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Charts simplify design of vertical formwork

Select or check sheathing, studs, and wales without tedious calculations

BY PAUL H. SOMMERS CHIEF ENGINEER ALGERNON BLAIR INCORPORATE MONTGOMERY, ALABAMA

HOW TO USE THE DESIGN CHARTS FOR VERTICAL FORMS

Start in the upper left-hand corner. The dotted line shows a sample problem which has already been worked out. 1. CHOOSE RATE OF POUR. Enter at the top of the upper left-hand chart. Go down from the selected rate of pour (Point A) to the line representing the expected concrete temperature (Point B). Many contractors select 50 degrees F for cool weather and possibly 70 degrees for warm weather. 2. CHOOSE PLYWOOD THICKNESS. Two choices are available in this chart. Draw a horizontal line from Point B across to the curve for the desired plywood thickness (Point D). Notice that the concrete pressure can be read at Point C, where this line crosses the border of the first chart. ALTERNATE: You could assume a pressure and start from a point at the edge of the chart, and then later adjust the rate of pour with temperature to establish the desired pressure level. 3. FIND STUD SPACING. From Point D draw a vertical line downward to the matching plywood thickness in the next chart at Point F. Read at Point E the maximum plywood span. THIS IS ALSO THE MAXIMUM STUD SPACING. Remember the assumption that plywood is used the strong way with face grain at right angles to the supports. 4. FIND SPACING OF WALES. From Point F draw a horizontal line to the left into the adjoining chart until it intersects the cur ve for desired size of stud (Point G). With the plywood and stud sizes selected, draw a vertical line downward from Point G to intersect the companion curve in the chart below at Point K. As you cross over the chart border, read the stud span at Point H. THIS IS ALSO THE WALE SPACING. 5. FIND SPACING OF FORM TIES. From Point K draw a horizontal line to the right to intersect the solid line representing the desired size of wales. Mark the Point M, and draw a vertical line down to the bottom of the chart at Point P. Read the maximum span of the wales here. THIS IS ALSO THE MAXIMUM SPACING OF THE FORM TIES. 6. FIND LOAD ON THE TIES. Where this last vertical line intersects a dashed line corresponding to the wale size selected, mark Point N and draw a final horizontal line to Point Q on the right. READ THE TIE LOAD at Point Q. 7. SAFETY FACTOR FOR TIES. In selecting the strength of a form tie, remember that ACI 347 recommends a safety factor of at least 2.0 for most work.

F

ormwork can be simple and straightforward when the designer recognizes the fundamental structural requirements of each member making up the forming system. Each part of the formwork must be within the allowable limits for bending, shear, and deflection. Once these requirements are met, a structurally sound forming system can be built. By combining what would otherwise be lengthy calculations into a series of charts the answers can be found in a few minutes. Such charts for vertical forming are given on the opposite page. A similar set of charts for horizontal forms will be published in an early issue. The charts provide a quick reference and are especially intended for the smaller contractors. They will also prove valuable to the estimator who is preparing bids for formwork, and they will aid the contractor or engineer in preparing or checking shop drawings. The charts for vertical formwork provide information on the following: · Rate of concrete placement · Plywood thickness · Size and spacing of studs · Wale size and spacing · Form tie spacing and load on ties The charts do not cover stiffbacks needed for alignment, nor do they provide for bracing needed for lateral stability. Remember that the vertical forms must be strong enough to resist wind or other lateral loads. It is recommended that they be capable of resisting at least 100 pounds per lineal foot at the top of the form. Certain compromises were made in establishing the charts to take care of as many situations and options as possible without making them complex and unreasonable. They are in compliance with the American Concrete Institute standard,

ACI 347-78. Most of the data for the charts were taken from ACI Special Publication No. 4, Formwork for Con crete.

Pressure acting on vertical forms

The lateral pressure of the concrete was based on formulas in ACI 347-78, Section 2.2.2. For these charts, maximum rate of placement is limited to 10 feet per hour. Fo rm w o rk with rates of placement faster than 10 feet per hour should be analyzed in greater detail.

DESIGN STANDARDS AND STRESSES

Listed below are the working stresses and some other assumptions used in preparing the charts.

SUMMARY Plywood sheathing

Plywood is assumed to be continuous over four or more supports, and it is used the strong way--that is with face grain parallel to the span. Bending stress f = 1930 psi Rolling shear vr = 80 psi Modulus of elasticity E = 1,500,000 psi These are stresses recommended by the American Plywood Association for Plyform Class I based on short term loading. Formwork for Concrete, ACI's Special Publication No.4, provides many design tables which reduce working time in the preparation of formwork shop drawings. Howe ve r, design charts like the one presented here can take even more mathematics out of the design work and reduce most calculations to simple arithmetic. It is hoped that this will help the craftsman or the small contractor to build forms with some engineering logic and not by empirical guesses.

Editor's note: Information in this article was condensed from a talk, "Simplified Design of Formwork," presented by Paul H. Sommers at a seminar on quality concrete construction sponsored by the ACI Kansas and Missouri Chapters at the ACI convention in Kansas City, September 27, 1983. Formwork for Concrete, by M. K. Hurd, referred to by Mr. Sommers, is also available from Concrete Construction Publications. It contains the complete text of the referenced ACI Standard 347-78.

Lumber or timbers

The wood members are assumed continuous over two or more spans. The allowable stresses are suitable for use with No.2 Douglas fir-larch subject to repeated usage or for No. 2 Southern pine which will have only limited reuse. Bending stress f = 1500 psi Ho ri zontal shear H = 140 psi Modulus of elasticity E = 1,700,000 psi

PUBLICATION #C840392

Copyright © 1984, The Aberdeen Group All rights reserved

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