modern'post+frame'structural'...

Modern'Post+Frame'Structural'Design'Prac6ces:'An'Introduc6on'

Presented(on(November(12,(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(InsPtute(of(Architects(ConPnuing(EducaPon(Systems((AIA/CES),(Provider(#G516.((!Credit(s)(earned(on(complePon(of(this(course(will(be(reported(to(AIA(CES(for(AIA(members.(CerPficates(of(ComplePon(for(both(AIA(members(and(nonZAIA(members(are(available(upon(request.(((

This(course(is(registered(with(AIA(CES(for(conPnuing(professional(educaPon.(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(construcPon(or(any(method(or(manner(of(handling,(using,(distribuPng,(or(dealing(in(any(material(or(product.(___________________________________________

QuesPons(related(to(specific(materials,(methods,(and(services(will(be(addressed(at(the(conclusion(of(this(presentaPon.((

(

Course'Descrip6on'((((This(session(is(intended(for(architects(and(designers(who(want(to(understand(basic(structural(design(methods(for(an(engineered(postZframe(building(system.(Without(delving(into(engineering(details(or(calculaPon(procedures,(it(covers(the(components(of(the(postZframe(building(system,(as(well(as(key(structural(design(concepts.(Two(approaches(are(highlighted(in(parPcular:(one(for(postZframe(systems(without(diaphragm(acPon,(the(other(for(postZframe(systems(with(diaphragm(acPon.(Procedures(for(designing(isolated(pier(foundaPons(for(postZframe(buildings(are(also(discussed,(as(are(technical(resources(available(to(the(design(professional.(

This!presenta,on!was!developed!by!a!third!party!and!is!not!funded!by!WoodWorks!or!the!so8wood!lumber!check;off.'

Learning'Objec6ves'

1.  IdenPfy(the(primary(structural(components(of(post(–frame((PF)(building(systems(

2.  Learn(basic(procedures(for(conducPng(structural(analysis(of(PF(systems(with(and(without(diaphragm(acPon(

3.  Define(the(design(approach(for(isolated(post/pier(PF(foundaPons(

4.  Recognize(postZframe(design(resources(available(to(architects(and(engineers(

•  Identify the primary structural components of post-frame (PF) building systems

•  Learn basic procedures for conducting structural analysis of PF systems with and without diaphragm action

•  Define the design approach for isolated post/pier PF foundations

•  Recognize 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?

Ques6ons?'This(concludes(The(American(InsPtute(of(Architects(ConPnuing(EducaPon(Systems(Course(

Harvey'B.'Manbeck,'P.E.,'PhD'NaPonal(Frame(Building(Assn.(hmanbeck@psu.edu(

This!presenta,on!was!developed!by!a!third!party!and!is!not!funded!by!WoodWorks!or!the!so8wood!lumber!check;off.'