visually observe structural integrity

• Weigh the existing gravel roof ballast to estimate roof dead load

D-2. FIELD OBSERVATIONS

Field observations of the existing structure are performed in a context to determine the extent of emergency shoring necessary to safely stabilize the structure. Contract scope and resource limitations preclude a detailed, comprehensive visual examination or structural analysis of the entire facility. Observations are hindered and complicated by areas which are concealed by other structures, coatings, insulation, ceilings, and equipment. Observations are also limited in what can be observed by the human eye.

In the judgment of WJA, the area of damaged roof trusses which requires emergency shoring is located between Grids F & G and 1 & 3. When reviewing the condition of roof trusses in adjacent areas, WJA observed no excessive deterioration or dangerous conditions that would require extension of emergency shoring beyond the area between Grids F&G.

Shoring is installed snug against the existing structure, but is not installed in a manner to restore the original correct geometry of the roof. In many cases, wood shims are added between the steel shoring beam and wood roof truss to compensate for various bottom chord elevations (See Photo P).

Abnormal Conditions:

• Excessive roof sagging, and severe bottom chord truss damage on multiple trusses between Grids F & G (see Photos F, H, I)

• The depressed region of failed roof trusses compromises the integrity of the roof drainage system to an extent that it creates a new ponding hazard (see Photo F)

• The presence of previous steel plate repairs to wood TJH roof trusses in the local vicinity of the partial roof collapse is abnormal (see Photos J, V)

• The absence of continuous bridging installed perpendicular to the trusses in accordance with Trus-Joist standards is abnormal.

• The roof ledger angle which attaches the metal deck roof diaphragm to the east exterior concrete wall is only anchored at approximately 8’ on center spacing, although the ledger angle holes are fabricated at approximately 2’-0” on center.

This is not related to the structural roof failure but could be a hazard.

• A TJH truss which supports a ceiling mounted heater in the west high-bay storage area near Grid AA.5 and 1.5 has bottom chord damage which apparently resulted from toe nails which are improperly installed into the truss chord. This is not related to the structural roof failure but could be a hazard if not repaired.

E. STRUCTURAL LOADS

E-1. GRAVITY LOADS

Dead Loads

The roof dead load is estimated as 20 psf in shored roof area including the weight of ceilings and other collateral dead loads.

Live Loads

Live loads include the load superimposed by the use and occupancy of the building, and exclude dead loads, wind loads, and impact and special loads.

The following design live loads have been used for design of the shoring system:

• Roof Live: 20 psf or 250 lb concentrated

• Roof Snow Load: 25 psf

E-2. SNOW LOADS

Snow loads for the structure have been estimated in accordance with ASCE 7-05. Snow loads are calculated in the same manner as a new structure for the purposes of this analysis.

• Ground Snow Load 35 psf

• Terrain Category C

• Exposure Partially Exposed

• Ce 1.0

• Ct 1.0

• I 1.0

• Flat Roof Snow Load 25 psf

E-3. WIND LOADS

Wind loads for this structure were calculated in accordance with ASCE 7-05.

• Basic Wind Speed 90 mph

• Exposure Classification C

• Importance Factor 1.0

• Enclosed Structure

The damaged roof trusses may not have sufficient strength to transfer code required net uplift wind pressures from the roof to the original primary structure. For this reason, the temporary shoring system is designed to transfer net-uplift wind forces from the roof system through the shoring into the slab on grade.

E-4. SEISMIC LOADS

Seismic Design Parameters

• Seismic Use Group I

• Performance Objective Collapse Prevention

• Site Class C (Assumed)

• Ss (MCE .2 sec SRA 2% PE 50 years) .30 g

• S1 (MCE 1.0 sec SRA 2% PE 50 years) .10 g

• Fa 1.2

• Fv 1.7

• Sds .24 g

• Sd1 .11 g

• Seismic Design Category B

F. SHORING ANALYSIS

F-1. SHORING ANALYSIS SUMMARY

The shoring system used for this project is the “Adjust-A-Shore” system manufactured by Thyssenkrupp-Safway (see Attachment 3.1). The system consists of two levels of stacked AS46 base frames (see Photos L, M, N, O), combined with upper level AS45 extension frames. Shoring posts are typically spaced at 7’-0” in the longitudinal direction, and 4’-0” on center in the transverse direction. The shored roof trusses are supported by W8x10 x 8’-0” stringers which run perpendicular to the roof trusses (see Photo P). The W8x10 stringers are supported by transverse W8x10 x 5’-0” cross beams which frame directly over the shoring posts spaced at 4’-0” on center. Height adjustments are accomplished by adjusting the upper extension frames, and also by adjusting threaded post screwjacks at the top and base of the shoring posts.

The shoring system is braced in both directions using a proprietary steel tubular bracing system manufactured by Safway.

Assumed Material Properties

• Structural Steel Beams ASTM A36

• Fy min 36 ksi

• E 29,000 ksi

• Allowable Shoring Post Load 11 kips per Safway

Load Combinations:

Load combinations are in accordance with IBC 2000 for allowable stress design:

• D

• D + L

• D + L + Lr

• D + L + S

• D + W + L + Lr

• D + W + L + S

• .6D + W

• D + .7E + L + Lr

• .6D + .7E

F-2. GRAVITY ANALYSIS

The capacity of the W8x10 shoring beams, shoring posts, and wood crushing perpendicular to grain at truss support locations are all relevant factors that were evaluated in determining the spacing of shoring support. Shoring components are designed for full roof dead + shoring dead + roof snow design loads (see Attachment 5.0).

WJA determined that four rows of continuous shoring spaced at 21’-6” maximum spacing would keep the stresses in the shoring system below allowable levels (see Attachment 3.0).

In local areas of severe truss chord damage, WJA required that RSCI install supplementary Simpson CS16 coil strap across the fractures to provide a continuous tension load path (see Photos Q, R, S, T, W, X). In addition, in several locations where horizontal chord splitting occurred, WJA required the addition of vertical metal banding around the truss chords to confine and add shear strength (see Photos R, T, U, X). The added metal strap reinforcing is an integral part of the emergency shoring stabilization, and is not intended to be any part of the permanent truss repairs.

F-3. WIND ANALYSIS

Wind analysis is limited to net uplift between the roof trusses and shoring system.

Positive uplift metal strapping was designed and installed between the roof trusses and the shoring support beams where required. Shoring posts were anchored into the concrete slab on grade at various locations to increase uplift resistance as well.

F-4. SEISMIC ANALYSIS

The presence of significant diagonal bracing in the shoring system, and positive anchorage between the shoring and slab on grade will provide adequate seismic resistance in our opinion.

G. CONCLUSIONS AND

RECOMMENDATIONS

G-1. CONCLUSIONS

• The extent of unsafe roof trusses which require emergency shoring is limited to an area between Grid lines F and G. An emergency shoring system was designed by WJA and successfully installed by RSCI to prevent total roof collapse in this local area.

• Now that an emergency shoring system has been installed, the roof structure appears to be safe from structural collapse provided that the roof is not overloaded beyond 25 psf roof design live load (see Attachment 6.0).

• Although the shored structure is safe from structural collapse, the presence of shoring frames, loose ceiling tiles, debris, dust, inadequate heating and ventilation, disconnected electrical wiring, and a compromised fire protection system result in special non-structural life-safety hazards. The presence of these hazards should be considered by the Building Official prior to issuing any level of

G-2. RECOMMENDATIONS

• We recommend that the Government install a new temporary roof drain in the failed, depressed roof truss area to provide positive drainage. In addition, standing water should be continuously monitored and removed in the depressed roof area starting immediately until a new temporary drain is installed (see Photos

F & G). We recommend that the Government protect the structure to limit roof live loads to 25 psf maximum.

• Fire protection sprinkler piping, mechanical ductwork, and electrical power have been disconnected in areas as necessary to install the emergency shoring system.

Penetrations through fire rated gypsum wall-board corridor ceilings have also resulted from the installation of the temporary shoring in areas. We recommend that the Building Official consider the special non-structural hazards that now exist with the introduction of the shoring system before allowing any level of

END OF REPORT

Attachment 1

Statement of Work

Attachment 2

Selected As-Built Drawing Data

Attachment 3

Shoring Data

Attachment 4

Field Reports

Photo A: Facility 2610 South Elevation Looking North - 9/18/2007

Photo B: Facility 2610 South Elevation Looking Northwest - 9/18/2007

Photo C: Facility 2610 West Elevation Looking Southeast - 9/18/2007

Photo D: Facility 2610 North Elevation Looking Southwest - 9/18/2007

Photo E: Facility 2610 East Elevation Looking Northwest - 9/18/2007

Photo F: Roof Depression & Water Accumulation in Broken Truss Area - 9/25/2007

Photo G: Emergency Temporary Sump Pump Drainage - 9/26/2007

Photo H: Broken Bottom Truss Chord - 9/26/2007

Photo I: Split Bottom Truss Chord - 9/26/2007

Photo J: Previous Repairs at Truss Bottom Chords - 9/26/2007

Photo K: Ceiling Removal in Childcare Rm for Emergency Shoring - 9/26/2007

Photo L: Shoring Base – Grid 1.3 Office - 9/26/2007

Photo M: Shoring Tower at Underside of Roof Trusses Grid 1.3 - 9/27/2007

Photo N: Shoring Base at Grid 1.8 Corridor – Grid 1.8 - 9/27/2007

Photo O: Shoring Base at Grid 2.7 Corridor – 9/27/2007

Photo P: Shims between shoring beam and truss – 10/02/2007

Photo Q: New straps across truss chord fractures – 10/03/2007

Photo R: New straps and banding across truss chord fractures – 10/04/2007

Photo S: New straps across truss chord fracture – 10/04/2007

Photo T: New straps across truss chord fractures – 10/04/2007

Photo U: New (2) 2x4 bottom chord reinforcing at broken truss chord–10/04/2007

Photo V: Previous repairs at truss bottom chords–10/04/2007

Photo W: New straps at fractured truss bottom chords–10/04/2007

Photo X: New banding and straps at fractured truss bottom chord-10/04/2007

Attachment 5

Shoring Calculations

Attachment 6

Shoring Certification