seismic retrofit training - bia
TRANSCRIPT
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SEISMIC
.RETROFIT
TRAINING for Building Contractors & Building Inspectors
PARTIOPANT HANDBOOK
__ ~~-~____________________~T~A_B_LE_OF_C_O_N_T_E_NT_S
LETTING STARTED Why We Are Here ............................................................ 1 Now You KIlow ................................................................ 2
EARTHQUAKE BASICS Factors Affecting Earthquake Forces ............................ 3 What is inertia force? ....................................................... 4 Two Structural Systems ................................................... 5 Cripple Walls .................................................................... 6 Effective Horizontal Force Resisting System ............... 7 Need for Seismic Retrofits .............................................. 8 Now You KIlow ................................................................ 9
SHEAR WALLS Shear Walls Resist Two Forces ..................................... 10 Location of Shear Walls ........................................... ...... 11 Weak Story Crawl Space ............................................... 12 Tall-Narrow Shear Walls ............................................... 13 Typical Shear Wall ......................................................... 14 Evaluating Existing Sheathing ..................................... 15 Sheathing Checklist ....................................................... 16 Fastener Locations ................................................;,....... 17 Full Scale Nail Chart ..................................................... 18 Predrilled Holes for Nails ............................................ 19 Predrilled Holes for Standard Lag Screws ................ 20 Uplift Force in Holdown .............................................. 21 Holdown Installation Checklist ................................... 22 Now You KIlow .............................................................. 24
r--
CONNECTIONS
Complete Load Path ...................................................... 26 Anchor Bolt End Distance ............................................ 27 Anchor Bolt Edge Distance .......................................... 28 Edge & End Distances (plan View) ............................. 29 Now You KIlow .............................................................. 30
')
FOUNDATIONS
Capping Existing Foundations .................................... 31 Now You KIlow .............................................................. 32
MISCELLANEOUS ELEMENTS Now You KIlow .............................................................. 33
PROTECTING YOURSELF
Job Site Checklist ........................................................... 34 Now You KIlow .............................................................. 35
~~-~_______________________G__E_n_IN_G__S_TA~R~TE___D ljf£ Why We Are Here
Why Limit Our Focus to Residential
Buildings?
• Large number of houses with weaknesses
• Retrofit is simple to install
• Larger buildings are more complicated
Training Objectives
In this training, you willleam...
• How earthquakes damage buildings
• Key retrofit components
• Installation Steps
• How to avoid typical errors
• Different materials available
• Bracing techniques for nonstructural items
• Protecting yourself
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GEnlNG STARTED Now You Know
1. Our focus is on 1-4 unit wood frame houses, not commercial buildings, apartments, or mobile homes.
2. There are literally thousands of buildings of this type that will need seismic retrofits in the immediate future.
3. You can use the building department's prescriptive standard to retrofit a house without an architect or engineer if the building meets the standard's strict requirements.
4. Incorrect retrofit installations are as bad as having no retrofit at all.
5. You are liable for damages that result from installation errors made by you.
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EARTHQUAKE BASICS Factors Affecting Earthquake Forces
Soil Under Building
Earthquake Magnitude
Distance from Epicenter
W hen an earthquake strikes an area, the ground below each building shakes in all directions. The magnitude of shaking
is not the sole factor that determines the amount of force felt by the building. The force will depend on the following factors:
• Magnitude of Earthquake - This is usually described by a number on the Richter scale.
• Distance from Epicenter - Buildings that are closer to the epicenter will usually feel stronger shaking than buildings that are further away.
• Soil Under Building - A building on hard, rocky ground will shake differently in comparison to one on soft clay soil.
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•
'~~-~_____________________E_A_R_TH_Q_U_A_K~E_B~A_S_IC~S ~ What is Inertia force?
Lnertia forces act in the opposite direction of ground movement. The concept of inertia orces can be explained by the example of a
person standing in a moving vehicle (as illustrated below).
Ace ele ration
Deceleration (Braking)
When the vehicle starts or accelerates, the vehicle moves forward as the person inside feels an inertia force moving him backward. Similarly, when the vehicle stops or decelerates, the person feels an inertia force pushing him forward.
t
•
•
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~--Foundations
EARTHQUAKE BASICS
Two Structural Systems
Gravity Force Resisting System
~~_~ Roof Joists
a..---Walls
t::====:;;;I---- Floor Beams
Horizontal Force Resisting System
Roof Diaphragm
Shear Wall
Foundation
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EARTHQUAKE BASICS Cripple Walls
it
M any older houses have first floors supported on short walls called "cripple walls." They go around the perimeter of
the house. These walls must also act as shear walls because these are the only walls at this level and they do not have sheathing on the inside face.
Earthquake forces are also larger at the lowest levels of a house. Most of these existing perimeter cripple walls do not have the strength or the connections to act as shear walls. Therefore, the cripple walls are usually very vulnerable during an earthquake.
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EARTHQUAKE BASICS
Effective Horizontal Force Resisting System
Shear Wall
Connection
Shear Wall
\---- Connection
1---Foun dation
For the Horizontal Force Resisting System to be effective, ALL portions of the building must be connected together to allow the
earthquake forces to transfer from the structural elements to the ground. This is called the "load path."
Structural failures occur when there are weaknesses in the elements or connections along this load path.
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---.----
•
~~-__---------------------E-A-RT-H-Q-U-A~K-E~B-A-S~IC~S~ Need for Seismic Retrofits
Reasons for Retrofitting
• Voluntary
- Increase Value
- Safety
• Code Requirement
• Insurance Requirement
Complete Seismic Retrofit
• Installing shear walls
• Installing anchor bolts
• Connecting shear walls to the building
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--- -~-"-"-"-.
EARTHQUAKE BASICS Now You Know
The shaking felt by a house depends on the magnitude of earthquake, distance from the epicenter, and composition of soil between the house and the epicenter.
2. Earthquakes cause inertia forces to act on a house in all directions.
3. Buildings have both Gravity and Horizontal Force Resisting Systems.
4. Cripple walls in older homes are not strong enough to withstand earthquake forces.
5. Weak stories cause significant damage during an earthquake.
6. Seismic retrofits consist of anchor bolts, shear walls, and connections that help to strengthen obvious weaknesses in older homes.
--"
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SHEAR WALLS
Shear Walls Resist Two Forces
Shear Forces
•
Upliff Forc~s
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I
SHEAR WALLS Location of Shear Walls
Interior Shear Walls
Exterior Shear Walls
Although a house has many walls, not all of them can act as shear walls. For a house to effectively resist earthquake forces, shear walls are required:
• In each story of a building and within the basement, garage, and crawl space
• On all exterior sides of the building and at some interior walls
• To be aligned vertically from the foundation to the roof if possible
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SHEAR WALLS Weak Story Crawl Space
Earthquake Force
Side Sway & Possible Collapse
k house has a weak story if any of its stories does not have enough strength to resist arthquake forces. Floors, basements,
garages, or spaces with few walls are considered weak or soft stories.
To strengthen and stiffen a weak story, an engineer or architect may require additional shear walls with heavily nailed plywood sheathing, hold owns, and additional framing and hardware.
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•
SHEAR WALLS
Tall - Narrow Shear Walls
Earthquake Force ,
,..... f /~/~______________~~~
r-I--------.....~......
Tall and narrow shear walls behave differently than those that are short and long. A wall that is too narrow will usually
be too flexible and may not have the required strength.
Tall and narrow shear walls have a greater tendency to uplift because they are not as wide at the base. There is less weight on the wall to resist uplift. Consequently, limits are placed on wall height in relation to the length of a shear wall.
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__ ~Z-~_________________________S_H_E_A_R_VVA_LL_S ljj£ Typical Shear Wall
Tap Plate(s) ""\
Siilor~ Sole: Plate:
Panel Edges End Stud
'Window Ope:ning
YOU add the following components to conventionally framed stud walls to form shear walls:
• Sheathing (consisting primarily of large pieces of plywood)
• Fasteners
• Additional framing (end posts, blocking)
• Holdowns (depending on forces calculated by an engineer or architect)
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SHEAR WALLS Evaluating Existing Sheathing
Type
Structural Plywood
Wood
Gypsum Board
Is each piece of sheathing...
1. Properly nailed along its edges?
2. Properly nailed to the end studs, top plate, & sill or sole plate?
3. Properly nailed to intermediate studs?
Laid diagonally across face of stud wall, it's acceptable if...
1. Each board extends from top plate to sill or sole plate.
2. Boards nailed to each stud they cross AND nailed to top & bottom plates.
Is each sheet. ..
1. Adequately fastened along edges of each panel?
2. Adequately nailed to:
End Studs
Top Plate of Double Top Plate
Sill Plate or Sole Plate
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.,
~~·____________________________ __S_HE_A_R_VVA_L_LS ~ Sheathing Checklist
Materials (Asl Yourself Trle~e Que~tlcn:;J
1. Is the specified sheathing 5-ply plywood? This is the recommended sheathing for strengthening wood frame houses.
2. Does each plywood panels grade stamp match the "specified" grade?
3. How long is the wall? Longer walls are stronger than narrow ones.
4. Did you combine sheathing materials? As a rule, don't combine different materials on the same wall.
5. Do you need to install gypsum board over new sheathing for fire protection?
Fasteners (Things to Rememr)er)
o Follow the spedfications on the supplied plans.
o Don't drive lag screws; you must screw them.
o Nails should have flush heads.
o Don't overdrive nails.
o Don't drive nails near the edge of sheathing.
o Use correct fastener as spedfied by the contract.
o Don't use screws/ staples unless specified.
o Don't apply coating to fasteners prior to installation.
a Predrill fastener holes if there is a potential for splitting.
Hardware (Warnings!)
Do NOT ... use hardware from a manufacturer that is not approved by the building official and engineer or architect!
Do NOT ... substitute lag screws for bolts!
Do NOT ... use fasteners that don't match exactly the manufacturer's specifications!
Stud Wall Without Sheathing
Earth quake ..-.i" I+-Excessive Force I· Side Sway ----. r --- ~-r-'T""'Ir--r-.-~--i
I I I I I I
Stud Wall With Sheathing
Eartfl quake ControUed Force Sidesway
...-- Resists Earthquake Force
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SHEAR WALLS Fastener Locations
Edge Nail Top Plate
Nail Intermediate Studs
Edge Nail Common Studs
Typical Sheathing
'--
Edge Nail To Blocking Around Opening
Edge Nail End Stud & Holdown Stud Edge Nail Sale Or Sill Plate
Eaddition to using the correct size nail, you must be sure to install them correctly. Due in
art to the shear volume of nails required in plywood sheathing, you will usually use a nail gun.
If you nail too closely to the edge of the sheathing, the nails may prematurely split the sheathing when resisting earthquake loads.
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SHEAR WALLS Full Scale Nail Chart
Wire Diameter Wire Gauge Length
20d
16d
12d
lOd
8d
6d
20d
16d
12d
lOd
8d
6d
Green Vinyl Sinkers
16d
8d
Joist Hanger
lOd
8d
.192
.162
.148
.148
.131
.113
.148
.135
.128
.128
.113
.099
.148
.113
.148
.131
6 4
8 31/2
9 31/4
9 3
101/4 21/2
111/2 2
9 4
10 31/2
101/2 31/4
101/2 3
111/2 21/2
121/2 2
9 31/4
111/2 23/8
9 11/2
101/4 11/2
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~~·~________________________S_H_EA_R__VV_A_L_LS ~ Predrilled Holes for Nails
COMMON
2Dd
16d
12d
10d
8d
6d
BOX
20d
16d
12d
10d
8d
6d
GREEN VINYL SINKERS
16d
8d
JOIST HANGERS
lOd
8d
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(less than 75% of nail diameter)
PREDRILL HOLE SIZE
1/8"
1/8"
3/32"
3/32"
3/32"
1/16"
3/32"
3/32"
3/32"
3/32"
1/16"
1/16"
3/32"
1/16"
3/32"
3/32"
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SHEAR WALLS Predrilled Holes for Standard Lag Screws
PREORILLED HOLES FOR STANDARD LAG SCREWS
O:~~~~- TREAD ED DIAMETER (0 T)
TREAD ED PORTION (L T ) LENGTH OF SHANK ( L S )
PREDRILL HOLE SIZE
LAG SHANK THREAD SCREW DIAMETER DIAMETER
3/4" 3/4" 1/2"
5/8" 5/8" 7 /16"
1/2" 1/2" 5/16"
7 /16" 7 /16" 1/4"
3/8" *3/8" 3/16"
5/16" *5/16" 5/32"
1/4" *1/4" 1/8"
II- Drilling of shank hole not required provided wood DOES NOT SPLIT.
PROCEDURE:
1. Drill thread diameter (DT) to a depth equal to length of lag screw (LT + Ls)'
2. Drill shank diameter (Ds) to a depth equal to shank length (Ls)' Do NOT over drill shank diameter.
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---------
SHEAR WALLS Uplift Force in Holdown
Shear Force --~.... =====--=== =======-- --------
C Shear Force
Tension In Holdo'Nn Rod
Earthquake forces acting on the top of the shear wall not only create shear forces in the wall but also uplift forces at the ends of the
wall. These forces tend to lift up one end of the wall and push the other end of the wall down. In some extreme cases, the uplift forces can even tip the wall over.
Usually, there will be some gravity load pushing down on the wall to resist some of the uplift. The amount of gravity load resisting uplift varies depending on the length of the wall and the framing of the floor or roof above the wall.
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SHEAR WALLS Holdown Installation Checklist
HOLDOWN INSTALLATION CHECKLIST SHEAR IO'ALL
\ FOUNDATION \
"-HOLOO\I\IN ROC. TlGIfTEN SNUG AFTER TIGHTENING SOLTS TO STUD OR POST
IILOCI(JNG OR JOIST UNDER END SnJD OR HOLDOWN POST
CHECKLIST
D Holdown post is correct size.
D Holdowns are correct size.
D Holdown bolts are correct size & length.
D Holdown rod is correct size & length.
o Nuts & washers are correct size & length.
o End studs or holdown posts are bearing on sill or sole plate.
D Hole drilled into foundation is the correct diameter & depth.
D Hold is cleaned of all dust & debris before the rod is epoxy grouted into place.
D The first bolt hole is drilled the required distance from the end of the end stud or post per holdown manufacturer.
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-SHEAR WALLS
Holdown Installation Checklist (cont.)
D The bolt holes are drilled the proper size (not greater than 1/16" larger than bolt diameter).
D Bolts in end stud or holdown post should not be countersunk. Route out adjacent framing to accommodate heads of bolt, but do not outer cut existing framing.
D Bolts are tightened to stud.
D Holdown threaded rod is an adequate length to extend from bottom of hole to holdown (or between holdowns).
D Nuts & washers are used to fasten rod to holdown per manufacturer. Rod should be an adequate length for the nut & washer to be fully engaged on rod.
D Holdown rods are tightened snugly.
D All framing shown on the drawings is installed.
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-SHEAR WALLS
Now You Know
1. Shear walls are vertical elements of the horizontal force resisting system.
2. Shear walls provide lateral strength for resisting horizontal earthquake forces and prevent the roof or floor above from swaying sideways.
3. They resist both shear forces and uplift forces.
4. Weak stories are floors, basements, or garages that only have a few shear walls or a number of short length shear walls.
5. Tall and narrow shear walls have a greater tendency to uplift.
6. Shear wall components include sheathing, fasteners, additional framing, and holdowns.
7. Some existing sheathing can be considered effective if they pass certain requirements.
8. Structural grade plywood (five-ply) is recommended for new sheathing.
9. You should not combine the strength of two different sheathing materials as a rule.
10. Nails are the most typical type of fasteners used for shear wall installations; don't substitute screws and staples.
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~~-___________________________ __S_H_E_A_R_VV_A_LLS ~ Now You Know (cont.)
11. There are several ways to avoid splitting the framing when you nail.
12. Always follow the hardware manufacturer's catalog for installation details.
13. The sheathing is attached with fasteners to existing stud wall framing.
14. Blocking may be needed along any unsupported edge.
15. An engineer may specify the addition of holdowns to the end studs of the shear wall to resist uplift forces.
16. Cripple walls often require sheathing and hardware installed in order to act as shear walls between the first floor and the foundation.
17. Ventilation reduces the potential for wood decay in repaired or retrofit areas.
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-CONNECTIONS
Complete Load Path
Roof Sheathing Top Connection
Shear Wall
Bottom Connection
~~ t--- Top Connection
~I---- Shear WallFoundation
\.-- Bottom Conn ection
The load path is like a chain; it is only as strong as its weakest link. When all of the elements above are connected, they form an
unbroken path from the roof to the ground that will transfer the diaphragm loads. A similar path must be traced to the ground from each of the floors.
'Without proper connections, these elements will separate or slide past each other during an earthquake.
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7 bolt diameters Min 7 bolt diameters Min
CONNECTIONS
Anchor Bolt End Distance
12 inch Max 12 inch Max
The foundation sills must be attached to the foundation with anchor bolts wellembedded into the concrete. The codes
require every piece of foundation sill to have at least two anchor bolts (every piece must have a bolt within 12 inches of each end).
Anchor bolts should be installed and tightened before the blocking is installed.
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~~-~________________________~C_O_N_N~E~C~T_IO_N__S ~ Anchor Bolt Edge Distance
Stud
Anchor Bolt Plate wash er
Foundation Sill
Foundation - __~
Sill Edge Distance
Foundation Edge Distance ---1---1
Nnchor bolts should be installed near the center of the foundation sill to maintain dge distance. These bolts also need
adequate edge distance to the side of the concrete to avoid blowing it out. Check with the manufacturer to get the minimum side cover on the bolts.
When drilling the holes for anchor bolts, it will sometimes be necessary to drill at a slight angle because of access problems. Make sure the bottom of the hole has the proper side edge distance.
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.. CONNECTIONS
Edge & End Distances (Plan View)
• Foundation Sill Plates
Edge Distance ~ End Distance
END DISTANCE
BOLT EDGE SILL SIZE DISTANCE MINIMUM MAXIMUM WIDTH
1/2" 3/4" 3-1/2" 12" 4"
5/8" 15/16" 4-3/8" 12" 4"
3/4" 1-1/8" 5-1/4" 12" 6"
NOTE: Some engineers or building departments may require larger minimum end distances.
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__ ~~-~b___________________________C_O~N~N_EC~T~IO_NS ~ Now You Know
1. The connective elements must be attached to each other in order to form a complete path for loads to be carried from the roof to the ground.
2. Rotted wood cannot resist earthquake forces as well as undamaged wood.
3. Use the same principles for creating load paths when retrofitting balloon-framed houses.
4. You must attach foundation sills to the foundation with anchor bolts wellembedded into the concrete.
5. Always try to eliminate a moisture source before replacing rotted wood otherwise you cannot prevent future deterioration.
6. Foundation sill cannot perform its function if the foundation is mislocated; you must notify the engineer or architect.
7. Always use proper bolt sizes and lengths for both wedge and epoxy anchors.
8. Protect yourself, your workers, and the owners from the epoxy fumes.
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-FOUNDATIONS
Capping Existing Foundations
Existing Brick
New Sill Plate -.~~.!I!!!.II!!!!I!!~..__&Anchor Bolts !!'.'! Concrete Cap
Foundation
Capping of the foundation involves adding concrete along the top of the existing footing and along the sides. Capping may
be needed if the existing footing is not strong enough to resist the forces of the anchor bolts. The concrete cap would help to insure that the anchor bolts were embedded into good, strong material.
Capping that extends down the sides may be specified when the new forces on the foundation are too great for the soil to resist. An extra width of concrete along the side of the existing foundation is used to spread out the forces on the soil.
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-FOUNDATIONS
Now You Know
1. Foundations transfer forces from the structural framing to the ground.
2. Foundations resist both horizontal sliding shear force and uplift force.
3. A shear wall requires a continuous foundation under its entire length.
4. Some older houses don't have foundations; others have foundations consisting of posts and piers or perimeter footing with interior posts.
5. Newer houses are constructed with a concrete slab cast on the ground.
6. If an existing foundation has deteriorated, the materials will not have any strength to resist earthquake forces.
7. You must beware of cracks found in a concrete foundation; have an engineer inspect them before installing a retrofit.
8. Foundation must be embedded into the ground.
9. There are only a few instances when you should disturb the soil around a foundation.
10. The two basic methods of retrofitting a foundation are capping and replacement.
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__ __
1":.
~~-~__________________~IS_C_EL_L_A_N7EO~US~E_LE~M~E_NTS'ij-£ Now You Know
t Brick chimneys in older homes are highly susceptible to earthquake forces; don't take the responsibility for recommending chimney bracing. Always defer to an engineer or architect.
2. Straps and brackets keep posts from separating from the roof or floor deck, but they do NOT increase the strength.
3. Automatic shutoff valves on gas lines help to prevent catastrophic fires.
4. To prevent a water heater from overturning, brace at the top and base and use flexible connectors.
5. Read FEMA's "Reducing the Risks of Nonstructural Damage: A Practical Approach" for more information on bracing nonstructural items.
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PROTECTING VOURSELF
Job Site Checklist
Safety on the Job Site o Use a respirator whenever there are airborne particles.
o Always wear hardhats.
o Rope off dangerous areas.
o Do NOT design the shoring yourself.
Documentation o Obtain a written contract
o Obtain a set of plans with drawings.
o Do a preconstruction walkthrough before bidding on the work
o Secure a building permit
o Take photographs of existing conditions.
o Outline your responsibility - notify owner of existing conditions.
o Document any change orders.
o Identify need and schedule for inspections - arrange them.
o Document and show all problems to engineer or architect
o Keep record of all inspections.
o Take photographs of all completed work
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-PROTECTING YOURSELF
Now You Know
1. Safety on the job site is important for both you and your employees.
2. Always be aware of the various concerns of the home owner.
3. Always document your work.
4. Don't mislead the home owner into believing that the house is "earthquake proof."
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