1997 Ubc Vertical Earthquake Term Ev

[UmarMakhzumi]

*SEFP Consistent Design*

*1997 UBC vertical earthquake term*

*Doc No: 10-00-CD-0002*

*Date: May 30, 2013*

*Article is ripped: Good one to share though* 

 

For Strength Design, E_v has the effect of increasing compression and tension/uplift effects on vertical load carrying systems. E_v is not applicable for Allowable Stress Design. The new term, E_v, was introduced in the 1997 UBC. UBC Section 1630.1 defines E_v as the load effect resulting from the vertical component of the earthquake ground motion.

 

For Strength Design, Ev is defined as 0.5CaID. For Allowable Stress Design, Ev is defined as 0.

C_a= seismic coefficient from UBC Table 16-Q

I = importance factor from UBC Table 16-K

D = dead load

 

UBC Section 1630.1.1 defines the earthquake load, E, as the earthquake load on an element of the structure resulting from the combination of the horizontal component E_h and the vertical component E_v.

E = Rh*Eh + Ev (UBC 30-1)Rh= redundancy factor defined in UBC Section 1630.1.1Eh = earthquake load resulting from either the base shear, V, or the design lateral force, FpSubstituting the definition of Ev into this equation results in:E =Rh*Eh + 0.5CaID (Modified 30-1)The 1997 UBC defines load combinations in Section 1612. Strength load combinations 12-5 and 12-6include E.1.2D + 1.0E +(f1L + f2S) (UBC 12-5)0.9D (1.0E or 1.3W) (UBC 12-6)Substituting modified equation 30-1 into these equations results in:1.2D + 1.0 Eh + 0.5CaID + (f1L + f2S) (Modified 12-5)(0.9 + 0.5CaI)D + Eh (Modified 12-6a)(0.9 - 0.5CaI)D - Eh (Modified 12-6b)

All terms with E_h are effects of horizontal earthquake components. These loads can be in any direction, for example, vertical loads on rigid frame columns, horizontal loads on columns, and diagonal loads on braced frames. Similarly, all terms with D, L, or S are effects of vertical loads or components. These loads can be in any direction, for example, vertical loads on beams, horizontal loads on rigid frame columns, and diagonal loads on braced frames.

Example:For typical California values of Ca = 0.40 and I = 1, the modified equations become:1.4D + 1.0Rh*Eh + (f1L + f2S)1.1D + Rh*Eh0.7D + Rh*Eh

The impact of the vertical earthquake component on modified Strength Design equations 12-5 and 12-6a is to increase compression effects on columns and foundations. The impact of the vertical earthquake component on modified Strength Design equations 12-6b is to increase tension and uplift effects on columns, anchorage, and foundations. There is no impact of the vertical earthquake component on Allowable Stress Design load combination equations.

[abdulqadeer29]

In etabs we use EQX , EQY for respective directions. so this earthquake force should have carry both its vertical and horizontal component with itself. so there is no need to define these modified equations when we are assisted by software. but during manual calculations we have to incorporate these modified equations.

Is my understanding correct?  kindly correct and explain me if i am wrong ???

[WR1]

AQ, you need to include additional vertical effects, in addition to the horizontal effects of EQx and EQy

In ETABS, if vertical component is applicable in your design then you must select appropriate settings from special seismic loading form and define rho and omega factors.

[WR1]

Another thought;

1. Vertical effects are not required in ASD analysis of UBC-97 but are required in ASCE 7-05.

2. Inclusion of vertical effects make no sense according to Gary R.. Searer;

SEAOC adopted the concept of vertical effects for the sole purpose of adjusting the seismic design in line with 1.4D factor of dead loads in earlier codes. This factor was changed to 1.2D due to better approximation of dead loads in the new codes. So in order to rule out the discrepancies vertical effects were introduced.

"Certain unintended consequences of this action were only discovered after the code was published"... For example one design that was safe in ASD suddenly became unsafe in strength design combinations. In near-fault areas of Ca=0.6, resistance to overturning decreases by a huge margin to 60% of dead loads.

The author explains that;

With the exception of a single story structure, ignoring live load totally in the resisting load combination is very very conservative.

In order to accelerate portions of the building rapidly upward, the upward forces must exceed gravity by a large amount, thus resisting weight is significantly greater than just 1.0 dead load.

The problem became more complicated with the inclusion of vertical effects in ASD in ASCE 7-05 in a try to align the ASD design with strength design as far as vertical effects were concerned.

He recommends using 1.2D +- E + f1L + f2S or 0.9D +- Eh.

Refer to:

 
2006 Annual Meeting of the Los Angeles Tall Buildings Structural Design Council  Alternative Procedures for Design of Tall Buildings
 POORLY WORDED, ILL-CONCEIVED, AND UNNECESSARY CODE PROVISIONS
Gary R. Searer, S.E. Consultant Wiss, Janney, Elstner Associates, Inc

https://www.scribd.com/doc/285836953/Poorly-Worded-Ill-Conceived-and-Unnecessary-Code-Provisions

 

[EngrUzair]

The article quoted above by Rana, can more easily be downloaded in full from the following link:-

http://peer.berkeley.edu/tbi/wp-content/uploads/2010/09/Poorly_Worded_Ill-Conceived_and_Unnecessary_Code_Provisions.pdf

Regards.