2011.05.11 - Design of Tilt-Up Wall Systems

Tilt-Up Construction

Part I - Planning

Greg Riley

Steven Schaefer Associates, Inc.

www.FindYourTechnology.com

Introduction

� Two part presentation:

� Part I – Planning

� Part II – Design

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� Target audience:

� “General” structural engineer that designs a wide variety of structure types. Engineer may or may not have tilt-up experience.

Introduction

� Definition:

� ACI 116R: “Tilt-up construction is a technique for casting concrete elements in a horizontal position at the jobsite and tilting them to their final position in the structure”

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tilting them to their final position in the structure”

Introduction

� Introduction to Tilt-up construction process:

1. Concrete slab on grade and exterior wall foundations (for the tilt-up walls) are placed.

2. The tilt-up wall panels are laid out on the slab on grade

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2. The tilt-up wall panels are laid out on the slab on grade (exterior face down on the slab). The formwork and rustication strips (architectural reveals) are connected to the slab on grade.

Introduction

� Formwork and reveals:

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Introduction


3. Bondbreaker is sprayed inside the forms on the slab on grade

4. Chairs, reinforcement, and embedded items (lifting/bracing inserts, embed plates for structural connections, etc.) are

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inserts, embed plates for structural connections, etc.) are placed.

5. Concrete placed, finished, and cured.

6. Forms removed

7. Crane connects to lifting inserts cast into interior face of wall panel and tilts panel up and places it on foundation

Introduction

� Panels with reinforcing:

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Introduction


8. Temporary braces are installed to support the wall panel prior to the crane releasing the panel

9. Structural connections made at foundation

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9. Structural connections made at foundation

10. Structural connections are made to diaphragm(s)

11. Braces are removed once LFRS complete

12. Non-structural work may occur at any time after erection

Introduction

� Lifting sequence:

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Introduction


10

Introduction


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Introduction


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Introduction


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Introduction

� History:

� Tilt-up has been used in the United States since the early 1900’s, but most of the development and momentum with using tilt-up as a construction technique is post- World War II.

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tilt-up as a construction technique is post- World War II.

Introduction

� History:

� Traditionally, tilt-up is thought of for large, warehouse structures

� The proper mindset:

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� Don’t limit yourself and the owner based on traditional ways

to construct something. Consider tilt-up and other

alternatives!

� Tilt-up can be economically and effectively used for a wide variety of structures: office buildings, theaters, churches, retail, etc. Also in non-building structures such as retaining walls, screen walls, signs, tanks, etc.

Introduction

� Industrial:

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Introduction

� Industrial/Warehouse:

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Introduction

� Office:

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Introduction

� Office:

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Introduction

� Office:

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Introduction

� Retail:

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Introduction

� The proper mindset (cont.):

� You are part of a design team.

� If at all possible, meet and discuss construction type possibilities with the architect and an experienced contractor early in the

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with the architect and an experienced contractor early in the process so a sound decision is made.

� Design-build obviously lends itself well to this or you need an experienced team for design-bid-build.

Step 1 – Is tilt-up a viable alternative?

� Crane access: Will there be plenty of room for the crane to move in and around the site? Truck crane costs can be ~ $3000/day, so efficiently using the crane is essential to the economy of a tilt-up building.

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essential to the economy of a tilt-up building.

� Building square footage: 5000-7000 sf is a decent rule of thumb for a minimum square footage size to economically consider tilt-up.


� Building wall height-to-floor slab width ratio is key. Ideally the panel height is less than ½ the building depth. Not a deal-breaker but definitely adds cost.

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� Building height:

� Don’t automatically disregard tilt-up if the building is multistory. A multistory panel isn’t any more difficult to design than a single-story panel.

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story panel.

� 3-story buildings are reasonable without upsizing the crane that would potentially be used. 4-stories would require a bigger crane than usual. For above 4 stories you will potentially need to stack the wall panels.


� Exterior elevations

� Will the exterior wall surface essentially be flat with some rustication joints?

� What percent of the wall is solid? As a rule of thumb, the walls

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� What percent of the wall is solid? As a rule of thumb, the walls should be at least 60% solid for tilt-up to be economical.


� Exterior elevations (cont.):

� Will curved panels be required?

� Can most of the panels be supported on foundations and not hung off adjacent panels?

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hung off adjacent panels?

� Modularity is important.

Tilt-up Comparisons

� Precast

� Is there a qualified precaster in the general area? Transportation costs aren’t cheap (especially these days).

� Transportation considerations also impact the precast panel

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� Transportation considerations also impact the precast panel dimensions that can be supplied. Precast panels are typically 8’ or 12’ wide, which can limit the openings and opportunities for architectural expression.

� The narrow panel width of precast panels can be a disadvantage compared to tilt-up if it is possible to use drilled piers and span the tilt-up panels pier-to-pier (no grade beam).

Tilt-up Comparisons

� Precast (cont.)

� More panel joints = more caulk and maintenance. Caulk joint life is ~ 5-7 years.

� Lead times: Lead times on precast and steel can vary. In

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� Lead times: Lead times on precast and steel can vary. In precast, you want the steel erected first and waiting for the precast. In tilt-up, the walls come first and the steel brought to the walls.

Tilt-up Comparisons

� Precast (cont.):

� Dimensional stability: Precast panels are frequently insulated. The result of the insulation between the exterior and interior panel face is a thermal gradient between the faces. The

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panel face is a thermal gradient between the faces. The resulting gradient can cause a bow in the panel. This bow results in problems where interior finishes are connected to the panels.

� Future flexibility: Tilt-up panels are typically somewhat easier to modify in the future for new or enlarged windows, doors, louvers, etc.

Tilt-up Comparisons

� Precast thermal bow:

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Tilt-up Comparisons

� Precast thermal bow:

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Tilt-up Comparisons

� Masonry:

� Dependent on local material costs and labor costs for masons.

� Building footprint and wall square footage is a big determinant. May need ~ 7,000-10,000 sf building footprint to make tilt-up a

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May need ~ 7,000-10,000 sf building footprint to make tilt-up a more economical choice.

� The taller the wall, the more economical tilt-up becomes relative to CMU.

� In general, a tilt-up building can be erected faster than a CMU building.

Tilt-up Comparisons

� Pre-engineered metal buildings (PEMB):

� PEMB’s are not really an apples-to-apples comparison with conventional structures.

� PEMB do not typically compare favorably to conventionally

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� PEMB do not typically compare favorably to conventionallyframed structures in terms of durability (future maintenance), future flexibility, and fire resistance.

� PEMB a great selection for owners whose primary desire is to have the lowest initial cost possible. Costs increase rapidly when trying to build in the features above into a PEMB.

Tilt-up Comparisons

� PEMB (cont.):

� It is possible to clad a PEMB with tilt-up. Requires additional coordination with PEMB manufacturer.

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Tilt-up Comparisons

� Wood structures:

� Similar to PEMB, not really an apples-to-apples comparison.

� If it can be done with traditional wood construction it is cheapest to do with wood.

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to do with wood.

Tilt-up Considerations

� Assume the design team has selected tilt-up for the construction of the project. Now what needs to be discussed and considered?

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� SEOR design vs. delegated design

� Lifting and bracing insert design. Most commonly done by insert manufacturer.

� Bracing design. Most commonly done by same engineer who

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� Bracing design. Most commonly done by same engineer who designed bracing inserts. They will specify a brace reaction to the slab on grade or potentially design the deadman/anchor if the brace is to the exterior.

� Is the slab on grade a structurally designed item or non-structural specified/chosen item?


� Foundations

� Strip foundations (trench formed) are the easiest (as compared to formed wall).

� Deep foundations are fine – no reason to disregard tilt-up as a

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� Deep foundations are fine – no reason to disregard tilt-up as a potential option if deep foundations are required


� Slab on grade (SOG) construction� Where are the panels going to be cast? Building SOG, casting

bed, stack casting? Contractor’s call based on available slab square footage and owners SOG desires.

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� Reference ACI publications for appropriate SOG design and construction techniques.

� 6” minimum


� Slab on grade (SOG) construction (cont.)

� Construction loads on the SOG will typically far exceed the in-use loadings for the slab on grade. Options:

� Keep tilt-up crane off slab

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� Keep tilt-up crane off slab

� Design thickened strips for crane travel

� Mandatory means/methods specifications for dunnage/cribbing under outriggers, keep outriggers off control joints (especially intersections), etc.

� You break it you bought it strategy. Sometimes you just have to repair or replace some slab.



� SOG is cast prior to building being enclosed. Pay special attention to slab and panel protection from the elements (wind, temperature, humidity). Plastic shrinkage cracking is especially

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temperature, humidity). Plastic shrinkage cracking is especially problematic (hot and windy days with high evaporation rates).


� Plastic shrinkage cracking on panels:

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� Bondbreaker is important between the panel and the SOG

� Make sure the bond breaker is compatible with the curing compound

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compound

� Combination bond breakers/curing compounds are available. Popular with contractors.

� Both bond breakers and curing compounds come in 2 general classes: Non-membrane forming vs. membrane forming.



� Joints are important as all joints/slab imperfections are reflected in the tilt-up panels

� Can avoid joints for casting, fill them, or attempt to cover them

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� Can avoid joints for casting, fill them, or attempt to cover them

� Lots of decent skim coats, etc. available on the market now. This is preferable to grinding imperfections

� Column locations: Covers with a thin layer of concrete on top, plywood/plastic covers, place slab over footing and core a hole for a column at a later date.



� The bottom line is that the SEOR on a tilt-up building needs

to pay more attention to the SOG and its details on than

they would on most other structures.

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they would on most other structures.


� Panels� How should the panel information be conveyed?

� Elevation format with reinforcing keys is conventional “structural detailing”

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� Tri-elevation format is closer to shop drawing level of detailing


� Tri-Elevation format:

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� Panels (cont.)

� Panel layout has many considerations - Set a maximum panel weight by balancing crane considerations with architectural considerations.

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considerations.

� Truck crane: Common panel weight limit is ~ 20T.

� Crawlers: Common panel weight limit is ~ 60T.

� You can upsize the size of the truck crane to increase the flexibility of the picks and minimize the number of set-ups.


� Panels (cont.)

� Quick panel thickness estimate in inches is vertical span in ft/4. Ideally keep h/t <50.

� Economy increases with modularity (e.g., window locations, joist

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� Economy increases with modularity (e.g., window locations, joist pocket locations, reveal strip locations, etc.)

� Ideally joists (and especially girders) don’t bear at panel joint locations

� Joist and girder bearing detail: Pockets vs. face mounted with seat angle


� Panels (cont.)

� Attempt to maintain ~ 2’ jamb widths (dimension from edge of opening to edge of panel)

� Avoid L- and T- shaped panels if practical as they’ll likely require

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� Avoid L- and T- shaped panels if practical as they’ll likely require strongbacks

� If possible, limit the reveal depths to ¾”. Make sure the reveals are chamfered to facilitate formwork removal and reduce tendency for crack formation

� Corner joint options: Lapped vs. mitered


� Panels (cont.)

� Chamfer corners of panels and openings for less spalls and cleaner look

� Joints between panels: ¾” pretty standard but ½” can be used

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� Joints between panels: ¾” pretty standard but ½” can be used for shorter panels.

� Concrete slump ~ 4”-5”. Use non-AE.

� Be extra careful with dimensions


� Panels (cont.):� Many panel finish options

� Paint

� LGMF/Board with stucco

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� LGMF/Board with stucco

� Form liners

� Thinset brick/veneer

� Exposed aggregate


� LGMF/Foam board feature:

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� Masonry feature:

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� Panels (cont.)� Insulation

� Industrial applications leave the lower 8’ left uninsulated

� Interior walls furred out and insulated in office/retail

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� Interior walls furred out and insulated in office/retail applications


� Panels (cont.)

� Sandwich panels

� Constructed with 2 wythes sandwiching an interior insulation wythe

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wythe

� May increase in popularity with the trend for increased energy efficiency

� Most popular now in extremely cold climates


� Panels (cont.)

� Sandwich panels

� Can be composite or non-composite

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� In non-composite, the inner wythe is structural

� In composite, a thermal gradient concern is present (similar to precast)

� If you use composite, make sure you are specifying a reliable connector product and have appropriate quality control measures in the field because it is big money to retrofit a failed system.

Questions prior to Part II?

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Questions prior to Part II?

Tilt-Up Construction

Part II - Design

John Ashbaugh

Steven Schaefer Associates, Inc.

www.FindYourTechnology.com

Tilt-Up Design Presentation Overview

� Code Evolution

� ACI 318, 14.8 - “Alternative Design of Slender Walls”

� Design Examples

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� Design Examples

� Misc. Design Topics

� Design Tips

Code Evolution

� Before Slender Wall Design Provisions

� ACI h/t limits resulted in uneconomical designs

� Example: Max h/t = 25 resulted in 14½” thick panel for 30’ tall bearing wall

1979 SEAOSC “Recommended Tilt-Up Wall Design”

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� 1979 SEAOSC “Recommended Tilt-Up Wall Design” (Yellow Book)

� Max h/t: 36 unstiffened bearing walls, 42 for stiffened bearing walls

� Included second-order effects

� 1982 SEAOSC/ACI “Test Report on Slender Walls” (Green Book)

� Full scale testing of tilt-up panels – showed stability under large deflections

� Report stated “no validity for fixed h/t limits”

� Report stated need for deflection limits (h/100)

Code Evolution

� 1988 UBC - “Alternate Design Slender Walls”

� Considered eccentric gravity load effects

� Considered P-delta effects

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� Included service load deflection limit (h/150)

� Basis of current ACI design procedure for slender walls

Code Evolution

� ACI 318-99, 14.8 - “Alternative Design for Slender Walls”

� ACI’s first “slender wall” design procedure

� Similar to UBC 97, but strength design

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� No h/t limits

� ACI 318-08, 14.8 - “Alternative Design for Slender Walls”

� Current “slender wall” design procedure

Code Evolution

� ACI 551.2R-10 – Design Guide for Tilt-Up Construction

� Expands on slender wall provisions of ACI 318 Section 14.8

� Provides a “comprehensive procedure for the design” of tilt-up wall panels

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wall panels

� Provides recommendations for various conditions not specifically covered in ACI 318

ACI 318, 14.8 – “Alternative Design of Slender Walls”

� Assumptions / Requirements – Section 14.8.2

� Simply supported axially loaded member subjected to out-of-plane lateral load, with max. moment & deflections at midspan

� Constant cross section over height of panel

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� Constant cross section over height of panel

� Tension-controlled (c / d < 0.375 – refer to ACI 318 R9.3.2.2)

� Vertical stress Pu/Ag at midheight ≤ 0.06fc’


� Assumptions / Requirements – Section 14.8.2 (cont’d)

� φMn ≥ Mcr Eq. (14-2)

where Mcr = fr S

'f

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where fr = 7.5 λ

� Concentrated gravity load distribution

� Bearing width + 2 vert / 1 horiz slope down to design section

� Not greater than spacing of concentrated loads

� Not extending beyond edges of wall panel

'fc


� Assumptions / Requirements – Section 14.8.2 (cont’d)

� Concentrated gravity load distribution: ACI 551.2R-11, Fig. 4.2

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� Design Moment Strength – Section 14.8.3

� φMn ≥ Mu Eq. (14-3)

� φMn is determined per ACI 318 Ch. 10

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φMn = φAse fy (d – a/2)

� Effective area of steel (Ase) accounts for increased bending moment resistance due to axial load

Ase = As + (Pu / fy) (h / 2d)


� Design Moment Strength – Section 14.8.3 (cont’d)

� Mu includes moment due to applied loads & due to P∆

� Fig. 3.1 from ACI 551.2R:

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� Mu can be determined using Iteration Method to account for P∆

� Mu = Mua + Pu∆u Eq.(14-4)

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� Mua = max. factored M at midheight due to lateral loads & eccentric vertical loads (does not include P∆)

� ∆u = Eq. (14-5)

crc

cu

IE

lM

48)75.0(

52



� Or, Mu can be determined using Moment Magnification Method

Μu = Eq. (14-6)

cu

ua

lP

M

5 2

−−−−

72

where Icr = Eq. (14-7)

and Es / Ec ≥ 6

crc

cu

IE

lP

48)75.0(

51

2

−−−−

3))(

2(

32 cl

cdd

h

f

PA

E

E w

y

us

c

s ++++−−−−++++


� Minimum Reinforcement - ACI 318, 14.3

� 14.3.2 - Min vertical reinforcement ratio

� (a) 0.0012 for #5 bars or smaller, fy ≥ 60 ksi

73

� (b) 0.0015 for other deformed bars

� (c) 0.0012 for WWR not larger than W31 or D31

� 14.3.3 – Min horizontal reinforcement ratio

� (a) 0.0020 for #5 bars or smaller, fy ≥ 60 ksi

� (b) 0.0025 for other deformed bars

� (c) 0.0020 for WWR not larger than W31 or D31


� Minimum Reinforcement - ACI 318, 14.3 (cont’d)

� 14.3.7 – In addition to min reinforcement, bars are required around windows, doors, and similar openings. Bars shall be anchored to develop fy at corners of opening.

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anchored to develop fy at corners of opening.

� Panel with 2 layers of reinforcement: not less than (2) #5’s

� Panel with 1 layer of reinforcement: not less than (1) #5


� Maximum Out-of-Plane Deflection – Section 14.8.4

� Max. out-of-plane deflection due to service loads (including P-D effects), ∆s, shall not exceed lc / 150

� If M > 2/3 M :

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� If Ma > 2/3 Mcr:

∆s = Eq. (14-8)

� If Ma ≤ 2/3 Mcr:

∆s = Eq. (14-9)

))3/2(())3/2((

))3/2(()3/2( crn

crn

cracr

MM

MM∆∆∆ −−−−

−−−−

−−−−++++

crcr

a

M

M∆)(

)(


� Max. Out-of-Plane Deflection – Section 14.8.4 (cont’d)

� Where…

∆cr = Eq. (14-10)gc

ccr

IE

lM

48

52

76

∆n = Eq. (14-11)

And Icr is per Eq. (14-7)

gcIE48

crc

cn

IE

lM

48

52


� Max. Out-of-Plane Deflection – Section 14.8.4 (cont’d)

� Eq. (14-8) accounts for a rapid increase in out-of-plane deflections when Ma > 2/3 Mcr

� ACI 318 commentary recommends the following load

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� ACI 318 commentary recommends the following load combinations for calculating service level deflections:

� D + 0.5L + 0.7W

� D + 0.5L + 0.7E

Reinforcing Steel Location

� One layer

� Vertical bars located at or near center of panel thickness

� Typical for solid panels

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Reinforcing Steel Location

� Two layers

� Vertical bars typically located minimum clear from each face

� Typical for panels with openings, and economical for some solid panels

� Provides significant increase in bending strength and stiffness vs. one layer

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� Provides significant increase in bending strength and stiffness vs. one layer

Design Example 1 – Solid Panel

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� Concrete & reinforcing steel properties:

� fc’ = 4000 psi

� γc = 150 pcf

'f

81

� fr = 7.5 λ = 474 psi

� fy = 60,000 psi

� Es = 29,000 ksi

� Ec = 57 = 3605 ksi

� Es / Ec = 8.044

'fc

'fc


� Panel wt. above design section:

(7.25 in/12)(150 pcf)(14 ft)(16.0 ft) / 1000 = 20.3 k

� For simplicity in this example we’ll only consider wind

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� For simplicity in this example we’ll only consider wind suction and only one design load case:

1.2D + 1.6W + 0.5 Lr

� Factored applied axial load at top of wall

Pua = 1.2 (0.45 klf x 14 ft) + 0.5 (0.6 klf x 14 ft) = 11.8 k


� Factored axial load at midheight of wall

Pum = 11.8 k + 1.2 (20.3 k) = 36.1 k

� Check vert. stress at midheight < 0.06fc’ = 240 psi

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Pum/Ag = 36,100 lb / (7.25in x 14ft x 12 in/ft) = 29.6 psi < 240

� OK

� Trial reinforcing: (18) #6 vertical bars, (1) layer centered in panel

As = 7.92 in2 & d = 3.625 in


� Check the design moment strength

Ase = As + (Pum / fy) (h / 2d)

Ase = 7.92 in2 + (36.1 k / 60 ksi)[7.25 in/(2 x 3.625 in)] = 8.52 in2

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Ase = 7.92 in + (36.1 k / 60 ksi)[7.25 in/(2 x 3.625 in)] = 8.52 in

a = = 0.895 in

c = a / 0.85 = 0.895 in/ 0.85 = 1.053 in

c / d = 1.053 / 3.625 = 0.291 < 0.375 �OK (tension controlled)

)/12)(14)(4(85.0

)60(52.8

'85.0

2

ftinftksi

ksiin

bf

fA

c

yse====


� Check the design moment strength (continued)

Icr = 3

))(2

(3

2 clcd

d

h

f

PA

E

E w

y

ums

c

s ++++−−−−++++

85

Icr = 8.044(8.52 in2)(3.625 – 1.053)2

+ (14 ft x 12 in/ft)(1.053)3 / 3 = 519 in4

φMn = φAse fy (d–a/2) = 0.9(8.52 in2)(60 ksi)(3.625-0.895/2)

φMn = 1462 in-k = 122 ft-k


� Check min. reinforcement per section 14.8.2.4

Mcr = fr S = 0.474 ksi [1/6 (14 ft x 12 in/ft)(7.25)2 = 698 in-k

φMn = 1462 in-k > Mcr �OK

86

n cr

� Check min. reinforcement per section 14.3.2

ρ = As / (bh) = 7.92 in2 / [(14 ft x 12 in/ft)(7.25 in)] = 0.0065

ρ = 0.0065 > 0.0015 �OK


� Check Mu using moment magnification (Eq. 14-6)

wu = 1.6 x 18 psf x 14 ft /1000 = 0.403 klf

Axial load applied to top of wall panel (previously calc’d)

87

Pua = 11.8 k

Factored moment, excluding P∆ effects:

Mua =

Mua = 0.403 klf (32 ft)2 /8 + 11.8 k (0.33 ft) / 2 = 53.5 ft-k

28

2ccuacu ePlw

++++



Axial load at midheight of wall, Pum = 36.1k (previously calc’d)

Factored moment, including P∆ effects:

M

88

Mu =

Mu = = 88.5 ft-k

< φMn =122 ft-k

� OK

144/)519)(3605)(48(75.0

)32)(1.36(51

5.53

4

2

inksi

ftk

kft

−−−−

−−−−

crc

cum

ua

IE

lP

M

48)75.0(

51

2

−−−−


� Check service load deflection with 1.0D + 0.5Lr + 0.7W

∆allowable = lc / 150 = (32 ft x 12 in/ft) / 150 = 2.56 in

Ig = (1/12) (14 ft x 12) ( 7.25 in)3 = 5335 in4

lM5 2)/1232)(698(52

ftinftxkin −−−−

89

∆cr = = = 0.558 in

Initial iteration service load moment (without P∆):

Msa = wl2/8 + (Paxecc)/2

Pa = 0.45 klf x 14 ft + 0.5 (0.6 klf x 14 ft) = 10.5 k

gc

ccr

IE

lM

48

5 2

)5335)(3605(48

)/1232)(698(54

2

inksi



� Check service load deflection (continued)

Msa = 2

)33.0(5.10

8

)32)(14018.0(7.0 2 ftkftftx++++

90

Msa = 24.3 ft-k < 2/3 Mcr = 38.8 ft-k … use Eq. (14-9)

∆s = (Msa / Mcr ) x ∆cr = (24.3 / 58.2) x 0.558 in = 0.233 in



Now including P∆ effects:

Ma = Msa + Psm ∆s

91

Psm = 10.5 k + 20.3 k = 30.8 k

Ma = 24.3 ft-k + 30.8 k(0.233 in/12) = 24.9 ft-k < 2/3 Mcr

∆s =(Ma / Mcr ) x ∆cr = (24.9 / 58.2) x 0.558 in = 0.239 in

Final ∆s = 0.24 in < 2.56 in �OK

Design Example 2 – Panel with Opening

92



� fc’ = 4000 psi

� γc = 150 pcf

'f

93

� fr = 7.5 λ = 474 psi

� fy = 60,000 psi

� Es = 29,000 ksi

� Ec = 57 = 3605 ksi

� Es / Ec = 8.044

'fc

'fc


� Panel wt. above design section acting on design strip:

(7.25 in/12)(150 pcf)(7 ft)(16.0 ft) / 1000 = 10.2 k

� For simplicity in this example we’ll only consider wind

94


1.2D + 1.6W + 0.5 Lr


Pua = 1.2 (0.45 klf x 7 ft) + 0.5 (0.6 klf x 7 ft) = 5.9 k



Pum = 5.9 k + 1.2 (10.2 k) = 18.1 k


95


Pum/Ag = 18,100 lb / (7.25in x 30 in) = 83 psi < 240 � OK

� Trial reinforcing: (5) #6 vertical bars each face, 1 ½” clr.

As = 2.2 in2 & d = 7.25 in -1.5 in – 0.75 in / 2 = 5.375 in





96


a = = 1.41 in

c = a / 0.85 = 1.41 in/ 0.85 = 1.66 in


)30)(4(85.0

)60(40.2

'85.0

2

inksi

ksiin

bf

fA

c

yse====



Icr = 3

))(2

(3

2 clcd

d

h

f

PA

E

E w

y

ums

c

s ++++−−−−++++

97

Icr = 8.044(2.4 in2)(5.375 – 1.66)2

+ (30 in)(1.66)3 / 3 = 312 in4


φMn = 605 in-k = 50.4 ft-k



Mcr = fr S = 0.474 ksi [1/6 (30 in)(7.25)2 = 125 in-k


98

n cr


ρ = As / (bh) = 2.2 in2 / [(30 in)(7.25 in)] = 0.0101

ρ = 0.0101 > 0.0015 �OK



wu = 1.6 x 18 psf x 7 ft /1000 = 0.202 klf


99

Pua = 5.9 k


Mua =

Mua = 0.202 klf (32 ft)2 /8 + 5.9 k (0.33 ft) / 2 = 26.8 ft-k

28

2ccuacu ePlw

++++





M

100

Mu =

Mu = = 40.0 ft-k

< φMn =50.4 ft-k

� OK

144/)312)(3605)(48(75.0

)32)(1.18(51

8.26

4

2

inksi

ftk

kft

−−−−

−−−−

crc

cum

ua

IE

lP

M

48)75.0(

51

2

−−−−


� Check service load deflection with 1.0D + 0.5Lr + 0.7W

∆allowable = lc / 150 = (32 ft x 12 in/ft) / 150 = 2.56 in

Ig = (1/12) (30 in) ( 7.25 in)3 = 953 in4

lM5 2)/1232)(125(52


101

∆cr = = = 0.559 in

Initial iteration service load moment (without P∆):

Msa = wl2/8 + (Pa x ecc)/2

Pa = 0.45 klf x 7 ft + 0.5 (0.6 klf x 7 ft) = 5.3 k

gc

ccr

IE

lM

48

5 2

)953)(3605(48

)/1232)(125(54

2

inksi




Msa = 2

)33.0(3.5

8

)32)(7018.0(7.0 2 ftkftftx++++

102

Msa = 12.2 ft-k = 146 in-k > 2/3 Mcr = 83 in-k … use Eq. (14-8)

∆s =

Where ∆n = = 9.18 in

))3/2(())3/2((

))3/2(()3/2( crn

crn

crsacr

MM

MM∆∆∆ −−−−

−−−−

−−−−++++

)312)(3605(48

)1232)(672(5

48

54

22

inksi

ftxkin

IE

lM

crc

cn −−−−====



∆s =

∆s = 1.31 in

)559.0)(3/2(18.9()125)3/2(672(

)125)3/2(146()559.0)(3/2( ininin −−−−

−−−−

−−−−++++

103

Now including P∆ effects:

Ma = Msa + Psm ∆s

Psm = 5.3 k + 10.2 k = 15.5 k

Ma = 12.2 ft-k + 15.5 k(1.31 in/12)

Ma = 13.9 ft-k = 167 in-k > 2/3 Mcr



∆s =

∆s = 1.62 in

)559.0)(3/2(18.9()125)3/2(672(

)125)3/2(167()559.0)(3/2( ininin −−−−

−−−−

−−−−++++

104

Continue to iterate using Eq. (14-8)…

Final ∆s = 1.7 in < 2.56 in �OK

Design Example 3 – Concentrated Gravity Load

105



� fc’ = 4000 psi

� γc = 150 pcf

'f

106

� fr = 7.5 λ = 474 psi

� fy = 60,000 psi

� Es = 29,000 ksi

� Ec = 57 = 3605 ksi

� Es / Ec = 8.044

'fc

'fc


� 12 ft wide “design strip” for concentrated gravity load

� Panel wt. above design section acting on design strip:

(7.25 in/12)(150 pcf)(12 ft)(16.0 ft) / 1000 = 17.4 k

107

(7.25 in/12)(150 pcf)(12 ft)(16.0 ft) / 1000 = 17.4 k


1.2D + 0.8W + 1.6 Lr


Pua = 1.2 (16 k) + 1.6 (22 k) = 54.4 k



Pum = 54.4 k + 1.2 (17.4 k) = 75.3 k


108


Pum/Ag = 75,300 lb / (7.25in x 144 in) = 72 psi < 240 � OK

� Trial reinforcing: (12) #6 vertical bars each face, 1½” clr.

As = 5.28 in2 & d = 7.25 in -1.5 in – 0.75 in / 2 = 5.375 in





109


a = = 0.75 in

c = a / 0.85 = 0.75 in/ 0.85 = 0.88 in


)144)(4(85.0

)60(13.6

'85.0

2

inksi

ksiin

bf

fA

c

yse====



Icr = 3

))(2

(3

2 clcd

d

h

f

PA

E

E w

y

ums

c

s ++++−−−−++++

110

Icr = 8.044(6.13in2)(5.375 – 0.88)2

+ (144 in)(0.88)3 / 3 = 1029 in4


φMn = 1655 in-k = 138 ft-k



Mcr = fr S = 0.474 ksi [1/6 (144 in)(7.25)2 = 598 in-k


111

n cr


ρ = As / (bh) = 5.28 in2 / [(144 in)(7.25 in)] = 0.0051

ρ = 0.0051 > 0.0015 �OK



wu = 0.8 x 18 psf x 12 ft /1000 = 0.173 klf


112

Pua = 54.4 k


Mua =

Mua = 0.173 klf (32 ft)2 /8 + 54.4 k (0.33 ft) / 2 = 31.1 ft-k

28

2ccuacu ePlw

++++





M

113

Mu =

Mu = = 53.2 ft-k

< φMn =138 ft-k

� OK

144/)1029)(3605)(48(75.0

)32)(3.75(51

1.31

4

2

inksi

ftk

kft

−−−−

−−−−

crc

cum

ua

IE

lP

M

48)75.0(

51

2

−−−−

Panel Base Connections

� Code language

� ACI 318, 14.2.8: “Transfer of force to footing at base of wall shall be in accordance with 15.8”

� ACI 318, 15.8: Forces at base of wall should be transferred

114

� ACI 318, 15.8: Forces at base of wall should be transferred using reinforcement, dowels, or mechanical connectors.

� ACI 318, Chapter 16 – Precast Construction

� R16.1.1: “Tilt-up concrete construction is a form of precast construction”

� 16.5.1.3 – Vertical Tension Ties: (b) “Precast wall panels shall have a minimum of two ties per panel, with a nominal tensile strength not less than 10,000 lb per tie”


� Code language (continued):

� 16.5.1.3 – Vertical Tension Ties: (c) When design forces result in no tension at the base, the ties required by 16.5.1.3(b) shall be permitted to be anchored into an appropriately reinforced concrete floor slab-on-ground.

115

floor slab-on-ground.

� R16.5.1.3: Base connections at shear walls “are designed to transfer all design forces and moments. The minimum tie requirements of 16.5.1.3 are not additive to these design requirements.”

� 16.5.1.4: “Connection details that rely solely on friction caused by gravity loads shall not be used”


� Code language (continued):

� ACI 551.2R Section 8.2 - In-Plane Shear / Resistance to sliding:

“Resistance to sliding forces can be obtained by a combination of friction between the bottom of panel and the footing, and connections to

116

friction between the bottom of panel and the footing, and connections to the floor slab or foundation (refer to ACI 318, Section 16.5.1.3)

“Where panels are subjected to seismic forces, the contribution of friction resistance may not be permitted by some buildings codes. In addition, connections between the panel and floor slab or footing is a compulsory requirement in many building codes, particularly for seismic forces.”


� Conclusions

� If calculated uplift, base connection shall be designed for uplift forces

� If no calculated uplift, minimum vertical tension tie connections shall be provided at panel base per ACI 318 Section 16.5.1.3

117

provided at panel base per ACI 318 Section 16.5.1.3

� ACI 318 Section 16.5.1.3(c): If wall panels anchored to slab-on-ground, is the slab-on-ground required to resist two 10,000 lb nominal tensile forces per panel? What is an “appropriately reinforced concrete slab-on-ground”?

� In-plane shear forces at base of footing can be resisted by a combination of friction and base connections, but not friction only


� Sampling of details from ACI 551.1R:

118



119

Panel & Floor / Roof Connections

� In addition to calculated gravity, uplift, in-plane lateral, and out-of-plane lateral forces, check minimum seismic anchorage forces (ASCE 7, Section 12.11 “Structural Walls and Their Anchorage”)

� Design connections according to provisions of ACI 318, App. D

120

� Ductility for seismic connections



121



122

Panel to Panel Connections

� Connections restrain shrinkage and thermal expansion/contraction… try to avoid

� ACI 551.1R recommendations:

� Design “with some degree of ductility to accommodate panel

123

� Design “with some degree of ductility to accommodate panel shrinkage and thermal expansion and contraction”

� Reinforcing bar anchors preferred over short headed studs

� Delay welding as long as possible to allow majority of panel shrinkage to occur

Panel to Panel Connections

� ACI 551.1R, Fig 7.14:

124

Design for Lifting & Bracing

� Analysis is often by supplier of embedded lift & bracing inserts

� EOR reviews submittal by embedded insert supplier

� Additional reinforcement &/or “stiffbacks” are sometimes required by embedded insert supplier

125

embedded insert supplier

� Multi-story, large openings with small jambs, and single layer reinforcement

� Review early in design to avoid costly measures later

� ACI 551.2R-10 states that further development of design procedures will be included in future editions

In-Plane Shear

� Typically resisted by tilt-up “shear walls”

� If high % of panel openings, may need to design as frames rather than solid shear wall elements

126

� Resistance to overturning (OT)

� Resistance to OT typically provided by panel wt., roof loads, and floor loads

In-Plane Shear

� Resistance to OT (continued)

� If panels alone do not provide OT capacity:

� Increase panel width &/or thickness

Anchor panel to foundation with tension tie connections, or

127

� Anchor panel to foundation with tension tie connections, or

� Connect 2 or more panels together to create a larger shear wall

� Resistance to Sliding

� ACI 551.2R states that resistance can be through combination of friction & connections to floor slab or foundation, but cautions that friction may not be allowed to resist seismic forces

Dock Walls / Retained Soil

� Lateral support is commonly provided by slab-on-ground, creating continuity as depicted in Fig 7.5 from ACI 551.2R:

128

Dock Walls / Retained Soil

� For large slab to roof span relative to slab to foundation span, continuity has often been neglected and panel is designed as simply supported from slab to roof.

� More rigorous analysis that considers continuity is presented in ACI 551.2R – Example B.6

129

ACI 551.2R – Example B.6

� Large horizontal forces in slab and at foundation must be fully developed

� Consider larger span from roof to footing that may occur temporarily during construction

Multi-Story Panels

� Example problem is presented in ACI 551.2R – Example B.5

� Moment diagram for continuous span is determined without considering P∆ effects.

� Moment magnifier is determined for each span per ACI 318

130

� Moment magnifier is determined for each span per ACI 318 Section 14.8.3 and applied to both max. positive and max. negative moments.

� Consider lifting stresses for larger spans that may occur temporarily during construction.

Large Concentrated Gravity Loads

� What are options if wall is compression controlled?

� Increase panel thickness

� Move panel joints &/or openings to provide more “b”

131

� Design & detail as “column” per ACI 318 Ch. 10

� Add pilaster at concentrated load

� Add steel column at concentrated load

Design Tips

� Repetition/modularity is key to efficiency & economy

� Look out for…

� Hanging/spandrel panels

132

� Panel to panel connections

� Panel hold-down connections at shear walls

� “L” and “T” – shaped panels

� Small jambs adjacent to large openings

� Heavy girder bearing on skinny jambs or panel joints

Design Tips

� Look out for… (continued)

� Lifting stresses for multi-story panels

� Dock wall & retained soil – details & load path at slab & footing

133

� Frequent openings may require moment frames (vs. shear walls)

� Tall & narrow shear wall panels (check OT)

� Reveal joints – may affects bar location for exterior face bars

� Out-of-plane deflection at small jambs adjacent to large openings

� Embedded plates near the ends of panels

Design Tips

� Look out for… (continued)

� Restraint created by panel to foundation (or slab) connections

� Corrosion projection may be required for panel to foundation connections

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connections

� Corner joint details at “big box” buildings (thermal expansion/contraction in roof)

� Too-small scupper size at parapets in “big box” buildings

� QC dimensions!!!!

� Site visit prior to first panel pour is recommended

Questions?

135

2011.05.11 - Design of Tilt-Up Wall Systems

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Transcript of 2011.05.11 - Design of Tilt-Up Wall Systems