Note: Descriptions are shown in the official language in which they were submitted.
JOIST GIRDER_BUILDJNG CONSTRUCTION
Back~round of the Invention
In recent years joist girder floor and roof
systems have become increasinyly more popular as a
structural system. Joist girders are a manufactured
S product and serve as a replacement for steel beams.
In general, economic benefits will result in the
substitution of joist girders for rolled beams in
floor and roof systems. Conventional engineering
practice is to design the joist girders as slmply
supported members, i.e. the ends of the joist girders
are free to rotate. The design procedure follows
design procedures established by the Steel Joist
Institute in Standard Specifications for Joist
girders adopted by Steel Joist Institute May 15,
1978, Steel Joist Institute, Richmond, Virginia.
Steel joists which support flooring material or roof
deck typically rest on the joist girders. The joist
girders in turn are typically supported on steel or
concrete columns. Typically, no attempt is made to
achieve beam continuity by connecting the ends of
the joist girders where they meet at a column.
The invention described herein relates to
the use of end ties for connecting the adjoining
ends of joist girders together, thereby providing
continuity between joist girders at the supporting
. ` '
" ~
, , . :
column. 'I'he purr)ose of usinq end ties ;s -to create
a horizolltal end force through -the ties to siynifi-
cantly reduce the axial forces in the upper and
lower chords so that lighter weight chord3 can be
emplo~ed to thus reduce the cost and weight of the
joist girders required to ~ccommodate the design
load.
Continui ty between adjoinlng structural
elements and beams has been used for many years.
For instance, steel beams are of-ten positioned over
the -tops of supporting columns in a continuou~ man-
ner, i.e. joined end-to-end. The use of continuous
stee] beams as opposed to simple span beams results
in the use of ~maller sized beams, thus reducing
weight and cost. During the ]ate 1950's plastic
steel design concepts were developed in order to
achieve an even qreater economic benefit in continu-
ous beam systems. This design concept is predicated
on a material propert~ characteristic of most
struc-tural steels. Specifically, the ability of
stee] to reach a given stress level (yield strength)
and then to 1OW plastically without an increase or
decrease in the sLress level. The plastic design
procedure makes use of this property by recognizing
that once a beam reaches yield levels at highly
stressed points the steel will "flow" and a re-
distribution of internal stresses wi]l occur. This
redistribution allows the designer to select beams
of less weight, which again reduces cost. In addi-
tion to the required steel behavior, I:he steel beammust possess certain geometrical cross~sectional
properties in order to permit the mentioned redistri-
bution to occur without premature beam flange of
beam web buckling. Should flange or web buckling
occur prematurely, then the beam will not reach its
'~2~
full predictcd load capacity and an inadequate
factor of safe-ty wowld exis1. Most s-teel beams
manufactured in the U.S. and Eoreign steel mills
have the required geometriral cross-sectional proper-
ti.es -to perm:it plastic design procedure.
Plastic design coneepts permit the selection
of a beam cross section to be hased on an ultimate
design moment of l6wL2; where w is the factored load
per foot (safety factor times design load), and L
is the beam span lengtll. This moment is the optimum
moment that can be used in design for a uniformly
loaded structural membe.r.
This optimum moment ean also be aehieved
~y using cantilever con.struction systems ~"drop in
systems"). These procedures have also been used
for many years with steel beams and also in some
cases with steel girde1~s. Unlike plastic design pro-
eedures, this method does not rely upon yielding
of the member or the reliance upor1.redistr.ibuklon
of stresses in the member i.n order to achieve the
optimum moment condition of l6wL , but rather by
~udiciously selecting the length of a eantilever
-from the support. Currently both the plastic design
technique and the eantilever construction method
are in common use for steel beams. rrhe Fish patent
2,588,225 illustrates the cantilever eonstruction.
Joist girders have not been designed using
plast.ie design procedures he~eause of very sp~cial
design precautions which must be fo].lowed. In l973,
Croucher and I proposed a eons~ruction in whieh
plastic design concepts eould be used for steel trus-
ses; Croucher and Fisher, AISC Engineering Journal,
First Quarter, 1973, Vol. lO, No. 2, pages 29 -32.
This eoncept required fixity of the ends of the
trusses to supporting eo].umns, with the yieldable
, ~.
.
~1,2~?D~
mechclnism being the end portions of the upper chord
This was made possi.ble by redesign of -the conventional
truss diagonal layout Howcver, slnce the required
geometrical layout ancl the eonnection requirements
5 are "non-standard" for fabrlcated trusses and for
st.eel joist girder fabricators, the procedure is not
readily used. Cantil.ever construction techni~ues
are occasionally used w.ith joist girders and trusses;
however, they have not met with wide aeceptance due
l.0 to connection costs and because they do not fit with-
ing standard product lines for ~oist gi.rder manu-
facturers.
By means of the present i.nvention, standard
joist girder yeometrical ].ayouts can be used, with
reduced chord sizes as compared to simple spans or
ful.ly continuous spans. I,oad (stress) redistribution
can be accomplished as in plastic design o beams with-
out eost penalty for connections or non-standard layout.
The tie connection an~les or plate.s can be designed
to yield a-t a predetermined moment so that a maximum
moment of l6w1, is created. The end result is a
significant weight savings in the joist girders
- without the penalty of high cost field connections
or ehanging existing standard geometrical layouts.
Tie plate connections that yield have been
used by desiyners of multi-story steel frames to
eonneet beams to columns. This eoncept of "semi-
rigid connections" or "wind eonneetions" has been
used to provide a given moment eapaeity at a beam
to eolumn joint. The conneetions are designed to
provide a given ~determined) moment resistance
from the beam to the eolumn. The present invention
is not used to transfer moment from a beam to
column, but rather to aehieve a load transfer
aeross the top of the column, i.e. ko transfer moment
~ ;
.
.~
-- 5 --
(force) from joist girder to joist girder.
Other prior art oE interes-t is Uni-ted States patent No.
3,793,790. In this patent the object is -to reduce the size oE the
column by using a deflection pad to reduce the column moment caused
by deflection of the lower chord of a joist girder under load.
Summary of the Invention
The invention provides in a joist girder construction
including a support element for supporting adjacent ends of joist
girders in which each joist girder has an upper chord, a lower
chord and vertical and diagonal members interconnecting said upper
and lower chords, the improvement to minimize the size of the
upper and lower chords of the joist girder for a predetermined
load comprising steel tie means connecting the adjacent ends of the
upper chords of said joist girders, said tie means including a non-
connected zone which affords plastic elongation and deformation of
portions of said tie means and said tie means being sized to yield
prior to said upper chord yielding and to transfer a sufficient
horizontal force through said tie means to reduce the chord force
said predetermined load causes within said joist girder, including
Eastening means for connecting said joist girders to said support
element, said fastening means retaining said joist girders from
separation from said supporting element but not interfering with
plastic elongation of said tie means, said fastening means afford-
ing relative sliding movement between said support element and said
upper chord.
The ties are of a predetermined size and can be in the
form of plates or angles and are attached to the adjacent ends of
the top chords of the joist girder by welding, bolting or other
, .
~2~;3~
- 5a -
suitable means. The ties have non-welded or unattached zones
intermediate the welded ends to obtain the benefit of plastic elon-
gation of the ties. Plas-tic elonyation will al]ow the ties to
reach and maintain a constant stress level and minimize premature
fracture of the tie connection. It is also necessary that the
joist girder seat not be tightly connected to the column or any
structure which would restrain the lateral movemen-t of the top of
the joist girder at the support location, which would minimize the
benefits of plastic elongation.
In addition, to obtain the maximum benefit of the
invention, adjacent ends of the bottom chords of the joist girders
must be connected together so that forces may be transferred from
one bottom chord to the other without significant elastic or
inelastic shortening. The ties of the invention thus allow the
joist girder to rotate or pivot about the bottom
i,,. , . :
chord at the support location, restrained only by
the ties connecting the -top chords. Based on a
given steel yield strength, the ties are
mathematical,ly si~ed to yield ~hen a gi,ven lo~d is
placed on the joist girder. I-laving reached the
yielded condition, a constant force is maintained
in the connec-tion -tie~ With the application of
additional vertical loads, the joist girders will
continue to def]ect and carry the additional load
as would a simple and conventionally supported
joist girder.
Further ob-jects, advantages and features
of the invention wlll be apparent from the disclosure.
Description of the Dr~ y~
Fig~ 1 is a fragmentary side elevational
view of a joist girder and column connection with
the top chord ties of the invention.
Fig. 2 is a plan view of t:he system shown
in E`ig. 1.
Fig. 3 is a plan view of a modified embodi-
ment of a tie.
Figs. 4A, B, C and D are force diagrams
~for difEerent conditions hetween the upper chords
of joist girders, with Figs. 4A and 4D representiny
prior art conditions and Figs. 4B and 4C illustrating
connections within the purview of the invention.
Fig. 5 ;s a fragmentary enlarged view of
the tie connection illustrated in Figs. 1 and 2.
.
Descri~t,ion of the Preerre~d Embodimerlt
~ lthoush l,he discl.osure hereoE is detai],ed
and exact to enabl.e those skille-J in the art to
practice the inventio~, the physical en~odiments
herein disclosed merely excmplify t'ne invention
which may be embodied in other specific structure.
While the best known embodiment has been described,
the details may be chanyed without departiny from
the invention which is defined by the claims.
Fig. 1 shows two ~oi.st girders 6 and 8
and an intermediate supporting element of column 10
along a single Eraming line in a structure. Most
structures which employ joi.st g.irclers would include
two or more of sucll frame lines. Joist girder seat
12 is attached by bolts ].3 to the column cap 4.
The bolts 13 secure the joist girders to the column
against wind uplit and facilitate assembly. The
bolts desirably extend throuyh 510ts 15 in the flange
9 of the gi.rcler sea-t or the column cap 4 which enable
plastic elongation of connecting ties as herein-
after described.
Also shown in Figs~ 1 and 5 is a steel
joist seat 55 resting on top of the joist girder.
The steel joist seat 55 is attached by bolts 56 to
the joist girder top chord. The bolts desirably
extend through slots 57 in the top chords 20, 22,
which also enable pla.sti.c elongation oE connecting
ties as hereinafter described.
Each of the joist girders 6 and 8 include
30 bottom chords 16, 18~ top chords 20, 22, vertical
members 23 and diagonal members 24. The bottom
chords 16, 18 are bolted or welded to a plate or
angle seat 30 which is fixed to the column 10. ~he
plate 30 can extend through the column 10. Alterna-
tively, the column 10 itself can provide the con-
' '
O
:
- 7(~)
nection between the lo~/er cllords 16 to t~o adjacent
joist girders.
In accordance with the invention, t:ie means
are employed to connect the acljacent ends of the top
chords 20 and 22. In ~he disclosed construction, the
means illustrated in Figs. l, 2 and 5 comprises short
lengths of angle stock 36, 38. The angle ties 36, 38
are welded to opposite sides of the top chords 20,
22 along weld zones 40, 42 along the top legs 47 of
the ties and the top edgP 49 of the upper chords 20,
22. The weld ~ones 40, 42 are separated by a non-weld
or plastic stretch æone 44 (Fig. 5). In Figs. 1 and
2, the vertical legs of the angle ties 36, 38 are
spac~d from the vertical leys of the top chords to
provide a space for the bolts 56. In Fig. 5, the
vertical legs are spaced from the top chords to ac-
commodate the bolts securing the steel joists and to
provide clearance Eor wide diagonals 24. In Fig. 5,
the mouth formed by the legs oE the angle ties is
facing the chords rather than facing outwardly as in
Fig. 1.
In the modi-fied embodirnent illustrated in
Fig. 3, the tie means is in the form of a plate 48
with weld zones 50, 52 connecting the plate 48 to
the top edges ~9 of the upper chords 20 and a non-
weld or plastic stretch zone 54. In the Fig. 3 embodi-
ment, the upper chorcls arc supported on seat angles
63 connected to the vertical sides of the column 10
rather than on the top of the column as illustrated
in Fig. l. The steel joist seat 58 is bolted at 61
or welded to the co]umn cap 59. Thus slotted holes
in the top chord of the joist girder are not required
for plastic elongation to occur in the ties. Slots
are required in the joist girder seat in Figs~ l and
5. The plastic stretch zone 54 is desirably equal
~ .
to 1.2W. For the angle stock, r/J i5 equal to the sum
of the adjoining lecJ lengths and for the plate 48,
W equals the width of the plate 48. The 1.2W parameter
is recommended in the de,sign o semi~rigid connections
for steel beams.
The funct,ion of the ties can be explained
using the force diagram~s 4~, 4~, 4C, 4D. The Figs.
4A and 4D illustrate the forcces in priox art joist
girder assemblies. Fig. 4C is also illustrative
of the truss design mentioned in Croucher and Fisher,
AISC Engineexing Journal, First Quarter, 1973, Vol.
lO, No. 2, pages 29 - 32. Figs. 4B and 4C illustrate
joist girder assernblies using the tie means of the
invention and a non-fixecl connect-on of the joist
girders to the supporting column, su-,h as with bolts
and slots as illustrated in the drawings. Fig. 4B
has lighter weight ties than Fig~ 4C and hence provides
less horizontal force than generated in the 4C con-
dition. However, the 4B conclition is arl improvement
over the prior art and within the purview of the in-
vention.
The chord force in the joist girder is equal
to the momen-t divided ~y the centroidal distance d
(the distance between the center of gravity of the
u~per and lower chords o the joist girder). A "simply"
supported joist girder without any tie plates or end
.
. .
,~ ,.
, ~ :
restraint which can rotate Ereely at its ends will
have a Eorce diagram as shown ln Fig. 4~ when sub-
jected to a uniformly distributed load or gra~ity
load. The maxlmum momen~ due -to this loacling will
5 OCCUl- at mid-span and will equal Ml 8wL , where w
is the load per foot of lenyt.h and L is the span
length. The chord force at the center of the joist
girder will be Ml divided by d. The size of the chord
selected depends upon the chord force. A joist
girder which is fully restrained at its ends, i.e.
welded or bolted rigidly to a column or to an
adjacent joist girder, will have a force diagram
as shown in Fig. 4D. The maximum moment will be
M~ 112wL2. The size of the chords for this situation
will be approximately fifty percent lighter than
for the "simply" supported joist girder illustrated
in Fig. 4A. This type of system is occasionally
used; however, the cost of fully welded or bolted
end connections may affect the cost benefits of
the chord weight savings.
By properly sizing the tie angles or tie
plates of this invention, the chord force can be
varied between the simple span case Fig. 4A and the
ully rigid case F'ig. 4D. As material is added to
thé connecting ties, thé shape of the force diagram
will change progressively, as shown in Figs. 4B and
4C. The optimum or balanced condition illustrated
in 4C can be achieved when the end moment equals
the interior moment M~ wL or the force transferred
through the ties equals the maximum chord force with-
in the joist girc3ers. This will result in minimum
chord forces and thus a minimum weight design for the
joist girder. In Fig. 4C, plastic elongation of
the ties provides the desirable optimum moment of
M 16wL . In a tie connection where there is no
. ' ~ .
_
,
. '
-- 10 --
plastic stretch zone, such a5 zone 44, because of
continuous welding of the ties to -the top chords
plastic 10w cannot occur. Thus redistributi.on of
forces cannot occur. Wi.hout redistribution,
designs must be predicated on the laryer force F2
(Fig. 4D) occurring in the tie connection and in
the joist girder chords. This requir~s more steel
in the chords as compared to F (Fig. 4C). Ilence
the steel savings is not as great as with theO 4C case.
In Fig. 4B, some horizontal forces are
present as compared with the "simply" supported
joist girder condition illustrated in Fig. 4A.
Hvwever, in Fig. 4s th~ chords would have to be5 sized larger than with the Fig. 4C tie condition.
Selection of the proper size of connecting
tie angles or plate to achieve optimum conditions
is accomplished as follows:
(1) The optimum end moment is first determined:
M-~wL
(2) Based on a selected depth of joist girder,
the force in the connecting ties is F=M/d.
(3) The area of connecting ties must equal
the force divided by the steel yield
strength.
(4) The connccting ties must then be attached
to each joist girder top chord in a manner
sufficient to transfer the force from
the top chords thxouyh the connection ties
and provide a plastic zone calculated to
be equal to 1.2W.
With the appropriate chord ties, significant
weight and cost savings result because optimum
moments are used, thus reducing the size of the
chords and hence the weight of and cost of the joist
3~ ~
-- 11 --
girders. In addition, standard joist girder geo-
metrical layouts are used which is advantageous -to
the manufacturer and a]so the ~ies are less costly
to use as compared -to full continui-ty connections.
~,
,
-
. . .
