An#Introduc+on#to#Structural#Design#of#Post#Frame#Building#Systems#
Presented(on(October(23,(2014(by:(Harvey(B.(Manbeck,((PhD,(PE(( (Consultant(to(NFBA(( (Professor(Emeritus,(Engineering(( (Penn(State(University(
“The(Wood(Products(Council”(is(a(Registered(Provider(with(The(American(InsRtute(of(Architects(ConRnuing(EducaRon(Systems((AIA/CES),(Provider(#G516.((!Credit(s)(earned(on(compleRon(of(this(course(will(be(reported(to(AIA(CES(for(AIA(members.(CerRficates(of(CompleRon(for(both(AIA(members(and(non\AIA(members(are(available(upon(request.(((
This(course(is(registered(with(AIA(CES(for(conRnuing(professional(educaRon.(As(such,(it(does(not(include(content(that(may(be(deemed(or(construed(to(be(an(approval(or(endorsement(by(the(AIA(of(any(material(of(construcRon(or(any(method(or(manner(of(handling,(using,(distribuRng,(or(dealing(in(any(material(or(product.(___________________________________________
QuesRons(related(to(specific(materials,(methods,(and(services(will(be(addressed(at(the(conclusion(of(this(presentaRon.((
(
Course#Descrip+on#
((((This(program(begins(with(a(brief(descripRon(of(post\frame(building(systems(followed(by(an(overview(of(key(concepts(for(their(structural(design.(InformaRon(is(presented(from(a(conceptual(standpoint(as(opposed(to(an(equaRon(and(computaRonal(standpoint.(Two(design(methods(are(addressed:(one(for(post\frame(systems(without(diaphragm(acRon,(the(other(for(post\frame(systems(with(diaphragm(acRon.((The(majority(of(the(program(is(focused(on(the(la`er.(The(presentaRon(shows(how(a(simple,(yet(powerful(and(readily(available(program,(DAFI,(determines(the(proporRon(of((design(lateral(loads(carried(to(ground(by(the(individual(post\frames(and(that(carried(to(ground(by(roof(diaphragms(&(shear(walls.(The(program(then(shows(how(isolated(post(foundaRons(are(designed(to(resist(lateral(and(uplib(forces.(Technical(resources(available(to(design(professionals(are(also(discussed.((
This!presenta,on!was!developed!by!a!third!party!and!is!not!funded!by!WoodWorks!or!the!so8wood!lumber!check;off.#
Learning#Objec+ves#
1.(((IdenRfy(the(primary(structural(components(of(post\frame(((((((((((PF)(building(systems(
2.(((Learn(how(to(conduct(structural(design(of(PF(systems(without((
(((((((diaphragm((acRon((3. Learn(how(to(conduct(structural(design(of(post\frame(
building(systems(with(diaphragm(acRon(4. Learn(how(to(design(isolated(post/pier(post(frame(
foundaRons(5. IdenRfy(post\frame(design(resources(available(to(architects(
and(engineers(
• Identify the primary structural components of post-frame (PF) building systems
• Learn how to conduct structural design of PF systems without diaphragm action
• Learn how to conduct structural design of PF systems with diaphragm action
• Learn how to design isolated post/pier PF foundations
• Identify post-frame design resources available to architects and engineers
LEARNING OBJECTIVES
• Wood industry’s counterpart to low profile (1 to 2-1/2 story) steel buildings
• Developed in late 1930’s for agricultural sector • Known as “pole building” in the past • PF has evolved to highly engineered wood
building system • PF has expanded to many commercial,
residential & institutional applications
POST-FRAME (PF) BUILDING SYSTEMS
TYPICAL PF BUILDING SYSTEM
Laminated or Solid-Sawn
Wood Columns
Roof Framing Trusses or
Rafters
Roof Purlins Typ. 2x4s
“on edge” or “flat”
Sheathing: • 26 to 29 ga Ribbed Steel OR • OSB or Plywood
Wall Girts Typ. 2x4 or 2x6
“flat”
PF BUILDING SYSTEM FOUNDATION OPTIONS
9
Isolated Pier Foundation
Continuous RC Foundation Wall
Thickened Edge of Concrete Slab
• 2-dimensional frame design method – Without diaphragm action
• 3-dimensional diaphragm design method – With diaphragm action
PRIMARY PF DESIGN METHODS
PF SYSTEMS WITHOUT DIAPHRAGM ACTION
Unsheathed walls
Unsheathed walls
PF SYSTEM WITH DIAPHRAGM ACTION
Sheathed Version of This Building
LATERAL LOADS: WITHOUT DIAPHRAGM ACTION
Wind direction Wind direction
Typical sway (Δ) of interior post frame at design lateral load = 5 to 8 inches
LATERAL LOADS: WITH DIAPHRAGM ACTION
Wind direction
∆1
Typical sway (∆1) of centermost post-frame at design lateral load =
0.5 to 1.0 inch
ADVANTAGES OF DIAPHRAGM DESIGN
• Smaller sidewall posts • Shallower post or pier embedment depths
• Benefits: – More economical design – Greater structural integrity – More durable post-frame structures
FULL-SCALE PF BUILDING TESTS
40 ft W x 80 ft L x 16 ft H
16 ft
5 ft
Hydraulic cylinder
Load cell
29 ga ribbed steel sheathing
DIAPHRAGM VS NO DIAPHRAGM ACTION
WHEN TO USE 2-D FRAME DESIGN METHOD
• Side or endwalls are open, or not sheathed • PF Building with L:W ≥ ≈ 2.5 to 3:1 • Connections and other structural detailing don’t
develop a continuous load path for transfer of in-plane shear forces – Through the roof sheathing – Between the diaphragm and the top of the endwall – Through the endwall or shearwall – Between bottom of the endwall and the endwall
foundation
EMBEDDED POST/PIER FOUNDATIONS
• Common post-soil fixity models for embedded post or pier foundations: – Constrained post or pier – Non-constrained post or pier
POST/PIER EMBEDMENT DESIGN Horizontal
movement permitted Horizontal movement prevented by
floor or mechanical connection
Non-constrained Constrained
d0
POST FOUNDATIONS-Simplified Model: NON-CONSTRAINED CASE
dw
Non-constrained post/pier
w
d
Constrained post/pier
Load Direction
Slab
Rotation Point
dw
Non-constrained post/pier
w
d
Constrained post/pier
Load Direction
Slab
Rotation Point
VG MG
Structural Analog for Determining Post Ground Surface Shear (VG) and Moment (MG)
• Fixed end at depth w below grade • w = face width of post bearing against soil
POST FOUNDATIONS-Simplified Model: CONSTRAINED CASE
dw
Non-constrained post/pier
w
d
Constrained post/pier
Load Direction
Slab
Rotation Point
dw
Non-constrained post/pier
w
d
Constrained post/pier
Load Direction
Slab
Rotation Point
Structural Analog for Determining Post Ground Surface Shear (VG) and Moment (MG)
VG MG
• Vertical roller at top edge of slab
• Fixed end at ground line
• Soil is homogeneous throughout the entire embedment depth.
• Soil stiffness is either constant (cohesive soils) for all depths below grade or linearly increases (non-cohesive soils) with depth below grade.
• Width of the below-grade portion of the foundation is constant. This generally means that there are no attached collars or footings that are effective in resisting lateral soil forces.
PRIMARY ASSUMPTIONS FOR THE SIMPLIFIED MODEL
• Used to determine ground surface shear, VG, and moment , MG when required conditions for simplified method not met
• Considers the load-deformation behavior of the soil surrounding the embedded post
• Soil – foundation load deformation behavior evaluated using soil spring models
UNIVERSAL MODEL – POST FOUNDATIONS
UNIVERSAL MODEL: SOIL LOAD - DISPLACEMENT BEHAVIOR
Soil Load (psi)
Soil Deformation (in.)
Ultimate Soil Strength, pu,z
(psi)
Slope = soil stiffness, Es (lb/in)
Elastic-Perfectly Plastic Soil
POST FOUNDATIONS-Universal Model: NON-CONSTRAINED CASE
z
t1
t2
t3
t4
t5
Fult,1
Fult,3a
Fult,2
MU
VU1
2
3
4
5
V
1
2
3a
4
5
3b
dRU
Point of foundation
rotation
dRU
Fult,5
Fult,4
Fult,3b
M
z
t1
t2t2
t3t3
t4t4
t5t5
Fult,1
Fult,3a
Fult,2
MU
VU1
2
3
4
5
V
1
2
3a
4
5
3b
dRU
Point of foundation
rotation
dRU
Fult,5
Fult,4
Fult,3b
M
POST FOUNDATIONS-Universal Model: CONSTRAINED CASE
z
Post contacts ground surface
restraint
t1
t2
t3
t4
t5
1 Fult,1
Fult,5
Fult,3
Fult,4
Fult,2
MU
VU
2
3
4
5
z
Post contacts ground surface
restraint
t1
t2t2
t3t3
t4t4
t5t5
1 Fult,1
Fult,5
Fult,3
Fult,4
Fult,2
MU
VU
2
3
4
5
DESIGN METHODS: 2-D POST FRAME
s x qwr s x qlr Wind Direction
W
H1
H2
Post-to-truss connections usually modeled as a pin
The post-to-ground reaction is modeled consistent with post embedment details. (Note that one post foundation may be constrained and the other non-constrained)
Each frame is designed to carry its full tributary lateral and gravity loads
s x qww
s x qlw
s x w
ASCE-7 Governing Load Combinations (ASD) • Dead + ¾ snow + ¾ wind (or seismic)
or 0.6 dead + wind (or seismic)
– Usually controls post design • Dead + snow (balanced & unbalanced)
– Usually controls roof-framing design
2-D DESIGN ANALYSIS
SIMPLIFIED 2-D PF DESIGN METHOD
Wind direction
V = roof truss vertical reaction
P = ½ (Resultant lateral roof load from truss)
Model post-to-soil interaction; for constrained pier foundation and simplified method, this is fixed end at ground line.
Then design the post for the design lateral load combinations
Specify dead & snow loads for truss manufacturer ½ (qww+qlw) x s
or Max(qww, qlw) x s
• Incorporates in-plane shear strength and stiffness of the roof and wall sheathing to transfer design lateral loads to the foundation
• Three-dimensional structural analysis method • Significantly decreases wall-post size and post-
foundation embedment depth
DIAPHRAGM DESIGN METHOD
• Key Definitions
- In-plane shear stiffness of the roof diaphragm panel, c
- Bare frame stiffness of the post-frame, k
- Design eave lateral load, P
PF DIAPHRAGM DESIGN
DIAPHRAGM TEST PANEL
Building width
Endwall
Test panel width, a
Test panel length, b
θ
Roof sheet end joint
Building length = LB
Roof span Test panel (basic element)
bsp = Slope length (roof diaphragm length)
ap
DIAPHRAGM TEST PANEL
Sheathing/ cladding
Rafter or truss top chord (strut)
Purlin (chord)
CANTILEVER TEST CONFIGURATION
Direction of corrugations
Cladding Purlin
Truss top chord
P = applied force
b = Test diaphragm length
a = Te
st
diap
hrag
m
widt
h
∆s
DIAPHRAGM TEST RESULTS, IN-PLANE STRENGTH & STIFFNESS
∆ P
P
c 1 ∆ ∆1
Diaphragm Test Panel Schematic
C = design shear stiffness (slope)
Ultimate Strength = Pult
Design shear strength = 0.4 Pult
Design unit shear strength = (1/b)0.4 Pult
DIAPHRAGM TEST PANEL
Building width
Endwall
Test panel width, a
Test panel length, b
θ
Roof sheet end joint
Roof span Test panel (basic element)
bsp = Slope length (roof diaphragm length)
ap
Test panel shear props from sheathing supplier
or from PFBDM
Roof diaphragm shear props deduced from test
panel props
• Shear stiffness of a roof diaphragm panel – test panel stiffness, c – roof panel width, ap – roof panel roof slope length bsp – roof slope Θ
ch = [c (a/b)] (bsp/ap)cos2Θ
DIAPHRAGM DESIGN METHOD – ROOF PANEL STIFFNESS
• In-plane strength is a linear function of diaphragm length, bsp
V = [unit shear strength](roof diaphragm length)
V = [0.4(Pult/b)](bsp)
DIAPHRAGM DESIGN METHOD-ROOF PANEL STRENGTH
DIAPHRAGM DESIGN METHOD- BARE FRAME STIFFNESS, K
P1
Model soil to post interaction using appropriate structural analog for
constrained or non-constrained pier
Pinned Connection
DIAPHRAGM DESIGN METHOD PF diaphragm design
procedures based on: 1. compatibility of post-
frame and roof panel eave deformations and
2. Equilibrium of horizontal forces at each eave
P = Design Lateral Eave Load
• Equilibrium of forces at each PF eave Pi = Pfi + Pri
– Pi = design eave load in ith PF – Pfi = portion of the design eave load carried by the ith PF – Pri = portion of the design eave load carried by the roof diaphragm panel
at the ith PF
DIAPHRAGM DESIGN METHOD
• Compatibility of roof and PF deformations at each PF eave
Δri = Δfi
– Δri = roof panel eave deformation at the ith PF (dependent upon ci, ki, and Pi )
– Δfi = Pfi/ki
DIAPHRAGM DESIGN METHOD
• DAFI program calculates – Eave displacement of each post frame – Portion of the design eave load carried by each post
frame – Shear forces carried by each roof diaphragm panel
in the building system – Available at no cost at
www.postframeadvantage.com
DAFI COMPUTER PROGRAM
• Total number of bays in the building • Design eave loads at each post frame, Pi
• Bare frame stiffness of each post frame, ki • In-plane shear stiffness of each roof diaphragm
panel, chi
DAFI INPUTS
DIAPHRAGM DESIGN METHOD
1(ch1) 3(ch3) 2(ch2)
Panel/PF structural analog of a 3-bay building
DIAPHRAGM DESIGN – STRUCTURAL ANALOG
1 42 3
PF 1 (k1) (k2) (k3) (k4)
Diaphragm Panel
P1 P2 P3 P4
DAFI: UNDEFORMED POSITION
2 1 3 4
Datum Datum
DAFI: DEFORMED EQUILIBRIUM POSITION
Datum Datum
1 2 3 4
DAFI COMPUTER PROGRAM
Pf4
Pf3
Pf2
Pf1
DAFI COMPUTER PROGRAM
V3#
V2#
V1#
• Can be used for post-frame building systems where: – Stiffness, ki, of the interior post frame elements are
not the same – Stiffness, chi, of the diaphragm panel elements are
not the same – Stiffness, ki of the two endwall post-frames are not
the same • Available at no cost to designers at
www.PostFrameAdvantage.com
DAFI: HIGHLY FLEXIBLE
DAFI: MINI DEMONSTRATION
• 48-ft-wide by 96-ft-long post frame • Post frames 8-ft o.c. • Number of bays —12 • Post-frame stiffness (k) — 300 lbs/in. • Endwall stiffness (ke) —10,000 lbs/in. • Roof diaphragm stiffness (C) —12,000 lbs/in. • Horizontal eave load at interior post frame —
800 lbs
DAFI: MINI DEMONSTRATION
Access DAFI by: • Going to www.postframeadvantage.com • Clicking onto “DAFI” icon • Running “DAFI” when prompted
NOTE: Recommend you use Explorer or Firefox browser
DAFI: MINI DEMONSTRATION
DAFI: MINI DEMONSTRATION
DAFI: MINI DEMONSTRATION
DAFI: MINI DEMONSTRATION
POST/PIER EMBEDMENT DESIGN
dw
Non-constrained post/pier
w
d
Constrained post/pier
Load Direction
Slab
Rotation Point
dw
Non-constrained post/pier
w
d
Constrained post/pier
Load Direction
Slab
Rotation Point
Lateral displacement Allowed at ground surface
Lateral displacement = zero At ground surface
• Post-embedment details must resist – Downward acting gravity loads – Shear force and moments from lateral loadings – Uplift post loads
– ANSI/ASAE EP486.2, Shallow post and pier foundation design
POST/PIER EMBEDMENT DESIGN
Two Design Approaches • Simplified Method • Universal Method
POST/PIER EMBEDMENT DESIGN: LATERAL LOADS
POST/PIER EMBEDMENT: LATERAL LOADS-SIMPLIFIED METHOD
MGVG
P
d
y
z
Ground surface
P
Post or pier with width b
Soil forces
R
Soil with ESthat
increases linearly with
depth z
Restraint
MGVG
P
d
y
z
Ground surface
P
Post or pier with width b
Soil forces
R
Soil with ESthat
increases linearly with
depth z
Restraint
Case 1. Soil properties vary linearly with depth below grade
Case 2. Soil properties are constant throughout embedment depth
MGVG
P
d
y
z
Ground surface
P
Post or pier with width b
R
Soil with ESthat is
constant with depth z
Restraint
Soil forces
MGVG
P
d
y
z
Ground surface
P
Post or pier with width b
R
Soil with ESthat is
constant with depth z
Restraint
Soil forces
Constrained at Ground Surface – Soil pressure dist.
POST/PIER EMBEDMENT LATERAL LOADS-SIMPLIFIED METHOD
Constrained at Ground Surface – Design Equation CASE 1: (Soil properties vary linearly with depth) pz = 4z2MG/d4b ≤ Soil Lateral Strength (at all depths)
pz = soil lateral pressure at depth z MG = ground surface post moment VG = ground surface post shear z = depth below grade d = total post embedment depth b = post dimension bearing against soil Soil lateral strength from geotechnical soil analysis or prescriptive design tables in PFBDM
POST/PIER EMBEDMENT LATERAL LOADS-SIMPLIFIED METHOD
Constrained at Ground Surface – Design Equation CASE 2: (Constant lateral soil properties with depth) pz = 3zMG/(d3b) ≤ Soil Lateral Strength
POST/PIER EMBEDMENT LATERAL LOADS-SIMPLIFIED METHOD
MGVG
P
d
dR
y
z
Ground surface
P
Point of rotation
Post or pier with width b
Soil forces
Soil with ES that does not change
with depth z
MGVG
P
d
dR
y
z
Ground surface
P
Point of rotation
Post or pier with width b
Soil forces
Soil with ES that does not change
with depth z
MGVG
P
d
dR
y
z
Ground surface
P
Point of rotation
Post or pier Soil forces
Soil forces
Soil with stiffness that increases linearly with depth z
MGVG
P
d
dR
y
z
y
z
Ground surface
P
Point of rotation
Post or pier Soil forces
Soil forces
Soil with stiffness that increases linearly with depth z
Case 1. Soil properties vary linearly with depth below grade
Case 2. Soil properties are constant throughout embedment depth
Non-constrained at Ground Surface – soil pressure dist
See ANSI/ASAE EP486.2 or PFBDM for Design Equations
Constrained at ground surface – Spring Model
POST/PIER EMBEDMENT LATERAL LOADS- UNIVERSAL METHOD
z
Post contacts ground surface
restraint
t1
t2
t3
t4
t5
1 Fult,1
Fult,5
Fult,3
Fult,4
Fult,2
MU
VU
2
3
4
5
z
Post contacts ground surface
restraint
t1
t2t2
t3t3
t4t4
t5t5
1 Fult,1
Fult,5
Fult,3
Fult,4
Fult,2
MU
VU
2
3
4
5
Foundation moment and shear capacity from basic mechanics
POST/PIER EMBEDMENT LATERAL LOADS- UNIVERSAL METHOD
Non-constrained at ground surface – Design Criteria
Foundation moment and shear capacity from basic mechanics
Design Articles in Frame Building News • Bohnhoff, David. 2014. Modeling Soil Behavior
with Simple Springs, Part 1: Spring Placement and Properties. Pages 49 to 54. April.
• Bohnhoff, David. 2014. Modeling Soil Behavior with Simple Springs, Part 2: Determining the Ultimate Lateral Capacity of a Post/Pier Foundation. Pages 50 to 55. June.
POST/PIER EMBEDMENT LATERAL LOADS- UNIVERSAL METHOD
POST/PIER EMBEDMENT DESIGN: UPLIFT RESISTANCE
Mass of soil in
shaded zone resists post withdrawal due
to uplift forces
Post must be mechanically
attached to the collar or wood
cleat Mass of attached collar or wood cleat
Bu
Governing Design Equations Weight of attached collar/footing + Weight of soil above attached collar/footing ≥ Design uplift load of post frame
POST/PIER FOUNDATION EMBEDMENT – UPLIFT LOADS
POST/PIER FOUNDATION EMBEDMENT – UPLIFT LOADS, U
dUh
dU
BU
γ (dU – h)
Shallow foundation
Deep foundation
Failure plane
Uplift resistance
dUh
dU
BU
γ (dU – h)
Shallow foundation
Deep foundation
Failure plane
Uplift resistance
Shallow vs. Deep Foundations
Shallow: du ≤ h Deep: du ≥ h (h dependent upon soil internal angle of friction)
Ultimate uplift resistance of soil above circular anchorage systems
Cohesive Soils: U = γdu(Bu
2π/4-Ap) + FcSuBu2π/4
du = post embedment depth
γ = soil density Bu = anchor diameter Ap = post cross sectional area Fc = breakout factor for soil uplift (1.2du/Bu) Su = undrained soil shear strength
POST/PIER FOUNDATION EMBEDMENT – UPLIFT LOADS
U-value equations provided in ASAE/ANSI EP 486.2 and PFBDM for additional cases
• Cohesive soils – rectangular uplift anchors • Cohesionless soils – circular uplift anchors (shallow and deep
foundations) • Cohesionless soils – rectangular uplift anchors (shallow and deep
foundations)
POST/PIER FOUNDATION EMBEDMENT – UPLIFT LOADS
POST/PIER FOUNDATION DESIGN: UPLIFT DESIGN
• Design Equations for Uplift Resistance of Embedded Posts with Uplift Anchors
1. Post Frame Building Design Manual (2014 ed.) (www.nfba.org or www.postframeadvantage.com) 2. ANSI/ASAE EP486.2, Shallow Post & Pier Foundation Design (www.asabe.org)
• ANSI/ASAE (ASABE) EP 484 – Diaphragm design procedures
• ANSI/ASAE (ASABE) EP 486.2 – Shallow post & pier foundation design
• ANSI/ASAE (ASABE) EP 559 – Requirements and bending
properties for mechanically laminated columns
– asabe.org or nfba.org
POST-FRAME TECHNICAL RESOURCES
POST-FRAME TECHNICAL RESOURCES
Provides structural design procedures, commentary & design examples for post-frame building systems
OTHER PF TECHNICAL RESOURCES • Post Frame Construction Guide • Post Frame Construction Tolerance Guidelines
OTHER PF TECHNICAL RESOURCES
DAFI
www.postframeadvantage.com
or www.nfba.org
WWW.POSTFRAMEADVANTAGE.COM
WWW.NFBA.ORG
National Frame Building Association 8735 Higgins Road
Suite 3000 Chicago, IL 60631
MORE PF DESIGN GUIDANCE?
Ques+ons?#
This(concludes(The(American(InsRtute(of(Architects(ConRnuing(EducaRon(Systems(Course(
Harvey#B.#Manbeck,#P.E.,#PhD#
NaRonal(Frame(Building(Assn.([email protected](
Top Related