IMPROVEMENTS IN TRUSSES FOR BRIDGES AND ROOFS
Year: 1885

page_text: 14
I, GEORGE HERNDON PEGRAM, of Wilmington, County of New Castle, and State
of Delaware, Civil Engineer, do hereby declare the nature of my invention for
IMPROVEMENTS IN TRUSSES FOR BRIDGES AND ROOFS," and in what manner the
same is to be performed to be particularly described and ascertained in and by the
following statement :-
My invention bas reference to Trusses for Bridges and Roofs; and consists in
certain improvements in their construction which are fully set forth in the
following specification, and shown in the accompanying drawings which form part
thereof.
It is claimed that trusses formed on the principles herein set forth combine
lightness, cheapness, durability and beauty, in a higher degree than in any of the
forms now in use. They are lighter because the members are arranged to carry
the loads with the least amount of material. The chords are far apart where there
is the largest moment to resist (as at the middle of a span supported at both ends,
and over the support in a cantilever) and approach each other at the point where
the moment is small. The truss is thus made deeper at the centre than in the
parallel chord bridges, and shallower at the ends. The sectional area is therefore
made more uniform and the theoretical weight more nearly approximated.
Beside this, the chord through its inclination relieves the web members of
shearing strains and thus reduces the weight of these members.
These advantages are gained in a degree in the curved chord trusses of the forms
now in use, which are known to be lightest. The advantages which the proposed
forms have over these are that the compression chord is made of short panels
which give it stiffness and therefore allows of a high unit stress, while in the
tensile chord the sections are made of the greatest practical length, thus reducing
the number of these tensile parts and also the work, as they may be eye-bars on
which the work would be, in a measure, independent of the length.
It is possible, and would indeed be the ordinary construction, to make the panels
of the compression chord equal, as well as those of the tensile chord, and this with
an advantage rather than a disadvantage to the web members.
In the forms of curved bridges now in use the web bracing consists of vertical
posts and inclined tie rods or posts and ties forming isosceles triangles in either of
which constructions the assumption of equal sections for one chord makes the
sections of the other chord unequal.

page_text: The greatest possible advantages are gained in the web members. At the middle
of a span the posts are small and it is necessary to make them as short as possible
in order to give them the necessary stiffness.
They have a low allowable unit stress and are not capable of transmitting the
shearing strain horizontally with economy. At the ends of a span the posts are
large and have a large allowable unit stress, and may be made to assist in
transmitting the shearing strain towards the piers.
In the form proposed by me, the posts near the middle are vertical or nearly so
and increase in inclination to the ends of the truss where they have the maximum
inclination. The lengths of all the posts may be made equal by modifying
the curvature of the chord to suit or with an arbitrarily assumed curve for the
chord by making the end inclined post about equal in length to the middle post.
This would ordinarily be done and we could have at pleasure a truss in which
the top chord panels the bottom chord panels and the posts are of equal lengths
respectively or a truss with the sections or panels of both chords equal respectively
and posts differing but little in length; or one in which the panels of one chord
are equal and those of the other chord differing but little in length, and in which
the posts are equal; or again a form in which the panels of one chord are equal
while the panels of the other chord and the posts, respectively differ but little in
length.
In the two ordinary forms of curved chord trusses herein before cited, the
vertical post truss has a large variation in the lengths of the posts, and the posts as
they become large and short and have a large allowable unit stress, are not made
to assist the web in transmitting the shearing strain towards the supports.
In the triangular braced truss there is also a great variation of length, and the
posts near the middle of the span are very long and slender and are further
subjected to reverse strains. This latter objection it is proposed to overcome by
the use of the tie rods as in the Pratt and Whipple trusses.
A glance at the figures shows that the forms have more beauty than those in
present use.
The saving in weight by making trusses deeper at the middle than at the ends is
easily demonstrated and generally recognized. In existing forms this saving in
weight is more than equalled by the increased cost of the shop work arising from
having posts and chord sections of various lengths. In the form proposed by me
it is possible to have all chord sections and all posts sections of equal length
respectively, or approximately so. There is also a saving in the number of parts
to be made, as the tensile chord requires less panels than in the other forms in
which the panels are of the same length, or nearly so in both chords.
Still further advantages arise from the fact that scientific principles are more
closely observed than in other forms of trusses, viz:-1st. In the compressive chord
it is desirable to make the length between supports as small as possible in order to
secure stiffness and admit of a high unit being used.
In existing forms this cannot be done without shortening the tensile chord panels
also which it is desirable to have long. In the form proposed the compressive
chord panels may be made of lengths independent of the tensile chord.
2nd. The posts at the middle of the truss having small sections and consequently
low allowable unit stresses should be vertical to make them as short as possible
as they are not capable of transmitting the shearing strain horizontally with
economy. At the ends of the truss however, the increased diameter of the posts
allows of a higher unit stress, provided it (the post) is not proportionally lengthened,
thus reducing the ratio between the allowable strain in the ties and that in the
posts and making it economy to have the posts assist in transmitting the shearing
strain towards the pier. By making the posts of the same lengths, decreasing or
slightly increasing in length from the middle or point of maximum chord strains to
the ends of the truss and increasing in inclination from said point to the ends, the
greatest possible advantage is taken of this principle,

page_text: 3rd. The chords are more uniform in section than parallel chord trusses and at
the ends have less waste of material, and also through their inclination relieve the
web of shearing strain.
Referring to the drawings:-
The compressive chord and posts are shown in heavy lines, while the tensile
members are in light lines.
The eleven figures have been properly proportioned for practical cases; other
forms might be devised but those shown are deemed to be the most important.
In Fig. 1 to 8 inclusive the chords are made of straight lines and circular curves
(a curved chord being understood to be one in which the panel points are in a
curve but which panel points may be connected by straight lines, according to the
general interpretation) and the center post is made equal in length to the end
inclined post.
Fig. 1 represents a thorough bridge truss in which the bottom chord is divided
into equal panels and the top chord is divided into equal panels of shorter length.
The tie rods are arranged as in the Pratt truss.
Fig. 2 is similar to Fig. 1 excepting that the chords are divided into nine in
place of eight panels, thus making a center panel in place of a center post and
illustrating the way the posts may be "nearly vertical."
The tie rods are arranged as in the Whipple truss.
Fig. 3 is a truss with both chords curved. The lower chord is divided into eight
equal panels, and arcs described from these panel points as centres using the middle
post length as a radius and cutting the top chord making sections thereof of slightly
different lengths.
Fig. 4 is a truss in which the bottom chord is curved and the top chord a
broken line, both chords being divided into equal panels respectively; and would
be well adapted to roofs.
Fig. 5 is a truss with both chords curved outwards, formed in the manner
described when speaking of Fig. 3. The dotted portion might be added to facilitate
the connection of the roadway with the abutments in deck bridges.
Fig. 6 is a truss for a deck bridge formed in the same general way as the others.
The dotted portions might be added advantageously in some cases.
Fig. 7 represents a cantilever.
Fig. 8 is a pivot span with broken line top chord.
Fig. 9 is a truss in which the bottom chord is divided into equal panels and from
these points of division arcs are described with the center post length as a radius
and the top chord panels made by starting from the top of the center post and
intersecting the arcs thus formed by arcs described with a chosen top chord panel
lengths as a radius, each intersection forming a centre for the next arc. In this
way the truss is made to have all bottom chord panels, all top chord panels and all
posts of equal lengths respectively.
Fig. 10 is a truss for a deck bridge formed in the same way as Fig. 9, in which
the top chord is taken as the starting line.
Fig. 11 is a truss in which the middle portion is formed as a Whipple truss with
parallel chords and the ends according to the construction given for Fig. 9.
Any of the figures for simple spans, (Figs. 1, 2, 3, 4, 5, 6, 9, 10 and 11) if
inverted and supported at the centre would represent a pair of cantilevers or a
pivot span.
A represents the compressive chord and B the tensile chord. C are the posts
and D the ties. The compressive chord A.has its panels A' A' A³ A' &c. of
shorter length than the panels B¹-B of the tensile chord B and the posts C
connect the said chords between the juncture of two adjacent panels, and as the
panels of one chord are longer than those of the other, the posts will be inclined
converging above or below the truss. It has been stated and can be easily proven
that the forms shown are lighter than the truss forms now in common use, and an
inspection of the figures, will show that they are cheaper in the shops, through the
small differences in lengths of the parts of one kind and by making the tensile

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members long and reducing the number of these parts. Other points of economy
come in with the facility with which changes may be made in the form to suit
special cases without extra cost.
Thus, Fig. 3 represents a bridge where height under the bridge is gained in the
middle as in a highway bridge over a canal. Fig. 4 would serve for a roof with
straight ousides and a vaulted ceiling below. With railway bridges, on a grade
the posts are required to be plumb, thus destroying the symmetry of an ordinary
Pratt or Whipple truss, but in the trusses proposed the various inclinations of the
posts would render it unnecessary to make any changes for grades. As regards
durability, it is believed that a bridge with curved chords will more readily adjust
itself to inequalities of expansion caused by changes of temperature.
The avoidance of reverse strains in the members is conducive to the "life" of the
material, and ability to adjust tension members, and keeping all parts in tight
working contact avoids the play and consequent wear which is common to all.
bridges having members subjected to reverse strains, i.e. alternating tension and
compression.
The compressive chord must have the shorter panels, and this rule applies in all
cases, for in either of the constructions shown in Fig. 1 to 11 inclusive except 7
and 8, the upper chord is the compressive member, but if we invert either of said
constructions and use them as pivot trusses unsupported at their ends, or so
arrange them that they are supported at their middle or using one-half of one truss
as a cantilever, then the lower chord becomes, the compressive member, and the
strains are transmitted to the central support. In the former case the posts will
converge above the trusses and in the latter below the same, and in both cases
the posts have an increasing inclination, said inclination increasing from the point
of maximum chord strains to the ends of the truss. From this it is seen that
in trusses supported at the ends the posts of least inclination will be at the greatest
distance from the supports or abutments while in trusses supported in the middle
or in cantilever spans they will be near or over the support as shown in Figs. 7
and 8. I do not limit myself to any particular arrangement of tie rods, as various
system may be used with this construction of upper and lower chords and posts,
neither do I confine myself to the use of panels of a uniform length in each chord, as
they may be made of different lengths without altering the principle involved. It is
also evident that the end panels of the lower or upper chord may be of any desired
length and independent of the length of the other or intermediate panels, to suit
skewed masonry and the method of support at the abutments.
It is however necessary to the construction of my improved truss that either one
or both chords be curved but not parallel and in which the respective panels in the
upper and lower chords are of different lengths excepting in the case of the end
panels as stated above. It is sometimes advisable to construct one portion of a
truss on one principle and another portion on another principle, hence my invention
comprehends a truss in which any portion of same is constructed under the herein-
before described principle.
Having now particularly described and ascertained the nature of iny invention.
and in what manner the same is to be performed, I declare that what I claim is
1st. A bridge or roof truss or a portion thereof having one or both chords curved
or broken lined, but not parallel, said chords being the greatest distance apart at
the point of the greatest moment. (being at the middle of a span supported at both
ends and over the support in a cantilever), and further being connected by posts
which are vertical or nearly vertical at the point of greatest moment and increase
in inclination towards the ends of the truss; the posts forming with the top and
bottom chords, quadrilateral panels, in each of which the section of the tensile
chords is longer than the corresponding section, of the compressive chord.
2nd. A bridge or roof truss or a portion thereof having one or both chords curved
or broken lined, but not parallel, said chords being the greatest distance apart at
the point of greatest moment. (being at the middle of a span supported at both

page_text: ends and over the support in a cantilever), and further being connected by posts
which are vertical or nearly vertical at the point of greatest moment and increase
in inclination towards the ends of the truss; the posts forming with the top and
bottom chords, quadrilateral panels, in each of which the section of the tensile
chord is longer than the corresponding section of the compressive chord; the
intersections of said chords and posts being connected by tie rods and counter tie
rods arranged substantially as in the Pratt and Whipple trusses, or in such a way
that the web members cannot be subjected to reverse strains by different positions
of the loads.
3rd. A bridge or roof truss or any portion thereof having one or both chords
curved, but not parallel, in which the panels of the compressive chord are of equal
lengths measured on the chord line, and in which the panels of the tensile chord
are of equal lengths, except the end panels of either or both chords which may be
of greater or less length than the intermediate panels, the said intermediate panels
of the tensile chord being longer than the intermediate panels of the compressive
chord "and in which the posts are verticle or nearly verticle at the middle of the
truss and increase in inclination as they approach the ends."
4th. A bridge or roof truss or any portion thereof having one or both chords
curved, but not parallel, in which the respective panels of the top and bottom
chords are of different lengths, and the respective points of connection of adjacent
panels in said chords being connected by posts of substantially equal lengths.
5th. A bridge or roof truss or a portion thereof having one or both chords curved
but not parallel, in which the panels of the compressive chord are of equal lengths
measured on the chord line and in which the panels of the tensile chord are of
equal lengths measured on the chord line and in which the posts are substantially
of equal lengths.