SUMMARY OF DEFLECTIONS FOR FLOOR BEAM LOAD
DETERMINATIONS by John Weaver, P.E.
Various field studies have indicated that AASHTO live load distribution factors are too
conservative for determining covered bridge floor beam deflections at specific locations. Results
at four different sites with various vehicle loads have yielded a variety of results for different types
of wood deck and beam combinations, however all results are a fraction of what would be
predicted by AASHTO load factors. Of course stiffness of beams relative to deck stiffness and
the presence of runner planks above the deck as well as magnitude of live loads and beam spacing
all contribute to structural behavior. Measurements indicate maximum deflections ranging from
19% to 58% of those predicted by AASHTO for concentrated live loads. The following table
summarizes the results:
Covered Bridge  Axel Load  Floor Beams  Deck
Type 
% of AASHTO Deflection. 
Worrall:  9.0 kip  6 x 13 3/4 @ 2'0"  5 1/2" nail lam.
deck  58% of defl. 
Larkin:  9.0 kip  4 x 10 1/4 @1'9"  4" plank deck  50 % of
defl. 
Brown:  10.48 kip  5 1/4 x 11 1/4 @ 2'0"  5 1/2" naillam w/runners
 19% of defl. 
Comstock:  2.4 kip  7 3/4 x 11 1/2 @ 4'0"  5 1/2" naillam.
deck  42% of defl. 
Additionally, Hoyle Tanner Associates, through the University of Maine, demonstrated in a
related recent laboratory study that a three beam (6" x 10") nail laminated deck model loaded
with a 2.4 kip load placed at the middle of the center beam produced deflection measurements
showing that the concentrated load was evenly distributed to the three supporting beams. Unlike
covered bridge floor beams, the lab model beams had unyielding supports and the entire system
was limited to three transverse beams (@ 3'6" spacing) with a nail laminated deck system,
however the results were interesting and relative to the results of field floor beam
investigations.
Although the above referenced studies are too limited in scope to establish new general
guidelines for floor beam load distribution factors, they do indicate that a specific site and load
condition study is required to determine a more exact load factor at (almost) any covered bridge
site. A more exact floor beam load factor would yield a more realistic live load capacity analysis
of existing beams and reduce size (and resultant dead load) of any replacement floor beams.
Attached are the records of member properties, loads and deflection results for the Brown,
Worrall and Larkin bridges listed above. Vermont Agency of Transportation has given permission
for publication of this data. Cross Sections and site data are copied from 1995 McFarland
Johnson (Phil Pierce) evaluation reports. Floor beam deflection data was derived from field
measurements and studies produced by McFarlandJohnson (Phil Pierce) 19931994. Note that
longitudinal distribution beams at the Worrall and Larkin bridges were loosened before
measurements (indicated here) were taken. However these same studies demonstrated that the
presence of the longitudinal beams, when connected, had no influence on the maximum magnitude
of the floor beam deflections measured.
Comstock site data was obtained by the author and assistant in April of 2002. Comstock
readings were taken with longitudinal distribution beams attached to the transverse floor beams.
Measurements were referenced from either a ledge surface beneath the bridge or tie crossbeams
above the bridge deck.
WORRALL COVERED BRIDGE
Type of Bridge: Plank lattice trusses with sawn floor beams.
Floor Beam = 6 x 13 3/4, assume MOE = 1,200,000 psi
I = 1/12 x 6 x 13.75 x 13.75 x 13.75 = 1300 in.^{4}, S = 189 in.^{3}
Deflection:
AASHTO transverse beam factor = 2.0/5.0 = 0.40
max. live load deflection = (Pa/24EI) (3 x L x L  4 x a x a)(@ center of beam span)
L = 15.83' x 12 = 190", a = (190 72)/2 = 59"
max. live load defl. = (4.50 kip x 59)( 3 x 190 x 190  4 x 59 x 59)/(24 x 1,200,000 x
1300)
max. live load deflection = 0.669" for entire axle load
40% of axle load yields 0.268" live load deflection @ center of span = AASHTO
predicted.
greatest floor beam deflection measured @ center of span = 0.155"
0.155"/0.268" = 58% of AASHTO predicted deflection.
Estimated Stresses:
AASHTO predicted = Pa/S = 4.50kip x 0.40 x 59/189 = 562 psi
By deflection measured = 0.58 x 562 = 326 psi
LARKIN COVERED BRIDGE
Type of Bridge: Kingpost trusses with sawn floor beams.
Floor Beam = 4 x 10 1/4, assume MOE = 1,200,000 psi
I = 1/12 x 4 x 10.25 x 10.25 x 10.25 = 359 in.^{4}, S = 70 in.^{3}
Deflection:
AASHTO transverse beam factor = 1.75/4.0 = 0.438
max. live load deflection = (Pa/24EI) (3 x L x L  4 x a x a)(@ center of beam span)
L = 14.00' x 12 = 168", a = (168 72)/2 = 48"
max. live load defl. = (4.50 kip x 48)( 3 x 168 x 168  4 x 48 x 48)/(24 x 1,200,000 x
359)
max. live load deflection = 1.576" for entire axle load
43.8% of axle load yields 0.691" live load deflection @ center of span = AASHTO predicted
greatest floor beam deflection measured @ center of span = 0.345"
0.345"/0.691" = 50% of AASHTO predicted deflection.
Estimated Stresses:
AASHTO predicted = Pa/S = 4.50kip x 0.438 x 48/70 = 1352 psi
By deflection measured = 0.50 x 1352 = 676 psi
BROWN COVERED BRIDGE
Type of Bridge: Plank lattice trusses with sawn floor beams.
Floor Beam = 5 1/ 4 x 11 1/4, assume MOE = 1,200,000 psi
I = 1/12 x 5.254 x 11.25 x 11.25 x 11.25 = 623 in.^{4}, S = 111 in.^{3}
Deflection:
AASHTO transverse beam factor = 2.0/5.0 = 0.40
max. live load deflection = (Pa/24EI) (3 x L x L  4 x a x a)(@ center of beam span)
L = 16.68' x 12 = 200", a = (200 72)/2 = 64"
max. live load defl. = (5.24 kip x 64)( 3 x 200 x 200  4 x 64 x 64)/(24 x 1,200,000 x
623)
max. live load deflection = 1.94" for entire axle load
40% of axle load yields 0.775" live load deflection @ center of span = AASHTO
predicted.
greatest floor beam deflection measured @ center of span = 0.145"
0.145"/0.775" = 19% of AASHTO predicted deflection.
Estimated Stresses:
AASHTO predicted = Pa/S = 5.24kip x 0.40 x 64/111 = 1209 psi
By deflection measured = 0.19 x 1209 = 230 psi
COMSTOCK COVERED BRIDGE
Type of Bridge: Plank lattice trusses with sawn floor beams.
Floor Beam = 7 3/4 x 11 1/2, assume MOE = 1,200,000 psi
I = 1/12 x 7.75 x 11.5 x 11.5 x 11.5 = 982 in.^{4}, S = 171 in.^{3}
Deflection:
AASHTO transverse beam factor = 4.0/5.0 = 0.80
max. live load deflection = (Pa/24EI) (3 x L x L  4 x a x a) (@ center of beam span)
L = 17.83' x 12 = 214", a = (214 57.5)/2 = 78"
max. live load defl. = (1.20 kip x 78)( 3 x 214 x 214  4 x 78 x 78)/(24 x 1,200,000 x
982)
max. live load deflection = 0.374" for entire axle load
80% of axle load yields 0.299" live load deflection @ center of span = AASHTO predicted
greatest floor beam deflection measured @ center of span = 1/8" = 0.125"
0.125"/0.299" = 42% of AASHTO predicted deflection.
Estimated Stresses:
AASHTO predicted = Pa/S = 1.2 kip x 0.80 x 78/171 = 438 psi
By deflection measured = 0.42 x 438 = 184 psi
