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National Builder, 8, 11, pp. 38-40, 51, 1959-12-01

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Lightweight trusses bear up better than conventional roofsHansen, A. T.

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LIGHTWEIGHT TRUSSES

BEAR UP BETTER THAN

CONVENTIONAL ROOFS

BY A. T. HANSEN

REPRINTED FROM

NATIONAL BUILDER VOL. 8, NO. 11, NOVEMBER 1959, Pp. 38-40, 51.

RESEARCH PAPER NO. 88 OF THE DIVISION OF BUILDING \RESEARCH

NRC 5481 Ottawa December 1959 Price 10 cents

Division of Building Research o f th

original publisher. The Division would

be of assistance in obtaining such p

Money Order or a cheque made payable at p

in Ottawa, to the Receiver General of Canada,

Research Council, Ottawa. Stamps are

acceptable.

A coupon system has been introduced to

make payments for publications relatively simple.

Coupons are available in denominations of 5, 25

and 50 cents, and may be obtained by making a

remittance as indicated above. These coupons

may be used for the purchase of all National

Research Council publications including specifica-

tions of the Canadian Government Specifications

Board.

Building Research finds

weight trusses bear up bett

onventional roofs

Builders have long appreciated the advantages of truss roof construction,

but found it too expensive to meet accepted standards. Now, scientific

tests by the Division of Building Research find lightweight nailed W-

trusses stronger than the strongest conventional roofs and make possible

CMHC acceptance of much more economical types of trusses.

The use of wood roof trusses in house

construction offers many advantages

over conventional roof framing systems.

When trusses are used, much of the

work involved in the roof framing is

done either on the ground or in a shop,

under safer and more satisfactory work-

ing conditions.

With trusses, the house may be en-

closed more rapidly, enabling further

construction to proceed independent of

weather conditions.

Truss construction allows

design the plan of the house, i

of bearing partitions. In recent years, the u

es has become more popular. As t

use increased, there was need fo r m

information on the con

quate trusses. Sufficient

isted to enable the safe design o f tr

but it was found that trusses des

according to accepted engineering

tice were in general much stronger an - By A. T. HANSEN Since it is possible to enclose the more expensive t o build than conve Research Officer, house without first erecting the parti- a1 roof frames. There is a n inconsis Building Practice (Housing) Section, tions, the finish flooring, ceiling and per- in the strength requirements of the tw Division of Building Research imeter wall cladding may be installed types of systems. However there National Research Council

without having to cut and fit around appear to be n o

partitions o r manipulate materials struction to be st

through a maze of studding. ventional constru

satisfactory performance. But, while the

strength of conventional constructions

can be calculated approximately, only

limited test records exist on the actual

strength of such structures.

In recognition of the need for further

information the Division of Building Re-

search, in co-operation with the Forests

Products Laboratory of the Department

of Northern Affairs and National Re-

sources, undertook an extensive testing program to investigate the strength of

conventional joist and rafter construc-

tions with a view to developing truss

designs with strength and deflection

haracteristics equal to those of good

onventional construction. DBR Tested Two

Basic Types of

onventional Roof Frames

Two basic types of conventional roof

rames were load tested, with various

after sizes and different types of end

upports. The rafters and joists were

paced I6 inches apart and collar ties

laced across each pair of rafters. Table

shows the different types that were

ested, the types of end supports, and

e average failure loads for each type.

cause the magnitude of the failure

varied so greatly, no average value

the tests that

would be representative of conventional

construction in general. And since it was

necessary to have a representative value

to use as a basis for assessing the ade-

quacy of truss construction, it was decid-

ed to select the most commonly used construction. This is the type in which

the rafters and joists bear on a common

wall plate. The most common rafter

size is 2 by 6 inches. Table 1 shows that

this construction failed at roof loads of

from 62 to 108 lb. per sq. ft., depending

on the type of end support. (The roller

support and fixed support used repre-

sent the two extremes of lateral resist-

ance that might exist in house walls.) I t

was decided that if trussed construction

had failure loads of a t least 100 lb./sq.

TABLE 1 - Failure loads for various types of conventional constructions with different end

supports - 24" span, 5 / 1 2 slope, space at 16" O.C. (Pounds per square foot of horizontal

projection of roof area)

Rafter Size

2 x 4

2 x 6

2 x 8

18

-

46

18

-

46

56

62

89

72

108

125

FIGURE 2 Lumber- Douglos Fir - Construction G r a d e

- Eastern Spruce - C.L.A. N o . 1 , M.L.B. NAILED N o . 4

"W" Plywood-All plates are I/*'' sheathing G:ade

2'-0" O.C. Douglas Fir Plywood

Nails - Al l nails - 3" common

- A l l rows of nails are staggered in the direc- tion of the groin to keep splitting to o minimum

- N a i l s may be either clinched or unclinched

Note - T o ensure maximum stiffness, the upper chords must b e in good bearing contact a t the peak .

ft., they would be strong enough for use Designs were developed and tested in any area in which this type of con- for spans of 24, 26 and 28 feet with

struction had been successfully used in slopes of 4/12 and 5/12, since these

the past. are the most commonly used in current

DUR Departs From building practice. The designs are shown

National Building Code in Figs. 1 and 2. The structural member

It was realized that, in making this size:: shown were estimated to be the decision, a departure from the National minimum sizes that nould provide ade-

13~11lcling Code of Canada was made in quate strength. Nailing requirements

two respects - first in accepting for were determined to provide adequate house construction, trusses of a lower over-all stiffness and strength. The re-

strength than if designed by timber en- sults of the loading tests are shown in

gineering methods and, second - in Table 2 (see page 6 ) . It may be seen requiring such trusses to have a greater that the failure loads for these trusses

strength than some of the accepted but were considerably greater than the 100

lower strength conventional roof flames. lb./sq. ft minimum limit established in

It was thought, however, that considering tests on conventional construction. In all factors, these departures were well most cases the trusses were stronger than

justified. the strongest conventional construction.

Types of Trusses

Tested by DBR Additional tests were carried out to

As a first step in the developinent of

roof truss designs, several types of truss-

es which had previously been developed

in the United States were built and test-

ed. This was done to determine which

type offered the most promise from the standpoint of strength, economy of con-

struction, and ease of fabrication by

those not experienced in truss construc-

tion. Nailed W-trusses were selected for

test.

determine the effect of locating parti- tions at various positions beneath the

trusses. Although partitions influenced the over-all deflection characteristics of

the trusses, their presence did nos have any significant adverse effect on the truss

performance. It was found, however, that the magnitude of loads carried by

these partitions might be quite large,

depending on where the partitions are lo-

cated. When the partition is located at

E A S T E R N SPRUCE T R U S S

N A I L I N G S C H E D U L E

the third point of the span, the loads may

amount to 2/3 of the total load on the

roof and ceiling. This is considerably more than the loads that are carried on

bearing partitions under conventional

roofs and would suggest that partitions

at this location under trusses should be

designed as load bearing. This would be

especially important if the partition is

TABLE 2 - Average results of tests on trusses ( s

flection at mid-span of more than 1/360 of the

equol to 1/360 of the span, ofter 1 hour of loo

limit is commonly used es the maximum deflection

atories and the arch are avail- r Builders Bul- Housing Corporation. Copies may be Mortgage and obtained either from any branch office