HASIG SIIBVE EFFIGIEROY IH £ 15 MATERIAL 7078-76 CLAD os 5 ULTIMATE TENSION LOAD, 10UQ POUNDS 9
020 052 040 050 Q63 Ơn O80 O30 IOOO
SHEET THICKNESS (INCH) 4125,
Fig, D2.23 (Ref 1) Static Strength of Typical Singie
Spot Welds In Tension Using Star Coupons
CENTER TO CENTER SPOT SPACING EN INCE!
Fig D2 24 (Ref 1) Efficiency in Tension for Spot Welding Aluminum Alloys
PROBLEMS:
sg (4) Illustrates a welded plate fittl
it fastened to a round steel tube Sot
tting plate and tube are steel Fry =
O00 What is the maximum desien load P which the fitting car Se subjected to 1? a fitting factor of 1.2 is used Fitting is not subjected to vibration or rotation 9+ nings cin £ tape LenS ST ee Tome
Sams as Problem L but tube and fitting ts naat-treated after welding to Fry = 150,060, By Mise Pe Cravese OTS), 7 k—— 14% —x WASHERS _#, (7 P Fig A
A -.051, 2024-T3 aluminum sheet carries an ultimate tension load of 700 Lbs per inch
It ts spliced by a lap joint involving one row of spot welds spaced at 0.5 inch Is
the spot weld strength satisfactory A 7075-T6 aluminum sheet *
ultimate tensile stress ot Zoo00 ost
sheet is to be soliced Design a scot
welded joint for a leo joint
arries an ‘The
In a wing section involving skin and
stringers, the shear flow in HN
panels to a particular stringer is 400
600 lbs per inch, and acting in the sane
direction Assuming no diagonal tension action due to skin wrinkling, wnat spot spacing ts required to fasten the stringe
to the skin 12 ths skin is 04 thick
Trang 2CHAPTER D2 SOME IMPORTANT DETAILS IN STRUCTURAL DESIGN BY WILLIAM F McCOMBS (DESIGN SPECIALIST - CHANCE VOUGHT CORP.) D3 1 Introduction,
In the design and fabrication of an air- plane the major components receive a thorough
vaview and evaluation Many of the smaller
parts, however, are designed at the last minute and, not receiving so much attention, sometimes
have faulty details It is these which
frequently lead to trouble in service and in tests This chapter represents an attempt to point out some of the more common details that
seem, somehow, to be overlooked from time to
time This should be of help to those involved in designing or dealing {n other ways with the
structural components of airplanes or of
similar types of structures With regard to specific details, many aircraft companies have standard methods of design The reader should always consult his company’s data on these, if available In the event such are not available, the following suggested practices should be of
practical help D3,2 Shear Clips
There are nundreds of these in a typical
military airplane They are used in joining
together both primary structural components
and secondary structural parts such as equip~ ment mounting brackets, etc The function of the shear clip is to transfer a shear load from one part to another It is not intended to transfer axial load or bending moment or twist, only shear
A typical example is shown in Fig 03.1 Here bracket, or beam, (a) is supported by beams (b) and (c) The load P is thus "beamed out" to (b) and (c}, passing as a shear load
through the clips into the webs of (b) and (c)
as 111ustrated
Fig D3 2
When a significant axial load or bending moment must also be transferred, additional
members must de provided or the shear clip
must be replaced by a heavier fitting This
is illustrated in Fig 03.2 Here a beam, (a),
Fig D3.2
is cantilevered off of a heavy plece of
structure, (b) The load, P, passes through
the shear clip as a shear load from web (a)
to (b} There is also the bending moment,
Px li, to be transferred Additional splice
plates, "S", are provided for this purpose
They transfer the moment in the form of axial
loads from the flanges of beam (a) to member
{b)
Shear clips are usually seen in two forms;
(a) bent up sheet metal or extruded
angles (the angle being anywhere
from 0° to 180° between the legs) (>) extruded "tees" These shear clips are shown in their The "ninimum acceptable” form in Fig DS.3 “Minimum” Type Shear Clips Fig D3.3
minimum requirement {s that eacn leg or an
angle type clip must have at least 2 ?
through it The "top” of the tee type E
and its leg must also have at least 2 fasteners
each, as snown in (e) and (f}
The load “balance” of an angle type shear
clip is illustrated in Fig D3.4 The corner
edge of the clip should be assumed to carry
Trang 3
only the shear being transferred, taken as
1000 lp, in this figure The net loads on the
fasteners are then as illustrated Once tne
loads are known, the clip and fasteners can
be checked for strength using standard methods
Example:
Final Balance
Obtained by Let a=.40", b=1,0" Adding Above _ 4 aang
Together Then Q= 1000 x~p = 400# Resuitant Rivet Load:
Rx,/500* +4007 = 6404 Use 2-5/32" Alum, Rivets
Then clip thickness required is t = 032", T5ST Alc,
Fig D3.5 illustrates what will happen if
only one fastener {1s provided in 4 leg of an
angle clip The single fastener cannot
(ignoring friction) balance any shear at the
corner In other words, it can receive only a shear from the web to which it is fastened This, in turn, puts a twist, P x a, into the
other leg of the clip and, hence, into the
other wed This is unacceptable, of course,
since a much thicker leg would be needed to
carry the torsion, and an undue twist would be
present in the other wed being Joined Of
course, Several fasteners, rather than Just
two, may be used when space allows
Fig, D3.4
Clips of type (a), (c) and (e) in Fig DZ.3 are more efficient than are types (Dd),
(4) and (7) The latter are used when this ts
all that space limitations will allow In all
cases the dimension "a" should be kept as small 3s practical installation will allow
jst For loads on longer leg in figure, leta=.4", bz1.0",
|) \rwist=10004 42400" 4 This is required to balance the 1000# which
is out of the plane of
the longer leg This is unacceptable 1000 14004 SH 400# An 'Ưnacceptable” Type Shear Clip Fig D3.5
Another type of deficiency sometimes arises when a minimum type shear clip is being
used This is illustrated in Fig D3.6 where
it has been necessary to "joggle” one leg of
the angle clip, say to fit over some locally
thicker part of the member being attached If
the joggle is 2 1cant one, say to the \ i ì i ì f + fi 1 ` nw ot oy t fo “À^« Fig D3.6
order of the clip’s thickness or greater, it
can considerably reduce the clip’s rigidity and
cause it to function as 2 "one rivet clips" with the adverse twist effects mentioned
previously In this case at least 2 fasteners
should be provided on one side of the joggle
tn the joggled leg, as illustrated in Fig DZ.6(b), to maintain rigidity and proper
functioning The lead should be assumed to
be carried by the 2 fasteners above the joggle,
similar to case (b) or (d) in Fig 03.3
Joggles are discussed further in Art D3.4 D3 3 Tension Clips
These are also quite numerous in military airplanes, being used to splice relatively light tension loads from one member to another The tension clip is a very inefficient type of
splice It has a relatively poor fatigue life,
particularly, and should be used only when the load 1s small and other design factors prevent the use of the more efficient lap shear splice
It is usually resorted to when some structural member such as a bulkhead web or flange or fitting cannot be efficiently “opened up" to let an axtally loaded member pass
through It is also frequently used to attach cantilevered brackets to bulkheads or ribs or
other structure
Consider Fig DS.7 Member (a) is, say,
on one side of a bulkhead and is to be spliced to member (b) on the other side There is an axial tension load to be transferred and since the bulkhead cannot be cut, a tension clip arrangement must be used as shown Angle clips
Trang 4
ANALYSIS AND DESIGN
Dant up shaet
(9) double aneles (back 1 back)
(2) elips cut from extruded tse sections
S$ the strongest and stiffest for
To obtain maximum strength and stiffness,
bolts should be used for attachment purposes, Allowadle load data is given in Fig D3.8 for
single angle clip arrangement illustrated
YIELD LOAD FOR SINGLE ANGLES
BOLT SPACING BOLT HEAD CLEARANCE - INCHES
2 IN AND OVER 30! 1,3 1N si 1.28 IN 40 LIN ‘3s Bw 9 04 08 9 200 400 600 300 1000 THICKNESS OF ANGLE - INCHES —- YIELD LOAD PER BOLT - LBS NOTES; 1 In these tests the angles procrudea
LoAD at either end beyond the © of the { boit a distance of 1/2 the bait BOLT HEAD spacing
CLEARANCE 2 For thick angles the bolt may be
eritieal
AN-4 SOLT 1 a Values are for room temperature
AN960 eee Jeet ANGLE use only Fig D3.8 (Ret, Vought Structures Manual} ip thick- 1 and begin ÿ tO smaller solts the thicknesses ying action ac the yoe clip arrangement P, Segin to "open-up"
sels an increasing tension
gad, <, and the "toss" of the clin bear down OF FLIGHT VEHICLE STRUCTURES D3.3 Toe = ve Boit? - |i Ũ a) Bolt Ye i &) Fig D3.9
mors on each other From statics, taxing
moments about ths center of pressure on the a+d) a)
P Obviously a small anough 201t will yleld
or fail in tension before a thick clip will yield or fail in bending near the washer
(Molin = P xe} There is also a prying action in the tse type clip, 2s illustrated
toes, 2= Px ( Thus 2 is greater than
This prying action is the reason why th designer should be cautious in using rivets
even for light tension loads, as is sometimes
done When rivets are used, as in mounting equipment brackets, 1t is best to use steel
types and carefully check the prying load
maintaining an ample margin of safety In
any avent, riveted clips are inferior and no
design data tor them is given here
Another point in using tension clips is frequently overlooked The structure to whicn
the clip is attached must te capable of taking
the loads applied to it These loads consist of the tension load from the bolt and the load
from the toe action Several examples are
shown in Fig D3.10 In these examples the
term “unacceptable” means that the allowabis
leads of Fig D3.8 are not applicable ỊP he 4P 4p Ị † S Ỷ | ®) (e) dy Ý ‡ Y (e) | @ '
Heavy Light Back to Eccentric Criss-
Back-Up Back-Up Back Clip Load — Cross
Structure Structure Clips Path Clips
Accept- Unaccept- Accept- Unaccept- Unaccept-
able able able able able
Fig D3 10
Sases (b), (da) and (¢) require a rearrangement
Trang 5
structure which is receiving the load from the Some aircraft companies have specific clips in order to achieve the full aliowables strength data and practices for the design of af The resultant load on the back- Joggled members This should, of course, te
uD of course, the applied consulted by the designer, if available Some
ve companies use a 6:1 joggle length to depth
Tension clips, aside from having a low
static strength and stiffness, also exhibit a
relatively poor fatigue life If the load is
of such a nature as to occur many times, say due to symmetrical flight conditions such as
pull-ups and gusts, there should be a large
margin of ety at limit load levels in the
neighborhood of 200% or more If the load is
due only to some non-recurring type of loading,
such as a crash condition or "jammed" system
load, the large margin of safety would not be
necessary
ther suggested practices
are: involving tension c11ps
1 Keep the bolt head as bend radius or fillet
possible
close to the radius as is
2 Avoid their usaze, if
repeated loadings are possible, when dominant
3 l7 part of the structure is continuous
across a joint and part is interrupted,
do not use tension clips to join the
interrusted structure - instead a heavier, stiffer, machined ?itting is required
D3.4 Joggled Members
A "Joggzle" is an offset formed in a member
© usually involves one ‘or more flanges of 2
amber or the "open" cross~section type
oggles are quite common in typical metal irplane structures
‡
+
+
They are used most often
is desired to fasten together two eting members without using an extra
the joint The jJoggle ts a compromise
S$ an extra part but the price pald is 4
strength and stiffness of the Joggled
IZ tne load in the member at the foint
ugh, the saving is justified If
extra part, instead of or in addition
joggle, must be used A typical joggled
ation 1s shown in Fig D3.11 where one an angle member has Deen joggled over
stened to another member lying tn the lane wee we t 2 9 n Qo ct đ H ô 100M ct oes cr se be a ow ou H o Skin or Floor is Q Usually Present, Fig D3 11 Angle Joggied Flange
ratio, others use a 3:1 ratio, or both may be used, Strength or stizfness data for one ratio
should not be used blindly for another Some
of this data indicates that when, in the case of angle members, the depth of the joggle is to the order of the thickness of the joggled
leg or more, the loss in strength is about
equivalent to the loss of the leg outboard of
the bend radius A "rule-of-thumb" design
practice is therefore suggested as follows Assume that the net effect of the Joggle, from a strength and stiffness standpoint, is
equivalent to a slot cut into the joggled leg that extends inward to the bend radius tangent
point This is illustrated in Fig 03.12 A {c) OS cà (a) ®) ft IN View A-A Joggied Assumed 4 : Member Slot" Effective Section Through Joggle Fig D3 12
With this assumption, the flat portion of
the joggled leg will carry no axial load across
the joggled area but will provide support for
the curved element The effective net section,
Fig D3.12(c), can then be checked using
standard methods of analysis for whatever forces are acting on it It is obvious that
the net section shown will have little strength
for carrying bending moment normal to the re- maining leg Thus, care should be taken to
insure that any axial loads are introduced as
near the corner as possible - which in turn means that at least two fasteners should be
used on each side of the jogzle
The above approach, considering the joggle
equivalent to a slot, will give the designer
& much better "feel" of what he is really doing when he specifies 2 joggled member
basic reason ror the loss of strength anc
Trang 6ANALYSIS AND DESIGN OF FLIGHT VEHICLE STRUCTURES
The axial loads in the joggled leg being
inclined to each other require a balancing
load Such a balancing load is not available
except a3 shear in the thin leg and this
results in the less of stiffness and strength
If the symmetrical leg of a tee member were
joggled there would be more stiffness than in the case of an angle, out the same approach,
though more conservative, 1s recommended in
the absence of test data
AS an example of the foregoing discussion
assume that an angle member is supporting
another member locally which is loaded by the
forces Q as showm in Fig 03.14 If @ skin is
present, as shown, part of the load can be
carried across the joggle by the gusset effect of the skin This can be approximated by
using the methods of calculating inter-rivet
buckling of skins discussed in another chapter
The rast of the load must be carried across by
the net effective section of the angle in the jJoggled area Edge M Membar, "b'" Fig D3 14
Thus if the total load at the joint were 2Q
and the load carrying ability of the skin were R, then the net section of the angle would be subjected to a load P = 2Q - R and a bending moment M = 23 x a- Rxb, The stress at the lower curved edge would be the sum of the
compressive stresses,
1= Ƒ~ +i P
Tyet section “Net Section be a a
For ultimate strength, f¢ could be carried up to Foy, conservatively, in the typical case
In order to realize the maximum strength
and stiffness, the load in the net section must
ve applied in the "corner" This is to prevent stresses due to bending out of the plane of the remaining leg This requires that a minimum
of 2 fasteners be provided to receive the load
at the joint The reasoning here {gs the same as discussed in Art D3.2 concerning minimum type shear clips, and the fastener loads can
be calculated in the same manner 2s discussed
there
If the load is too large for ths net
D3.5
section to carry, then an additional member
should be provided locally Two ways of doing
this are shown in Fig 02.15
Sometimes local requirements are such as
to necessitate both legs of an angle member
being joggled In such cases it should be assumed that the angle has no significant load carrying ability at the Joggle Thus, the existence of any significant load at the Joint would require an additional member and the
angle should be ended just short of the joint
rather than joggled up onto it Add Member Cut from Tee Extrusion Add Unjoggied Angle Member Fig D3 15
The suggested effective net sections of members having other types of cross-sections
are shown in Pig D3.16 where the legs
indicated by dotted lines are joggied In gemeral if the joggle is slight, considerably less than the thickness of the joggled leg, its effect can be ignored, but proper fasteners should still be provided as discussed The
smaller the length to depth ratio used for
joggling, the greater the effect of the Joggle Joggled members lose stiffness and strength
when subjected to tension loads as well as
when under compression (but any skin present is, of course, much more effective as a gusset
than when in compression)
fn “ " tị
fig, D3 18 D3.5 Fillers
As the name implies, fillers are used to
fill up a void It is when they become a part
of the structural load path that they need particular attention Fillers also represent
an item that is quite common in typical large or complicated metal airplane structures
AS an example consider Fig 93.17 Here
two tees, "2" and "b" carrying axial lead,
P, are seen to be spliced togetaer by 2 pair
of angles "c" Since the lower leg of "Q" 1s
thicker than that of "a", 2 filler is needed
This filler ts part 2f the structural load
Trang 7"b0, nam nh đợt = t Load E=— View A-A fe Extended € Filler At Filler Fig D317 path, ?rom 7c" Yeslize 2ull st filler must oe "ext fasteners provided *
into memder “a", I said to bea "“Zloat later, a floating f cause a loss in fas
o "a" In this case, to
enzth of the fasteners, extended” “and additional
o tle the extended portion this is not done, it 1s z" filler As explained ler, 1f thick enough, will er strength t + " the 9 in 11 ten
In the above example let the total load
@ 8000 lbs and assume that 2000 lbs of from "c™ to "a" by the Tiller area The into filler by ce taken 4: fasteners in the load put
ing pressure can tne Q3 e1 c†t TÚ e2 a ? = “Filler Prasten * +t “pinier * “2 204 „€4 + 06 2000 x = 860 lb
Sufficient fasteners should be out in th
vended part of the filler to transfer this o los into member "a"
Thus, whenever a thick filler 1s inserted tetween two members being spliced together in
shear, the 7iller should be assumed to ve a part of one of them The part of the total
lice load {t will carry can be calculated as
illustrated above The fasteners can then oe 3znsidered as being in two sets One set must
the
ve the strength to splice total shear
rom the Single member to the combination us member" The ot Another examcle us = Hi Shear (Steel) Rivets 32 1 7075-T6 Slum Alloy Sheet Mtl, (All Members} Strengths: Rivet Shear = 1820+ Bearing In 072 = 16304 Bearing In 081 = 1840# Fig D3 18 ral with the lower So be integ
1 Rivets required to splice
"a" to combination 2000 ibs from clus filler: 3000 4
No Rivets = TE2O = 1,65, or Zz
2 Load carried by filler (to be spliced to Tp"): te 3 = : 2ooo( O72 = P = ——— = Frtiier = S000KE SET = a 1500 lbs 3 Fasteners required to transfer Pp ta "bt No Fasteners = ie = 292 or 1
Total Fasteners required =2+ 123,
Splice is adequate since 3 rivets are present
Had no filler been present, 2 fasteners would have sufficed Case Il P = 5000 lbs Repeating the same steps as Ụ require 3000 5 r 1 No Rivets required lê20 2.75 or 3 2 + Load in Filler = 5000x% 7 _._-Ắ .078 207 + ,072 =
3 No Rivets required = ae = 1.54 or 2
Thus the 3 rivets are required to transfer load, P, from "a" and 2 additional rivets are
needed to unload the filler into member "bd" The filler should be
additional fasteners dotted lines in Fig
extended over “b" and 2 added as shown by th
D2.18,
edly, the abOVe nracedure is
2 ; ĐỤt It provides 2 quick way of
3 tne effect of the filler when the ft thickness {s less than soout 15%
of the fastener diameter, {ts presence can
te ignored
The effect of the filler {ts to reduce
tne allowable Tength of the fastener The
reason for this can se sesn trom Fig 03.1 wherg tne oresence of the filler causes greater
prying leads and nence mora tension in the
Trang 8ANALYSIS AND DESIGN OF FLIGHT VEHICLE STRUCTURES — - a —> Sa ee Larger Prying
Pryin y=“ Forces Due to
zoree Eccentricity of Floating Filler
—
“Normal” Eccentricity
and Prying Forces, & Smaller
with No Fillers 3 ap Pyne Forces
with Structural (Extended) Filler Fig D3 19
structure around it That is, one should not
use a soft aluminum filler between high heat-
treated steel parts or a ohenolic or fiberglas
filler between aluminum parts The need for
fillers arises not only from design consider-
ations but frequently from manufacturing
problems In these latter cases "mis~-match"
between parts sometimes occurs in assembly To prevent expensive re-work, structural fillers
must be used to make the spliced area adequate
In these cases detailed attention is necessary in the occasional instances when floating fillers cannot be avoided, the fasteners should
have quoted allowables well in excess of the
Shear being transferred locally, if the riller
is of Significant thickness It is common
practice also to use a donding agent (glue) in addition to the fasteners in installing ?iliers
D3.6 Cut-outs in Webs or Skin Panels
The aircraft structure 1s continually faced with requirements for opening up webs
and panels to provide access or to let other
members such as control rods, hyaraulic lines,
electrical wire bundiss, etc., pass through
The destgner or liaison engineer should be
familiar with some of the various methods of providing structurally sound cut-outs
There are several ways of providing cut-
outs Three will be mentioned here These
are:
iding suitable framing members around the cut-out
Providing 2 doubler or “bent" where framing as in (a} cannot de done
e rụ ing standard round flangec
which have published 2110wab1as
cussed in chapter on beam
T
wy ou
o
(a) ming Sut-Outs in wads
AS an example assume that a deam wed
requires a cut-out as shown in Fig C3.20,
Fig D3 20
Before the cut-out was made the members shown by solid lines (flanges, stiffeners, weds) are present The members "a" and "b" are added to
frame the cut-out, as shown by the broken
lines
There are 2 ways to determine the loads
in the area framed around the cut-out The first is to assume a shear flow equal and opposite to that present with no cut-out (qd,
in the figure above) and determine the
corresponding balancing loads in the framed area Adding this load system to the original one will give the final leads and, of course, q = 0 in the cut-out panel The other method is to use standard procedures assuming the
shear to be carried in reascnable proportions
on each side of the cut-out The first method will ce illustrated here
All shear flows are the edge members (on the
in this discussion
shown as they act on
flanges and stiffeners}
If there were no cut-out there would be 2 constant shear flow, dg, in all of the panels,
as shown in Fig 03.Zla Next 4 shear flow
equal and opposite to that in the center panel of Fig (a) 1s applied to the center panel of Before Cut-Out | tag IP ime lJ 7218501] f 1/240 jh a0 it 1/249 | 17/1686 {7/44 Ij'118qo- 1
Self- Balancing Internal Loads (Due
Trang 9
Fig (b) Since this represents a self-
balancing lead system, no external reactions outside of the framing areas are required Tats is an important concept and the reader
These will add or subtract, depending upon their directions, to any loads
present before the cut-out was made
(as in the case of the beam flanges)
should think about it The loads in the framed
areas due to dg in (b) are next determined
To eliminate redundancies, it is usually
assumed that the seme shear flow exists in the
panels above and below the cut-out Tt 1s also
assumed that shear flows are the same in the panels to the left and right of the cut-out
a}
s)
“
the shear flow in the panels above
and below the center panel must statically balance the force due to
do, oF
Since IF, = 0, 40x%7 = 4x (5+3)
q = 7/8 I
the shear flows in the panels to the
left and right of the center panel
must also statically balance the
force due to qq
Since IFy = 0, 49x12 = ax (12+ 12)
q= 1⁄2 qo
the shear flows in the corner panels must also balance the force due to
the shear flow in the (any) panel between them Considering the panels in the right hand bay
BF, = 0; 1/2 dg X7 = 9x (5+3) 1
2 3X”? 4 s“Cg—° Tạ %
the final shear flows are gotten by
adding the values in (a) and (b)
together, algebraically Note that:
1 the shear flow in the center panel
(the cut-out) 15 q = 49 ~ dg =O, as it should be
2 the shear flows above and below and to the left and rignt of the cut-out add, giving a number
greater than the original qo
4 the shear flows in the corner panels are smaller than the original value of do
This is the way the changes always
occur in the area framed about a cut-
out
Finally, and importantly, there are
axial loads developed in all of the
framing members due to the cut-out
The axial loads due to the
can be gotten from Fig (b)
total axial loads in all of the members These are can be gotten from (c) illustrated in Fig 03.22 cut-out Or the P<P, -L2"x1g4o 7 P^P, +12"xïg đọ — Pa giên l1-8q l9 l6qg `
Axial Load Distribution in Upper Flange from Fig D3 21¢ fa) 9/18 qq 1-7/8 dg 9/16 qg 1-1/2 do \ 172 q Pa 2X Pxà 2 (1-1/2 9 - Hao 440) Compression
Axial Load Distribution in Framing Member Above Cut-Out Obtained from
D3.21e (Same Result Could be Gotten from Fig D3 21b) (b) 9/1640 | [1-1/8 PL =8"x(1-1/8qg~ 9/16 a9) 1-1/2 | axial ` -5(21/16 qọ) Tension ~1⁄2 qo ep =3"(21/16q9) Compression 3/46 qu|Ù , „ „z4 SoŸ`1-1/8qo
Axtal Load Distribution in Stiffener Bordering Cut-Out on Left Side, from Fig D3 2ic (Same Result Obtained from D3, 21b}
Fig D3 22
Once the internal loads are known, the members can be checked for strength
standard methods of stress analysis using
Tne cut-out could have been framed without extending the framing members into
the bay on the right of the cut-out This
case is tllustrated in Fig 03.23
Had the 7" deep cut-out veen required at the 5ottom of bay, the framing could have
been done with only one member (as c ould the
preceding cases a1s0) as illustrated in Flg
p3.24 This represents the minimum
adequate framing for any cut-out 7
there must be a minimum of one redis
of nat is, tribution
bay on one side of the cut-out and at least ‘two redistribution bays on the other side,
ane there must be the framing members de- fining the days
always be loaded axially These framing members will
Note that in the previous examples in
Case (b) the sum of the loads on all edge members (framing members) is Zero
loads are needed for equilicrium 7
Trang 10ANALYSIS
AND DESIGN OF FLIGHT VEHICLE STRUCTURES D3.9
Pa ty it Joe it aio 1 | PL in Fig DS.25 ao HR do dt G0, il đọ I 20 — = 1 Ps “fap hao củ —- ' P l—Doubier | 2 = II CE, = tee tt P as ha 0 Â, 1 am = 0 À~.W/ Làn ret eR ' \A =) | yuggh © % fF Teas Pea I 0 | v 20 | t 2 yo 4 | te n y.aoh 0 Yb 5g M8 | ‡ T (c) Final Distribution do Ì | Go Fig D3 23 {Shear Resist Web) () Fig D3, 25
P, ° \ = == = PL A8 shown in (a) the doubler whose thickness ts
% | do it qo hk đo | la yet to be determined is made to fit around the
tag (hoa | cut-out aS shown Reasonable internal radii are
Pe ee ———==—'—~ ° in the cut web and doubler at the corners to
keep stresses due to curved beam bending
== reasonable (see Chapter Cll) Attachments are
| 1/8 do {t 7/8 qo { ° provided as shown to pick up the basic shear | 23a 1 9 (c} Final Distribution =< „ ae & Ii Fig D3 24
always the case when a set of self-balancing shear flows are applied to a flat panel structure or to 4 3 dimensional box structure with 2 cut-out on any Side The reader should study the examples closely Although the method is shown only for a ‘let beam it is also applicable to any structure with a cut-out,
such as the box beam of Article AZl.3 This
has actually been illustrated in Solution No & of that article and the reader should review
it at this time
Sometimes framing 7 ars for a cut-out
are not conveniently available as were the
stiffeners and flanges of the beam used in the
previous examples In such cases they must,
of course, be provided
(0) Framing Cut-Outs with Doublers or Bents
Frequently a cut-out in the web of 4 beam must be so deep that it removes nearly all of
she web In this case the method previously
described cannot be used Instead the “brute-
forces” approach is necessary and a heavy doubler, or bent, is provided around the
cut-out to carry the shear This is illustrated
flow in the web The
shown in loading imposed on the doubler ts (o), mamely the shear flow đọ
Strictly speaking, the doubler should be
analyzed as a frame With reasonable symmetry
the loading in (c} can be assumed at the
center of the frame That is, one haif of
the total shear, q, x h/2 is resisted in the
top of the frame, one half in the bottom and a pin joint (no bending moment) exists at the
cut The bending moment axial loads and shears at any section of the frame follow as a matter
of statics For example,
At A-A,
= Ww do a
MSVx LIÊN +
(there may also be a little relieving motient due to do)
peeve 492
Fy = do X Z
The thickness of doubler required to take the
loads can thus be determined using standard methods of stress analysis Tne doubler
should have sufficient out of plane stiffness,
also, to provide simple support for the beam
web, as discussed in Chapter ClO This will
normally be provided oy the thickness required for strength purposes
Sometimes the nature of the cut-out is such that the frame (doubler) can be deeper In such a case, the
Trang 11
agumed to carry 2 greater
§ Ý, and the lower
n “This is illustrated
h not extend
93 2B and @ the | upper part is
the total shear and
as high as in biện
assumed to carry 4/5 of
she lower part only 1/5
The cases illustrated are for shear
resistant webs If 2 tension field is present the doubler must also be designed for the end- bay effects discussed in Chapter Cil V24/5 qoh 4o /8 4eh H[ ao { qo ! V=1/5 qọh Fig D3 26
(3) Providing access With Standerd Round Flanged Holes or "Donut Doublers®
Frequently a cut-out size requirement is
such that a standard round flanged hole will
provide the needed open space and strength In such cases either of the following van be
done when the beam is of the shear resistant
type:
I replace the web, locally, with a panel having a standard round nole that nas
a 45° flange, aS discussed in Chapter 010
2) Gut the required diameter nole into
the wed and attach a “donut” doubler
(ring) that has the same (or greater) stiffness in a direction normal to the
web as does the flange of (1) above
In either case the allowable shear can be
calculated as discussed in Chapter s10, Th se
allowebdles apply only to round hol not to
elliptical or rectangular noles nies Pianged
edges, which are weaker
Item (2) above is illustrated in Fig
03.27 Note that the nole spacing “b" of
Snapter C10 will oe quite large i? only one
hole is 2resent and {s not near the end of the
beam The role spacing “b" is, of course, used
in determining the allowable shear if no
stiffeners are 0rasent
I? a beaded web ts cut, a panel can
inserted locally, containing 2 nole with 2
yeaded flange, as discussed in Chapter C10
and these allowaoles used conservatively
If the beam 1s of che tension Ỹ method (c) does not apply Basic i Beam | Ring „ " Basic Beam aa Doubler ' 1 Ñ t 8 #
Cross Section of Ring
Doubler has same or Greater Out-of- Plane Stiffness (Igz} as Does 4§° Flange of Required Round Lightening Hole Fig D3, 27 View A-A heavier frame, as in method (3), the
that can take
tension field effects should be used
D3.7 Special Cases of Beam Design
There are several cases involving beam design not discussed in Chapters 710 and C11,
Since these cnanters are intended primarily
to present fundamental design principies The
designer will encounter {n practice, nowever
the following situations which include
ordinary straight beams and beams in the form
of bulkheads or frames:
a) Curved beams
b) Flanges with local changes in
direccion ("bent" flanzes)
€) Flanges sunject to
which tend "normal"
<9 Dand them
loads
4) requiring “stabil ization"
tor the plane of
puckling ow
the web
{a; Curved Beams
tion 27 2 curved Seam 28 curvature is the bending m {llustration purcoses of
When the outer (usp
in tension and the inner on
snere will te 2 collapsing (co
loading on the wed 2¢ smown in
because a "hooo” loading Is required <%&
tne ?1 trị % nward, Sonar
other load, P, in the
anda Ir
£P = 2⁄R
Trang 12ANALYSIS AND DESIGN OF FLIGHT VEHICLE STRUCTURES c Outer Flange roe vo TY Lo TS” Ty “ Inner “\ Pi Flange Pi xử ice R {a} A inner SPE Flange
wis, of course, lar
since R is smaller, ger at the inner flange,
t t 9 ater flange
This compressive loading tends to collapse
the wed [2 rrying shear,
13 the usual ca: ; 8n inter- ten formula wht ar stresses shou s Kling pp vo G th ld be used + This 18 t
stresses quoted for beaded pansls
flangsa noles in Cnapter C10 do
cply These sllowables were for straight
s with no normal loads, as present in curved deams Say PU Bos * Độ oF ct vỡ
@ curvature is enough to cause
eant compression in the web, stiffeners
be provided to take this load This can as shown in Pig D3.29 Solid Web & Stiffeners in Curved Portion \ \ Py is determined by assuming the flanges to be straight between stiffeners This gives the value of 9
ternal load analysis
int Than the stiffener
loads will be,
Pst, = 2Pa tan 9
Peta = 2P¡ tan 9
Any difference in stiffener end loads will be sheared into the web through the attaching fasteners
The flanges should be designed Zor ths
axial loads, P, in them Some allowance for bending moment, M, (Fig 03.30) can be made
by taxing Mas Pxex
-?Pxex 1/2 at the 5s These secondary effect:
when obviously smal?,
1/2 at the center and tciffener junction: S are sometimes ignored in practice Poe 2 P == <= Py M= Fig D3 30
"e" 1s the eccentricity between stiffener junctions due to tae flanges being curved rather than straight The same applies to the inner flange, except that it is in compression in this discussion
when the outer flange is in compression
and the inner one in tension, all of the
loadings are reversed, as in Fig D3.28(c)
and (d) The web is then subjected to tension
loads and there is no collapsing problem No
stiffeners required, cther than for the
"normal" reasons However, in vractice, most
to some degree, and
webs will then, of
Trang 13
ener is then sheared into the
iffener is not provided, some g present unless the kick load
web in compression, in which case it
le if thin If the beam is machined
stock the stiffener can be machined
? Iz the beam is "built up" from
eparate flanges and eb parts a stiffener can 2 made up from a tee nember attached to the 1 and an angle member attached to the web
the tee This is illustrated in Fig
The kick load path is from the flange
see to the angle to the web
rs
Fig D3 32
Tha loading shown will produce an up load on the bolts, putting tension into the leg of she tee and the stiffener angle A reversed loading would sush down on the tee producing
sompression The above tee and angle
combination could, of course, be replaced by
a ngle machined 7itting The important thing
is that an adequate stiffener, attached to the
outstanding legs, de present if all of the
outstanding leg is to function effectively (e} Flanges Subjected to Normal Loads Tending to Bend Them
Frequently the ?langes of a beam are
sudjected to loads which pull outward or push
inward It is important in each case that the
flange be “backed up” by a stiffener The
affect is the same as in the case just
discussed, (b), involving kick loads due to
mt flanges A similar stiffener arrangement
can be used
(4) Flanges Requiring Stabilization against
Sut-of-Plane Suckiing
In the cases of flange design discussed in
Chapters ClO and Cll, it was assumed that any
flange in compression was stabilized, or prevented from Duckling as 4 column, oy some
supporting member This member was usuaily
shown there as a “loor or a skin to which the
flange was attached Thus the flange could
not buckle since it was restrained in one
plane by the beam wed and in a plane normal
to this Dy some Sxin
Occasionally, nowever, shis out-of-plane
(normal to the web) supporting member ‘s not
inherently present and must be provided A
typical example ‘ts the inner flange of 2
?uselaze bulxhead or frame (the outer flange is stabilized, usually, dy the outer skin) The inner flange will usually be subject to 4
compression load over much of its length for
some design condition Since the flange will
have little stiffness of its own, its L/p will be small and it will buckle at very 10w
compressive stress levels as a long column
It is then necessary to provide supporting
members, aS shown in Fig 03.23, to reduce the 5 unsupported length and bring the buckling
stress up to an effictently nigh level, nearer
to the local crippling strength of the flange Outer Skin Frames with Inner 4 Flanges Stabilized
Against Out-of- Plane
Buckling Using Axial Members (Tubes) and Fittings or Intercostals Flange Must be Attached to Supporting Member in Stiff Manner Fig D3 33
The supporting member will not be subject
to any appreciable load but it must have
sufficient stiffness to prevent column buckling The required stiffness criteria will not be
discussed here but the reader should consult Rer (1) or similar textbooks to obtain such criteria Since the supporting members are 31zed by stiffness rather than strength re- quirements they can usually consist of light tubes or intercostals Their weight is
usually less than would result from beefing-up
the bulkhead flange for out-of-plane strength The stiffness of the supporting member must,
of course, include the end fitting attaching it to the flange
D3.8 Structural Skin Panel Details
"he general principles involving the
design of structural sain or floor panels are
covered in Chapters ClO and Cll Chapter S11 concerning buckling panels, in particular,
should be thoroughly understood Sy the
designer In addition to this information,
several details of design are oresented below
a) Rectangular holes
5) Recessed panels
c) Installation of long axial members
on panels
da) Spot welding sheet metal dcuolers
e) Tension skin splices
Trang 14ANALYSIS AND DESIGN OF FLIGHT VEHICLE STRUCTURES
(a) Rectangular Holes
t+ is frequently necessary to cut
rectangular holes into load carrying panels to
provide doors or to mount equipment These holes must, of course, be framed as discussed
1n Art 03.6, Zqually important from a fatigue
standpoint 1s that the internal corner radil
not be too small or cracks will eventually
start there AS an arbitrary design requirement it is suggested that the corner radii be
maintained at R = 35 inches or larger with the
normally obtained finish In those cases where
a@ smaller radius is absolutely necessary, it is suggested that the lower limit of R= 10 be maintained, and that an f-40 finish be
specified around the edge of the panel at the
comer
(bp) Recessed Panels
it is sometimes necessary to recess a
structural panel locally in order to mount equipment, as shown in Fig D3.d4 Corner Members Transierring Load P to Beams P = Out-of-Plane Kick Load + 2g, %4, (Self Balancing) pp (d = depth of recess) Balancing Kick Loads distribution Structure by Beams Provided Fig D3 34
The recessed panel can continue to carry its shear load but there will be out-of-plane "kick" loads These are resisted by the
framing members "a" and "b" as shown in the
figure and carried over ta some beam or frame
that can redistribute them {nto the main side
panels If the recess must be very deep and
its size 15 not too large, it may be better
to omit the panel and simply frame the
resulting hole This is more likely to be
the case if highly buckling skins are involved
The recessed panel cannot, of course, carry
tension stresses, only shear The hole in the
basic skin at the recess must not have sharp corners, as discussed in (a) above
(c) Installation of Axial Members on Skin
Panels
Whenever a local axial member, meaning one
lying in the direction of the main bending stresses of the overall structure (i.e fore~
and-aft in a fuselage), is installed, care
should be observed This member will tend to
D3.13,
become "erfective™ That is, 1t will develope
axial stresses as 1t strains along with the skin, particularly in tension Most of the
load picked up will enter through the
fasteners near the ends of the members The member tends to strain the same amount as the
panel it is attached to though it never is as
much due to the flexibility of the fasteners Thus the total load will be P = f_ skin x
Anember ag an upper limit, but only 60% to 80% of the skin axial stress is normally
developed in the member The larger the member
and the greater its length the more axial load it will develope Most of this load can be
considered to enter through the outer (end) 20%
of the fasteners at each end High loads and relatively larga bearing stresses will thus be present here When these are present in
the skin panel along with high skin tension
stresses, the fatigue life of the skin panel
will suffer Therefore to keep these induced
fastener loads lower, axial members (other
than primary structure) should be kept as short as 1s practical If their area is large
then their ends should be tapered There are
methods for evaluating these effects more
specifically, but they are beyond the scope
of this article The possible deleterious
effect should be anticipated as it can some- times cause cracks in panels
whenever a member is installed on 2
buckling (diagonal tension) shear panel it must be strong enough to prevent the possi-
bility of "forced crippling” failure due to
the action of the buckling skin The reader
should consult Chapter Cll in this respect for design criteria
(4) Spot Welded Doublers
Frequently some bay or area of a skin or floor panel must be made thicker than other bays because of higher local shear flows For
example, @ skin of 040 thickness might have
to be made 064 in a bay because of higher
leads as shown in Fig DS3.35 Rivets Along Edge of Doubler 040 Skin 025 Doubler Fig D3 38
One common way of achieving this, for large panels, is to spot weld (or sometimes
to bond) an 025 doubler to the 040 skin
The spot welds should be put in at a close specing to make the combination act as unit
than as two separate skins, which would
Trang 15
It 1s considered goed practice to
along the edge of the doubler,
ularly if the panel is of a tension field
esign with 1ts accompanying buckles
fg) Tension Skin Splices
Skin splices should be kept to a minimum number, but they are common in aircraft
structures The splices of major structural
skins should be given due consideration since they always contain stress concentrations and
limit the fatigue life
There are two factors present in tension splices which reduce the fatigue life:
a) There is the besic tension stress in the skin being spliced and the stress concentration due to the holes
b) There is also the bearing stress in
the holes, due to the fastener splice loads, which worsens the situation That is, tus combination of tension
and high bearing stresses in the skins is worse than tension stresses only
To keap the fatigue life as large as possible, the following practices should be
observed:
a) When more than one row of fasteners per side is required, as is the usual case:
1 Do not "stagger" the fastener
pattern but keep the holes in
line, as shown in Fig D3.36
This gives a lower stress concen- tration than staggered holes It is an interesting fact that two or more holes in line in the direction of the load gives a lower stress concentration factor than does a single hole
2 Avold using more than 3 rows of
attaciments per side
>} When possible use a double shear splice
(a splice plate on each side of the skins) This ig frequently seen in
centerline splices of wing skins
‘hen the skins are machined or chem=-
milled they can be left thicker at
the splice to reduce the tension and bearing stresses locally, as shown in
Fig 03.37
c) Maintain a fastener spacing in each
row and between rows of approximately
four times the fastener diameter (the
rows should be xept as close together as is practicable) Poor Practice (Holes Staggered) Good Practice (Holes in Line} Fig D3 36 Z Splice Bolts — Se +
Cross-Section of a Double Shear Tension Skin Splice With Skins Machined Thicker at the Splice Area (Angle Shown Due to Wing Dihedral and Thickness Taper )
Fig D3 37
D3.9 Additional Important Structural Details
The following list of details 1s pre-
sented with a minimum amount of comment as
representative of "good practice" In
addition to the following list, the details listed in Chapter B25 "Fatigue Analysis and Fail Safe Design” should de observed
1 Avoid mixing hole-filling and nen-hole- filling fasteners In the same pattern (i.e aluminum and hi-shear type rivets or steel bolts), when this cannot be
avoided, ream the holes for the non-
hole-filling fasteners to insure their picking up load better than a plain
drilled hole, with its "slop", would
produce
2 When using less than four non~hole-filling
fasteners tn a pattern, use reamed holes
to insure a better distribution of load
among the fasteners
3 When using fasteners in thin sheets
where the value of D/t (fastener diameter to sheet thickness {s greater than 5.5)
Trang 16
ANALYSIS AND DESIGN OF FLIGHT VEHICLE STRUCTURES D3.15
conservative extrapolations The thin at Py
sheet tends to “buckle” around the hole “ph at relatively low bearing stresses 42
4 Do not use hi-shear type fasteners in |
joints where the bottom skin 1s dimpled (he
5 Maintain an arbitrary margin of safety of D Typ +
.15 in shear joints for fastener patterns ha.o04 to allow for uneven distribution of loads (Typ)
Fig D3 38
6 Do not use spot welds to attach buckling
skins to their supporting frames unless 2 46) Por even a closed section to operate "one-shot" structure, such as a missile, efficiently the torsion should be capable is involved Even in these cases do not of being distributed into all sides of the
use spot welds on either side at the closed section This may require a
joggled area of a jJoggled member; use "bulkhead" type end fitting, or a much
rivets at the joggle thicker section Locally where the torsion
is put tn The action is similar to a
7, Do not use a “long string" of fasteners fuselage bulkhead distributing a twist
ina splice In such cases the end loading into the skins of the fuselage
fasteners will load up first and yield or a wing rib distributing an applied early Three, or at most 4, fasteners twist into the skins and spar webs per side is the upper limit unless a
carefully tapered, thoroughly analyzed 11 Compressive buckling does not necessarily
splice ts used Such a design can be mean failure it means failure only if
fashioned but 1s beyond the scope of this there is no other member to keep taking
article, additional load Shear buckling simply
indicates that additional load must be 8 Carefully insure against feather edges in carried as diagonal tension as discussed
all fitting design Re-entrant surface in Chapter Cll Of course, the members
intersections must have their edges supporting the web must be able to with-
rounded or else fatigue cracks will in- stand the ensuing so-called “secondary”
evitably begin in such places Any angle effects,
between surfaces less than 70° can be
considered a feather-edge These commonly When the compression skins of a fuselage
eccur in design if not watched, in or wing buckle they will carry no addi-
drilling and other machining operation tional compression load but the stringers
call-outs and flanges are still capable of this,
as discussed in Chapter Al9 The buckled
9 The permanent buckling data presented in skins can, however, carry additional
Chapter Cll applies to structures similar shear load through tension ffeld action
to fuselages where the sub-structure rings Thus to achieve a light efficient design are closely spaced as compared to the the designer should have a thorough radius of curvature of the skin That is, understanding of the factors involved the ratio of skin support spacing to after members have buckled as covered in radius of curvature 1s small, considerably other chapters of this book
less than one When the spacing of the
sub-structure becomes larger, permanent 12, Probably the most single important item
buckling should be considered to occur at regarding detail structural design is the
approximately the same shear stress that matter of equilibrium If the designer
produces initial buckling; that is when will show.the load equilforium ?or every
the above ratio approaches unity part of his assembly, most errors will be
prevented The majority of all structural
10 Avoid the use of "open section” members strength problems occur simply because the
when torsion 1s present Open section
members are extremely flexible compared
to closed sections as can be seen from
Chapter A6 For a given torsional stiff-
ness, an open section member will be far heavier than 2 closed one In the example
below, member "b" is 65 times as stiff as member "a" for pure torsion applied at
each end (see also the example in Chapter laws of statics nave not been observed,
and it is usually in the smaller detail
parts that the time is not taken to do
this It takes a considerable amount of experience to safely substitute the
“eyeball” for the slide rule and data
book The beginning detail designer and
many others who have been at it fora
Trang 17
in all areas of structural design The only Safe course in such circumstances is alaays
to show the member loads in static balance
Tor every part of a structural assembly When this is done 1t will aiso give the designer a better feel as to how the structure ts actually deflecting under load This can be of sig
nificant help in anticipating problems where
members are joined together and therefore must push, pull or pry on each other when loaded,
SOME IMPORTANT DETAILS IN STRUCTURAL DESIGN REFERENCES :
1 "Theory of Elastic Stability", Timoshenko