As the name implies, signposts are post bearing structures that offer information or guidance to people. They are a prominent feature of our highways, streets, city centers, villages, and areas of public gathering. Signposts are usually placed strategically away from obstruction as they are intended to show information like route direction, warnings, route assurance, traffic signs, commercial advertisements, etc.
EN 12899-1:2007 requires that signposts made of steel structures should conform with EN 1993-1-1:2005 (Eurocode 3). One of the major concerns in the design of billboards and signposts is the risk of failure under wind load, which has serious economic and safety consequences. A failed highway sign structure can cause injury to pedestrians, damage vehicles, and obstruct traffic. As a result, such structures that are exposed to the public must satisfy all needed safety considerations. Additional risks of vehicles colliding with sign structures should also be checked, with passive protection provided for such structures.
Actions on Sign StructuresWind action on signposts and billboards can be evaluated according to EN 1991-1-4:2005 (Eurocode 1 Part 4). ASCE 7-10 code of practice can also be used for the evaluation of wind load on sign structures. The National Annex to BS EN 12899-1:2007 recommends suitable wind loads for the majority of signs in the UK. Whilst is it more conservative than performing a full analysis, it is simpler and quicker.
Other forces that may need to be taken into account when designing sign structures are point loads and dynamic snow load (not applicable in Nigeria). The UK National Annex recommends that signs should be able to withstand a force of 0.5 kN applied at any point. This represents the load that might be exerted by, for example, a glancing blow from a vehicle mirror, a falling branch or malicious interference with the sign. This point load is the critical factor only for very small signs, but for signs mounted on a single support, it causes torsional forces that need to be considered.
For large billboards, live loads and the weight of services should be accounted for the in the design.
Design exampleProvide adequate sections for a sign structure with the configuration shown below. The sign post is located in an area that is 76 m above sea level with a wind speed of 35 m/s.
Mounting height above ground hm = 2.0 mWidth of sign face l = 3.0 mHeight of sign face b = 2.0 mTotal height H = hm + b = 4.0 mHeight to centroid of sign area z = hm + b/2 = 3.0 mDepth of post buried above foundation hb = 200 mm
Basic wind velocityLet the basic wind velocity from wind map = vb,map = 35 m/sThe altitude of site above sea level A = 76 mAltitude factor calt = 1 + 0.001A = 1 + (0.001 × 76) = 1.076vb,0 = vb,map ⋅ calt = 35 × 1.076 = 37.66 m/s
Assess Terrain OrographySite is not very exposed site on cliff/escarpment or in a site subject to local wind funnelling. Therefore co = 1.0
Determine Design Life Requirement
p = design annual probability of exceedencep = 1/design life = 1/25 = 0.04 (for signs design life is 25 years)K = Shape parameter = 0.2n = exponent = 0.5
Basic Wind Velocityvb = cdir ⋅ cseason ⋅ vb,0 cdir = directional factor = 1.0cseason = season factor = 1.0vb = 1.0 × 1.0 × 37.66 = 37.66 m/s
See AlsoPostpartum Summer Clothes and Outfit Ideas from a Mom of 4 — A Mom Explores | Family Trel Tips, Destination Guides with Kids, Family Vacation Ideas, and more!Comments Theme By The World In 35Mm47.55555, -122.55555List of the 300 Largest Automotive Companies in Germany| Incl. Revenues10 minute mean wind velocity hing probability P for an annual exceedence is determined by:
vb,25 years = vb ⋅ cprob vb,25 years = 37.66 × 0.96 = 36.15 m/s
Mean WindThe mean wind velocity Vm(z) at a height z above the terrain depends on the terrain roughness and orography, and on the basic wind velocity, Vb, and should be determined using the expression below;
Vm(z) = cr(z). co(z).Vb
Where;cr(z) is the roughness factor (defined below)co(z) is the orography factor often taken as 1.0
cr(z)= kr. In (z/z0) for zmin ≤ z ≤ zmaxcr(z)= cr.(zmin) for z ≤ zmin
Where:Z0 is the roughness lengthkr is the terrain factor depending on the roughness length Z0 calculated using;
kr = 0.19 (Z0/Z0,II)0.07
Where:Z0,II = 0.05m (terrain category II)Zmin is the minimum height = 2 mz = 3 mZmax is to be taken as 200 mKr = 0.19 (0.05/0.05)0.07 = 0.19cr(3)= kr.In (z/z0) = 0.19 × In(3/0.05) = 0.78
Therefore;Vm(3.0) = cr(z). co(z).Vb = 0.78 × 1.0 × 36.15 = 28.197 m/s
Wind turbulenceThe turbulence intensity Iv(z) at height z is defined as the standard deviation of the turbulence divided by the mean wind velocity. The recommended rules for the determination of Iv(z) are given in the expressions below;
Iv(z) = σv/Vm = kl/(c0(z).In (z/z0)) for zmin ≤ z ≤ zmaxIv(z) = Iv.(zmin) for z ≤ zmin
Where:kl is the turbulence factor of which the value is provided in the National Annex but the recommended value is 1.0Co is the orography factor described aboveZ0 is the roughness length described above.
For the structure that we are considering, the wind turbulence factor at 3 m above the ground level;
See AlsoArticles about 47.55555,+-122.55555 on Dwell.comIv(60) = σv/Vm = k1/[c0(z).In(z/z0)] = 1/[1 × In(3/0.05)] = 0.244
Peak Velocity PressureThe peak velocity pressure qp(z) at height z is given by the expression below;
qp(z) = [1 + 7.Iv(z)] 1/2.ρ.Vm2(z) = ce(z).qb
Where:ρ is the air density, which depends on the altitude, temperature, and barometric pressure to be expected in the region during wind storms (recommended value is 1.25kg/m3)
ce(z) is the exposure factor given by;ce(z) = qp(z)/qbqb is the basic velocity pressure given by; qb = 0.5.ρ.Vb2
qp(60m) = [1 + 7(0.244)] × 0.5 × 1.25 × 28.1972 = 1345.66 N/m2
Therefore, qp(3m) = 1.345 kN/m2
Determination of force coefficient (Table NA 2 BS EN 12899)λ = effective slenderness ratio of sign or aspect ratioλ = l/b = 3.0 / 2.0 = 1.5Therefore cf = 1.30
Calculation of the total wind force (Clause 5.3 of EN 1991-1-4)Fw = cscd ⋅ cf ⋅ qp(ze) ⋅ Aref (Aref = area of sign)cscd = 1.0 (for sign posts)
Fw = 1.0 × 1.30 × 1.345 × 3.0 × 2.0 = 10.5 kN
Partial Factor for Action γF (Table 6 EN 12899-1:2007 (E))ULS (bending and shear) γF = 1.5SLS (deflection) γF = 1.0γf3 = 1.0
Design Wind Force on the signFw,d = Fw ⋅ γF ⋅ γf3Fw,d (ULS) = 10.5 × 1.5 × 1.0 = 15.75 kNFw,d (SLS) = 10.5 × 1.0 × 1.0 = 10.5 kN
Ultimate Action EffectsUltimate design bending moment per post, MEdMEd = Wind force × lever arm to foundation / number of postsMEd = Fw,d (ULS) ⋅ (z + hb) / nMEd = 15.75 × (3.00 + 0.2) / 2 = 25.2 kNm
Ultimate design shear per postVEd = Wind force / number of postsVEd = Fw,d (ULS) /2 = 15.75 / 2 = 7.9 kN
The 0.5 kN point load on the sign is not critical, since it is less than the wind action and there are no torsional effects with 2 posts.
Try circular hollow section CHS 139.7 x 8 (S355)
A = 33.1 cm2; Wpl = 139 cm3; Ix = 720 cm4
Section classificationε = √235/fy = √(235/355) = 0.81Tubular sections (Table 5.2, sheet 3 of EN 1993-1-1:2005):d/t = 139.7/8 = 17.46Limit for Class 1 section = 50ε2 = 40.740.7 > 17.46; section is Class 1
Member resistance at ULSAccording to Table 7 of EN 12899-1:2007 (E), the material factor of safety for steel is γm = 1.05Moment Capacity MRd = fy⋅Wpl/γm = [(355 x 103 x 139 x 10-6)/√3)]/1.05= 46.99 kNmMEd/MRd = 25.2/46.99 = 0.536 < 1.0 Okay
Shear capacity VRd = Av(fy/√3)/γm Av = 2A/π = (2 x 33.1)/π = 21.027 cm2VRd = 21.027 × 10-4 × [(355 x 103)/√3)]/1.05 = 430.96 kNVEd/VRd = 7.9/430.96 = 0.018 < 1.0 Okay
Calculation of Temporary deflectionThe wind velocity for calculating the temporary deflection (SLS) criterion is 75% of the reference wind velocity, as it is based upon a 1 year mean return period. The 0.96 factor below reverses the cprob conversion from 50 to 25 year return period used above (Clause 5.4.1, note 1 EN 12899-1).
Fwd(1 year) = Fwd (SLS) x 0.752/0.962 = 10.5 x 0.752/0.962 = 6.41 kN
Uniformly distributed load along sign face = Fw,d (1 year) / bFw,d (1 year) / b = 6.41/2.0 = 3.2 kN/mwhere b = height of the sign face
Maximum deflection at top of sign (bending), δ
E = 210000 N/mm2I = 720 cm4n = number of posts = 2Other parameters are as defined above
δ = [3.2/(24 x 210000 x 720 x 104 x 2)] x [3(4000 + 200)4 – 4(2000 + 200)3 x (4000 + 200) + (2000 + 200)4] = 34.3 mm
Deflection per linear metre = δ’ = δ/(H + hb) = 34.3/(4 + 0.2)= 8.16 mm/mMaximum temporary deflection taken as class TDB4 = 25 mm/m8.16 mm/m < 25 mm/m. Therefore deflection is okay.
References The Institution of Highway Engineers (2010): SIGN STRUCTURES GUIDE Support design for permanent UK traffic signs to BS EN 12899-1:2007 and structural Eurocodes